How Things Work - Additional Demonstrations
Section 4.4 Elevators
96. A Jackscrew Elevator
Description: A person stands on a metal plate that's supported from a jackscrew automobile jack. Another person furiously turns the crank that rotates the nut that supports the jackscrew and the metal plate slowly rises upward.
Purpose: To show that, while a jackscrew elevator is possible, it isn't very practical.
Supplies:
1 jackscrew automobile jack, rigidly attached to plates at its top and bottom and equipped with a crank to turn the nut that lifts the screw
Procedure: Place the jackscrew elevator on the floor and have someone stand on the upper plate. Then have another person turn the crank so that the screw slowly rises upward. Point out that the person turning the crank is doing work on the crank (pushing it in the direction that it's turning) and that this work is lifting the person on the plate. A small force exerted over a long distance is providing a large force exerted over a short distance. Note, however, that it takes a long time to lift the person on the plate, making this jackscrew elevator rather impractical.
Explanation: The jackscrew and the nut that lifts it are really inclined planes (ramps) that are wrapped into cylinders. The person turning the crank is effectively sliding one inclined plane across the other and causing the inclined plane that is the jackscrew to rise upward. They are obtaining mechanical advantage—a small force exerted over a long distance is providing a large force exerted over a small distance.
97. A Hydraulic Elevator
Description: You rest an uninflated balloon on the table and hold a small weight on its top. You then inflate that balloon and the weight rises.
Purpose: To show that a solid object can rise up on a trapped volume of hydraulic fluid.
Supplies:
1 rubber balloon
1 small weight
Procedure: Place the uninflated balloon on the table and hold a small weigh on top of it. Now carefully inflate the balloon. The balloon will inflate, carrying the weight upward with it.
Explanation: As you blew high pressure air into the balloon, you did work on that air and the air in turn did work on the weight, lifting it upward against the force of gravity.
98. A Bigger Hydraulic Elevator
Description: A person sits on a large plate that's on top of a plastic garbage bag. When air is pumped into the garbage bag by a vacuum cleaner (running backward), the bag inflates and the person rises into the air.
Purpose: To show that even a small pressure inside a large container can exert enormous forces on the walls of that container.
Supplies:
1 heavy-duty plastic garbage bag (or better yet, several of them, one inside the other)
1 stiff plate that's almost the size of the garbage bag
1 vacuum cleaner that can be run so that it pumps air into its hose
duct tape
Procedure: Tape the plate to the top surface of the plastic bag and then seal the open end of the bag around the outlet of the vacuum cleaner hose. It's best to seal most of the bag's open end to itself with the duct tape (like closing a zipper) and then duct tape the small open corner of the bag to the hose. Place the assembly on the floor or on a large, sturdy table and have a person sit on the plate. Hold that person's hand so that they don't fall over as the bag inflates. Now turn on the vacuum cleaner so that it inflates the bag. The person will rise into the air as air enters the bag and inflates it.
Explanation: Although the air pressure inside the bag is only slightly above atmospheric pressure, it acts on the whole surface area of the plate. The upward force that the plate experiences, due to the high pressure below it than above it, is more than the person's downward weight and the plate accelerates upward.
Alternative Procedure: Inflate a dozen or more rubber balloons and put them on the floor underneath an inverted table. If the area under the inverted table is packed densely enough with balloons, 4 or 5 students will be able to stand on the table without popping any of them. Make sure that someone stands outside the table and holds onto its legs to keep it from tipping over as people climb onto the table.
99. A Hydraulic Jack
Description: Pumping the handle of a hydraulic jack many times causes its main piston to rise only a small distance. However, the piston exerts an enormous force on the object above it. (We have a hydraulic jack that crushes the object above it against a fixed plate—we typically crush a broken electronic device such as an old calculator.)
Purpose: To show that a small force exerted over a long distance on the small piston of a two-piston hydraulic system can exert a large force over a small distance on the large piston of the system.
Supplies:
1 hydraulic jack system, with a hand-operated pump
1 heavy object to put on the jack (or an object to crush, if you have a plate bolted in place above the large piston)
Procedure: Put the jack on the floor or table and place the heavy object on top of its large piston. Explain that moving the handle of the pump up and down pushes a small piston in and out of a narrow hydraulic cylinder. Pushing the handle down pressurizes the hydraulic fluid and squeezes it out of the cylinder, through a one-way valve, and into the large cylinder beneath the large piston. The pressure that you create in the small cylinder by pushing the piston into it also appears in the large cylinder, where it pushes upward on the large piston. But because the large piston has much more surface area than the small piston upon which you push, the upward force that the large piston experiences is much greater than the downward force you exert on the small piston. That's why you can lift a very heavy object with the large piston while exerting only a modest force on the pump handle. Show them that this is so by pumping the handle and lifting the heavy object. Point out that you must move the pump handle a very long distance to raise the heavy object even a small distance. That's because the mechanical advantage in the hydraulic system is allowing you do the work of lifting the heavy object—a large upward force exerted over a small distance—by exerting a small downward force over a very large distance.
Explanation: Pushing the large piston out of the large cylinder requires much more fluid than is provided by pushing the small piston into the small cylinder. Thus you must push the small piston into the small cylinder many times, replenishing its fluid each time from a reservoir, before the large piston moves a reasonable distance out of the large cylinder.
100. A Simple Cable-Lift Elevator
Description: A chair is lifted from above by pulling on a rope that's attached to it.
Purpose: To show that tension in a cord can convey an upward force to an object that's attached to that rope.
Supplies:
1 chair
1 rope sling that attaches to all four legs of the chair and provides a single loop above the chair to which you can attach a rope
1 rope
Procedure: Attach the rope sling to the chair and attach the rope to the sling. The rope should descend to the chair from the space above the front of the lecture hall. Go up to the top of the rope and lift the chair upward from above at constant velocity. Announce that to make things simple, you will assume that the rope weighs nothing. Point out that you are pulling upward on the rope with a force equal in magnitude to the weight of the chair and that the chair is pulling downward on the rope with a force equal in magnitude to its weight. The rope is thus conveying your force to the chair and the chair's force to you. It has a tension in it equal to the magnitude of the force you and the chair exert on it (not twice that amount).
Explanation: The rope acts as an intermediary between you and the chair. When you pull upward on the rope with a certain force, the rope pulls upward on the chair with that same force (assuming the rope itself doesn't have any weight or mass). The force "passing through" the rope is called the tension in the rope.
101. A Simple Cable-Lift Elevator with a Pulley
Description: A chair is lifted from above by pulling down on a rope that's attached to it and that passes over a pulley near the ceiling.
Purpose: To show that tension in a cord can convey a force to an object, even when that rope passes over a pulley and the directions of the two forces are no longer the same.
Supplies:
1 chair
1 rope sling that attaches to all four legs of the chair and provides a single loop above the chair to which you can attach a rope
1 rope
1 pulley attached near the ceiling
Procedure: Attach the rope sling to the chair. Drape the rope over the pulley near the ceiling and attach one end of it to the top of the rope sling. Again, announce that to make things simple, you will assume that the rope weighs nothing. Pull downward on the rope so that the chair rises at constant velocity. Point out that you are pulling downward on the rope with a force equal in magnitude to the weight of the chair and that the chair is pulling downward on the rope with a force equal in magnitude to its weight. The rope is thus conveying your force to the chair and the chair's force to you, even though the pulley is changing the directions of those forces. The rope has a tension in it equal to the magnitude of the force you and the chair exert on it (not twice that amount).
Explanation: The rope acts as an intermediary between you and the chair. When you pull on the rope with a certain force, the rope pulls on the chair with that same force (assuming the rope itself doesn't have any weight or mass). The force "passing through" the rope is called the tension in the rope. But the pulley is changing the directions of the force so that, while the tension remains constant throughout the rope, the direction of the force that you exert on the rope isn't the same as the direction of the force the rope exerts on the chair.
102. A Cable-Lift Elevator with a Multiple Pulley
Description: A chair is lifted from above by pulling down on a rope that's part of a multiple pulley system extending from the ceiling to the chair. Even though a person is sitting in the chair, it takes only a modest force exerted on the rope to lift the chair upward.
Purpose: To show that tension in the cord passing through a multiple pulley can be used several times to exert enormous inward forces on the two ends of a multiple pulley system.
Supplies:
1 chair
1 rope sling that attaches to all four legs of the chair and provides a single loop above the chair to which you can attach a rope
1 multiple pulley system that hangs from the ceiling
Procedure: Attach the top of the multiple pulley to the ceiling. Attach the rope sling to the chair and connect the top of the rope sling to the bottom of the multiple pulley. The end of the multiple pulley's cord should extend downward from the upper portion of the multiple pulley. Have someone sit in the chair. Now pull downward on the rope. The chair and person will rise upward. Point out that you are exerting a relatively small force on the rope and thus creating only a modest tension in the rope. However, because that same cord extends many times between the top and bottom of the multiple pulley, the inward forces exerted by its tension is used several times. If there are 5 rope segments between the two ends of the pulley, then the tension is used 5 times and the force pulling upward on the chair is 5 times the tension in the rope. Note that although you only have to exert a modest force on the cord to lift the chair and person, you must exert that force over a long distance to lift them a short distance.
Explanation: You obtain mechanical advantage with the multiple pulley—a modest force exerted on the rope over a large distance exerts an enormous force on the chair over a small distance.
103. Lifting Yourself with a Cable-Lift Elevator
Description: You sit in the chair of the previous demonstration and lift yourself upward.
Purpose: To show that you can even lift yourself with a multiple pulley.
Supplies:
The setup for the previous demonstration
Procedure: Sit in the chair of the previous demonstration and ask whether you will be able to lift yourself upward. The answer is yes. Point out that you are holding in your hand an additional segment of the multiple pulley—should that make it easier or harder to lift yourself upward? Now pull down on the rope in your hand and you will begin to rise upward.
Explanation: When you sit on the chair and pull downward on the end of the cord of the multiple pulley, there is one additional segment of cord pulling upward on you and the chair. It takes even less tension in the cord to lift you and the chair than it would if someone else were lifting you by pulling on the cord.
104. The Value of a Counterweight
Description: You operate a toy elevator system with an elevator car on one side and a counterweight on the other. As the elevator car rises, the counterweight descends and vice versa.
Purpose: To show how a counterweight can store energy as the elevator car descends and then provide energy to lift the car upward the next time.
Supplies:
1 toy cable-lift elevator—consisting of a "car" and a counterweight, both suspended from the same string that passes over two horizontally separated spools that turn easily on a support
2 pretend passengers (small weights)
Procedure: Start with the elevator car low and the counterweight high. Put the two passengers into the elevator car and begin to raise the elevator car by turning one of the spools. Point out that while the cord is doing work on the elevator car by lifting it upward, the counterweight is doing work on the cord by pulling its other end downward—energy is flowing from the counterweight to the car. Now begin to lower the elevator car. Point out that while the elevator car is doing work on the cord by pulling its end downward, the cord is doing work on the counterweight by pulling it upward.
Explanation: The counterweight provides some of the energy needed to lift the car upward as the car rises and it stores some of the energy released by the car as the car descends.
Section 5.4 Vacuum Cleaners
138. A Fan in a Pipe
Description: A small fan located between two sections of pipe causes the pressure to rise in one pipe and drop in the other.
Purpose: To show that a fan increases the total energy of the air passing through it.
Supplies:
1 small "boxer" fan (a computer fan)
2 segments of pipe that fit tightly against the outer edges of the fan
2 pressure gauges (manometers) for the two pipe segments
Procedure: Attach the two segments of pipe to the two sides of the fan and note that both pressure gauges read atmospheric pressure. Now start the fan. The upwind pressure gauge will drop, showing that the air in that portion of pipe has converted some of its pressure potential energy into kinetic energy so that it can flow toward the fan. The downwind pressure gauge will rise, showing that the air in that portion of pipe has an increased total energy—both its kinetic energy and its pressure potential energy are greater than they were before you turned on the fan.
Explanation: The fan does work on the air that passes through its blades and increases the total energy of that air. This increased total energy is reflected in a rise of both the air's pressure and speed.
139. Chalk Dust in the Air
Description: Two chalk erasers are pounded together, releasing a cloud of dust that hangs in the air. A piece of chalk is released and drops quickly to the table.
Purpose: To show that chalk dust isn't supported by buoyant forces—it's supported by viscous drag forces.
Supplies:
2 chalky erasers
1 piece of chalk
Procedure: Smack the two erasers together and observe the cloud of chalk dust that hangs in the air. Now drop a piece of chalk and observe that it falls quickly. Note that the chalk is far more dense than the air it displaces, so that neither the piece of chalk nor the chalk dust is support by buoyant forces. Note instead that the chalk dust is supported by viscous drag forces—as the chalk begins to descend through the air, the air molecules exert an upward viscous drag force on it and support it against the force of gravity.
Explanation: Chalk dust has so much surface area relative to its volume that its motion is dominated by air resistance. Because of the dust's small size, the air flow around it is generally laminar and the only drag force it experiences is viscous drag—the molecular friction that occurs when the dust moves relative to the air. In effect, the dust pulls the surrounding air with it because of viscous interactions in the air. This pulling of the air slows the dust's decent and limits its downward speed to only a few millimeters per second (its terminal velocity).
Section 6.5 Clothing and Insulation
174. Poor Conductors of Heat - Glass
Description: You hold a glass rod in your hand and heat its other end until that end melts.
Purpose: To show that glass is a very poor conductor of heat.
Supplies:
1 glass rod (about 20 cm is fine)
1 gas burner
1 piece of paper (to burn)
matches
water (to put out the burning paper, if necessary)
Procedure: Light the burner. Hold one end of the glass rod in your hand and place the other end of the rod in the burner flame. After a few seconds, the rod will begin to melt. Note that the end you are holding is still cool. Note also that you can barely see the red glow emitted by the hot glass—it's emissivity is very low in the visible (it's transparent and doesn't couple well to visible light). Now remove the glass rod from the flame and touch it to the paper (be sure that nothing else flammable is nearby). The paper will burst into flames. Carefully extinguish the paper.
Explanation: Glass is a poor conductor of heat primarily because it has no mobile electrons—it's an electric insulator. Heat flows so slowly through glass that you can heat one end of the rod red hot while the other end of the rod remains cool.
175. Which Feels Hotter, Glass or Metal?
Description: Your students touch two chilled plates with equal temperatures. One is glass and the other is metal. The metal plate feels much colder than the glass plate.
Purpose: To show that thermal conductivity plays a role in our perception of temperature.
Supplies:
1 glass plate (or a plastic plate)
1 metal plate (copper would be best, but aluminum will do, too)
ice
Procedure: Chill both plates in the ice so that they have equal temperatures. (Or you can even use room temperature plates, assuming that the room isn't too warm.) Remove the plates from the ice, dry them completely, and have the students touch them. The metal plate will feel much colder than the glass plate.
Explanation: You perceive an object as cold because it extracts heat from your skin. The surface of the glass plate quickly warms up as heat flow into it from your skin, so the rate at which heat flows out of you soon decreases. As a result, the glass doesn't feel very cold. The metal plate conducts heat well, so that you will be unable to heat its surface significantly. Heat will continue to flow rapidly out of your skin to the plate, so the plate will feel very cold.
Follow-up: Why is it more hazardous to touch your tongue to a metal surface in freezing cold weather than it is to touch it to a glass surface?
176. Glass Wool - Insulating with Air
Description: You place a coin on a thick layer of glass wool that you're holding in your hand and heat that coin red hot with a blowtorch. Your hand remains cool.
Purpose: To show that air trapped in a fibrous material is an excellent thermal insulator.
Supplies:
1 thick pad of glass wool (I use the glass fiber wrapping material that's used with large hot or cold water pipes)
1 coin (a solid copper penny works well; but don't use a recent penny—after about 1982—because it will be made of zinc and will melt)
1 hand-held propane torch
matches
water (in case you have to cool anything quickly)
Explanation: Because the glass fibers trap the air and prevent it from undergoing convection, the only way that heat can flow from the coin to your hand is via conduction through the air and glass. Since neither of these materials is a good conductor of heat, that heat flow is very slow. Even though the coin is red hot, your hand remains cool.
177. Countercurrent Exchange in Your Arm
Description: You immerse your hand in a bucket of ice. Even though your hand becomes quite cold, your body remains warm.
Purpose: To show how heat exchange processes in your arm allow your hand to become much colder than your body without wasting very much thermal energy.
Supplies:
1 bucket of ice
2 skin thermometers (thin plastic strips with liquid crystals inside and numbers on their surfaces that indicate the current temperature of your skin)
Procedure: Place one of the skin thermometers on your hand and the other on your upper arm. Note the temperatures at these two locations. Then immerse your hand in the ice. Despite the continuing flow of blood to and from your hand, only your hand becomes cold. The temperature of your hand drops substantially while the temperature of your upper arm remains unchanged.
Explanation: As blood flows toward your hand, it gives up heat to the blood returning from your hand in a process called countercurrent exchange. The temperature of the blood decreases on its way to your hand and increases on its way back.
Description: A thermopile measures the heat emitted by a warm surface. You put various materials over that warm surface and the amount of thermal radiation detected by the thermopile decreases.
Purpose: To show how various barrier layers reduce radiative heat transfer.
Supplies:
1 warm, black surface
1 thermopile
1 plate of glass
1 sheet of aluminum foil
1 sheet of cloth
Procedure: Point the thermopile at the warm surface and note how much thermal radiation that surface is emitting. Now insert the glass, aluminum foil, and cloth in between the surface and thermopile, one at a time. In each case, the amount of thermal radiation will decrease.
Explanation: Glass is a good absorber and emitter of room-temperature thermal radiation (infrared light). Since the glass is cooler than the warm surface, the glass absorbs more of the warm surface's thermal radiation than the glass emits itself. The thermopile sees the glass's weaker thermal emission rather than the warm surface's stronger thermal emission. Aluminum foil is a good reflector of thermal radiation. It blocks the thermal radiation from the warm surface and allows the thermopile to see a reflection of its own weak thermal radiation. The cloth absorbs hot surface's thermal radiation and emits thermal radiation of its own. Since the cloth exchanges heat readily with the air around it and is at room temperature, it emits less thermal radiation than the warm surface.
179. A Thermos Bottle or Dewar
Description: A Thermos bottle or dewar holds a very hot or very cold liquid and keeps it hot or cold for a long time.
Purpose: To show that it's possible to stop virtually all heat transfer in some cases.
Supplies:
1 Thermos bottle or dewar flask
liquid nitrogen or another hot or cold liquid
Procedure: Pour the liquid nitrogen into the Thermos bottle or dewar flask. After a few seconds of violent boiling, the liquid will settle and boil relatively gently. The liquid will take a very long time to boil away completely.
Explanation: The Thermos bottle or dewar has a double wall structure, with a vacuum in between. The double wall makes it difficult for conduction to transport heat to or from the contents of the bottle. The walls are made of a material with a poor thermal conductivity (either glass or stainless steel). The space between the two walls contains a vacuum, so that convection can't carry heat between the walls. And in many Thermos bottles or dewar flasks, the inner surfaces of the double walls are mirrored to prevent radiation from carrying heat between the walls. With almost no way for heat to flow to or from the liquid in the container, it remains hot or cold for a very long time.
Section 6.6 Thermometers and Thermostats
180. An Illustration of Thermal Expansion - Cans and Rubber Bands
Description: Several beverage cans are connected with rubber bands to form a lattice. When they're "heated" to higher temperatures and vibrate relative to one another, their average spacings increase—the lattice expands.
Purpose: To show why heating most substances causes them to occupy more volume.
Supplies:
2 or more beverage cans
rubber bands
Procedure: Place the cans upright on a table and arrange them to form a lattice. Now attach adjacent cans to one another with rubber bands—loop each rubber band around two adjacent cans. When you hold up this lattice and don't push on it, the lattice is analogous to a solid at absolute zero. But if you begin to stretch and release the rubber bands, so that the cans begin to vibrate against one another, the lattice is analogous to a solid at a finite temperature. The more vigorous the vibrations, the hotter the solid. If you now observe the average spacings between the cans (the atoms), you'll see that they become larger as the system's temperature increases.
Explanation: At absolute zero, the atoms in a classical solid are all in equilibrium and don't move. This equilibrium is stable, so that if you displace one of the atoms, it will experience a restoring force. However, this restoring force isn't symmetric—the repulsive forces between atoms that are too close are stiffer than the attractive forces between atoms that are too distant. As a result, atoms that are vibrating when the temperature is above absolute zero spend more time too far apart than they spend too close together. As a result, the lattice expands. The same situation holds for the can/rubber band analogy: the repulsive forces between squeezed cans are stiffer than the attractive forces of stretched rubber bands. Thus as the cans vibrate back and forth more vigorously, their lattice expands, too.
181. Expansion of Metals - a Ball and a Ring
Description: A metal ball is too large to fit through a metal ring when they are both at the same temperature. But heating the ring and/or cooling the ball makes it possible for the ball to pass easily through the ring.
Purpose: To show that metals expand when heated.
Supplies:
1 ball and ring set (from a scientific supply company)
1 gas burner
1 container of liquid nitrogen
matches
Procedure: First show that the ball can't fit through the ring—it's too large for the ring. Now chill the ball by dipping it in liquid nitrogen. The ball will contract and it will then be able to fit through the ring. Allow the ball to warm to room temperature and repeat the experiment. However, this time light the burner and heat the ring until it's almost red hot. While it might seem as though heating the ring should make it expand and shrink the hole inside it, the entire ring will expand outward and the hole will become larger. The ball will now fit through the ring.
Explanation: Raising the temperature of most metals moves their atoms farther apart on average and causes them to increase in size in all directions. Hollow spots, such as the hole in the ring, expand in size as the overall metal expands. While having the innermost layer of atoms in the ring expand inward would move those atoms farther from the atoms in the next to innermost layer, it would also move the atoms of the innermost layer closer to one another. In thermal expansion, every atom moves farther from its neighbors, and that can only occur when all the atoms expand outward from the center of the object. This rule applies even to the innermost layer of atoms in the ring—they move outward from the center of the ring.
Follow-up: Drill a hole in an aluminum plate. The diameter of that hole should be just too small for an aluminum rod to enter it. Now chill the aluminum rod in liquid nitrogen and insert the contracted rod in the hole. Allow the rod to warm up. It will be impossible to remove the rod from the hole.
182. Expansion of an Metal Tube with Temperature
Description: When steam flows through an aluminum tube, the tube's temperature increases and so does its length.
Purpose: To show that metals expand when heated.
Supplies:
1 aluminum tube (roughly 5 mm in diameter and 30 cm long)
1 hose
1 steam boiler
1 stand for the steam boiler
1 gas burner
matches
1 flat weight
1 paper pointer with a pin through it
tape
Procedure: Use the hose to attach the aluminum tube to the boiler. Lay the aluminum tube along the edge of the table or a heat-resistant surface and tape the hose end of the tube firmly to the table. Insert the pin of the paper pointer under the open end of the aluminum tube and lay the weight over the tube to press it against the pin. As the open end of the tube moves toward or away from the hose end, the pin will rotate and the pointer will change its direction of pointing. Place the steam boiler on the stand, light the burner, and allow the boiler to make steam. As the steam flows through the aluminum tube, the tube's length will increase and the pointer will turn.
Explanation: The steam will heat the aluminum tube and cause it to expand a small fraction of its length. The pointer makes that small expansion more visible.
Alternative Procedure: Make the simple plastic ruler thermometer of described in the opening of Chapter 6.
183. Glass/Liquid Thermometer
Description: When a common glass/liquid thermometer is heated or cooled, the level of liquid inside it rises or falls, even though both the glass and the liquid are experiencing the same changes in temperature.
Purpose: To show that liquids normally expand or contract more with temperature changes than solids do.
Supplies:
1 glass/liquid thermometer
1 container of hot water
1 container of ice water
Procedure: Observe the reading of the thermometer at room temperature. Immerse the thermometer in hot water and watch as the level of liquid inside it rises. Point out that both the glass and the liquid are expanding with temperature, but that the liquid is expanding more rapidly than the glass. Now immerse the thermometer in ice water and water as the level of liquid inside it falls. Again, the liquid is contracting more rapidly than the glass.
Explanation: When the temperature of a solid changes, its atoms vibrate more vigorously and their average spacings increase. However, they don't normally rearrange much. When the temperature of a liquid changes, its atoms not only vibrate more vigorously, so that their average spacings increases, but they also tend to rearrange and adopt less tightly packed formations. That's why liquids expand more rapidly with temperature than solids do. In the case of the glass/liquid thermometer, the rapidly expanding liquid is force to flow up the thin tube inside the glass because the liquid expands more rapidly than the hollow volume inside the glass does.
184. A Rubber Band Thermometer
Description: A weight hangs from the end of a rubber band. When you heat the rubber band, its length decreases and the weight rises.
Purpose: To show that not all materials simply expand with temperature.
Supplies:
1 rubber band
1 weight
1 support for rubber band
1 heat gun or hairdryer
Procedure: Attach the rubber band to the support and hang the weight from the rubber band. The rubber band should be stretched almost to its elastic limit. Note how far the rubber band has stretched. Now heat the rubber band without touching it. The rubber band will shrink and pull the weight upward.
Explanation: The rubber band contracts upon heating because the long organic molecules inside it develop more and more kinks as their temperatures rise. In a cold, unstretched rubber band, these molecules are wound up into random coils. Stretching the rubber band unwinds those coils. However, the random coils are the more thermodynamically favorable arrangement so the molecules recoil and shorten when you either relieve the tension on the rubber band or heat the rubber band up.
185. A Bimetallic Strip Thermometer
Description: A thin metal strip, made of a sandwich of two metals, bends whenever its temperature changes.
Purpose: To show that different metals expand differently with temperature and that this can be used to make objects that bend with temperature.
Supplies:
1 bimetallic strip (typically iron and brass)
1 gas burner
matches
1 container of ice water
Procedure: Examine the bimetallic strip, pointing out that it's made of two different metals that have been bonded together. Note that at room temperature the strip is flat. Now light the burner and heat the strip gently. It will curl in one direction as the outer metal (brass) surface expands more rapidly than the inner metal surface (iron). Now immerse the strip in ice water and watch as it curls the other way. The surface of the strip that expanded more rapidly when heated (brass) also shrinks more rapidly when cooled and becomes the inner surface of the curling strip.
Explanation: The characteristics of the forces between atoms varies from metal to metal, so that different metals have different coefficients of volume expansion. The strip is made of two different metals with different coefficients of volume expansion. As a result, it only remains flat at one temperature.
Description: A bimetallic strip is used as an electric switch, opening a circuit when it becomes hot. When the circuit is used to power a heater near the bimetallic strip, the strip repeated opens and closes the circuit. A light bulb attached to the circuit also blinks on and off.
Purpose: To illustrate how both a light blinker and a thermostat work.
Supplies:
1 bimetallic strip
1 switch mount for the bimetallic strip (see below)
1 powerful battery
1 nichrome wire heater
1 small light bulb
wires
Procedure: Mount the bimetallic strip so that it barely touches a metal contact while the strip is at room temperature and bends away from that contact when it's somewhat hotter than room temperature. Connect one terminal of the battery to the bimetallic strip. Connect the metal contact to one terminal of the nichrome wire heater and also to one terminal of the light bulb. Place the nichrome wire heater very close to the bimetallic strip (or touching it, if the nichrome is insulated). Now connect the other terminals of the nichrome wire heater and the light bulb to the other terminal of the battery. Current will begin to flow through the bimetallic strip and metal contact. It will continue through both the heater and the light bulb before returning to the battery. The light bulb will light and the heater will heat. When the bimetallic strip's temperature exceeds some value, it will bend away from the metal contact and current will stop flowing. The light will go out and the heater will stop heating. After a few seconds, the strip will have cooled enough to straighten out and will again touch the metal contact. The light will turn back on and so will the heater. The system will switch on and off indefinitely.
Explanation: This blinker arrangement oscillates indefinitely because whenever the heater is on it soon turns itself off and whenever the heater is off it soon turns itself on.
187. Liquid Crystal Thermometers
Description: Numbers on a flat plastic strip change colors as the strip's temperature changes. A larger sheet of plastic changes colors as you rub your hand across it.
Purpose: To show how temperature can affect the ordering of liquid crystals.
Supplies:
1 liquid crystal room or aquarium thermometer (a flat plastic strip thermometer that measures temperatures near room temperature)
1 sheet of liquid crystal film that changes colors in the temperature range only slightly above room temperature
Procedure: Show that the liquid crystal thermometer is reading room temperature—that only one or two of its numbers are brightly colored. Then warm the thermometer with your hand and show that the different numbers appear and disappear as the strip's temperature rises. Each number appears when the liquid crystal it contains achieves the proper ordering characteristics. Now examine the liquid crystal film. At room temperature, it should be mostly colorless. But when you begin to heat it with your hands, it will become brightly colored. As its temperature increases, the liquid's order becomes such that it reflects visible light.
Explanation: These temperature sensitive films contain chiral nematic liquid crystals that naturally form spiral structures within the film. The pitch of these spirals is temperature dependent. When the temperature of a particular liquid crystal mixture is such that its spiral pitch is equal to the wavelength of visible light, that liquid crystal will reflect some of the visible light. The liquid crystals in the various numbers of the thermometer are slightly different and achieve the right pitch at different temperatures. In the large film, only the portions of the film that are with the right temperature range reflect visible light.
188. Color-Changing Toys
Description: Toys ranging (from shirts to pens) made from special plastics change colors when heated by body heat, friction, or contact with hot water.
Purpose: To show another type of crude thermometer.
Supplies:
1 color changing toy (BIC makes a set of color changing pens call "wavelengths")
Procedure: First examine the toy or pen while it's at room temperature. Then warm the toy or pen by holding it or rubbing its surface (sliding friction). The toy or pen's color will become much lighter.
Explanation: The plastic contains tiny bubbles. Each of these bubbles contains a mixture of chemicals, one of which melts at a temperature slightly above room temperature. At room temperature, that chemical is solid and the remaining liquid chemicals are brightly colored. But when the plastic is heated and the solid chemical melts, it interferes chemically with the coloring molecules of the liquid. The liquid loses its color. Upon cooling, the melted chemical solidifies again and the liquid's color returns. The plastic often contains a second, temperature-insensitive dye that becomes visible when the other color vanishes at elevated temperatures.
189. Thermocouples as Temperature Sensors
Description: You measure the voltage developed between the two wires of a thermocouple when that thermocouple is immersed in liquid nitrogen or heated over a burner.
Purpose: To show that mobile electrons not only make metals good conductors of heat, they also gives metals interesting electric properties when the metals are exposed to temperature gradients.
Supplies:
1 thermocouple (two different wires of, for example, iron and constantin, that have been twisted or welded together at one end)
1 sensitive voltmeter (or thermocouple readout)
1 gas burner
matches
1 container of liquid nitrogen
Procedure: Form the thermocouple by removing about 1 cm of insulation from each end of the two different wires and twisting the pair of wires together at one of their ends. Attach the voltmeter to the free ends of the two wires. With everything at room temperature, the voltmeter will read zero volts. But when you heat or cool the twisted end of the thermocouple, the voltmeter will read a small non-zero voltage (in the tens of millivolts range).
Explanation: When you heat or cool the twisted end of the thermocouple, there is a temperature gradient along each wire. Because the mobile electrons in each wire are moving fastest at the hotter end, they tend to migrate to the colder end and make the colder end of each wire negatively charged (the Seebeck Effect). However, the extent of this negative charging depends on the wire. By comparing the charging of two different wires, you can determine the temperature difference across the wires.
190. Thermistors as Temperature Sensors
Description: The electric resistance of a thermistor decreases as its temperature increases.
Purpose: To show that semiconductors become better conductors as their temperatures rise.
Supplies:
1 thermistor
1 ohm meter
hot water
ice water
Procedure: Connect the two wires of the thermistor to the ohm meter and determine its resistance at room temperature. Now immerse the thermistor in hot water (or simply pinch it in your fingers) and observe its decrease in resistance. Finally, immerse the thermistor in cold water and observe its increase in resistance.
Explanation: The thermistor is a semiconductor that would normally not conduct current if it weren't for thermal energy. At absolute zero, the semiconductor would have all of its valence levels filled and all of its conduction levels empty and it would be unable to transport electric charge. Thermal energy transfers electrons from filled valence levels to unfilled conduction levels and makes it possible for the semiconductor to transport charge—to carry current. The more thermal energy (i.e. the higher the temperature), the more easily the semiconductor carries charge.
191. Copper wire as a Temperature Sensor
Description: The electric resistance of a coil of copper wire decreases as its temperature decreases.
Purpose: To show that metals become better conductors as their temperatures fall.
Supplies:
1 coil of thin copper wire
1 light bulb (one that requires a fair amount of current)
1 battery
1 container of liquid nitrogen
wires
Procedure: Form a complete circuit by connecting one terminal of the battery to one end of the coil of copper wire, the other end of the coil of copper wire to one end of the bulb, and the other end of the bulb to the other terminal of the battery. The bulb should glow dimly because the current should be losing most of its energy in the copper wire. Now immerse the coil of copper wire into liquid nitrogen. As its temperature drops, the copper will become a better electric conductor and the light bulb will become much brighter.
Explanation: As current flows through room temperature copper wire, the individual charges collide with the vibrating copper atoms and transfer some of their energies to the copper atoms. When the copper is chilled to low temperature, the copper atoms vibrate less and their increased order makes them less likely to be hit by moving charge. Copper's electric resistance drops as its temperature drops and it becomes a better conductor.
Section 8.1 Electronic Air Cleaners
227. Removing Dust from the Air
Description: You clap chalky erasers together and note how slowly gravity removes the chalk particles from the air.
Purpose: To show that gravity is slow and ineffective at removing tiny particles from the air.
Supplies:
2 chalky erasers or another source of visible, nontoxic dust
1 piece of chalk (or a bulk form of whatever dust you choose)
Procedure: Clap the erasers together and show that it hangs in the air for a long time. Drop the piece of chalk to show that it falls rapidly—buoyancy isn't supporting the chalk dust, air resistance (viscous drag) is. Discuss the concept of terminal velocity; of the falling particles experiencing upward forces that balance their weights when they reach very small downward velocities relative to the air. Point out that to be drawn through the air at larger terminal velocities, the particles need to be exposed to stronger forces than gravity—for example, to Coulomb forces!
Explanation: Small particles have such large surface-to-volume ratios that their interactions with air dominate their dynamics. To pull them through the air at more than a snail's pace, they need to be exposed to forces stronger than those of gravity.
228. Electric Charge and Coulomb Forces
Description: Two pith balls hang from threads. One of them is given negative charge by a negatively charged Teflon rod and the two objects repel one another. The other pith ball is given positive charge by a positively charged acrylic rod and the two objects also repel one another. Finally, the two pith balls are carefully brought toward one another. They suddenly draw together and touch, showing that they attract one another.
Purpose: To demonstrate the strong repulsive and attractive forces between electric charges and to show that there are two types of electric charges: positive and negative.
Supplies:
2 silvered pith balls hanging from threads and supports (we sometimes use carbon-coated latex rubber balloons, which work very nicely but age badly and must be made fresh for each use. The carbon-coating is done with Aerodag colloidal carbon spray and makes the balloons electrically conducting.)
1 Teflon rod
1 Acrylic rod
1 piece of silk
Procedure: Set the two pith balls so that they hang about 40 cm apart. Rub the Teflon rod with the silk, a process that will transfer negative charge to the Teflon and leave the silk positively charged. Touch the Teflon rod to one of the pith balls. The pith ball will immediately repel the Teflon rod. Demonstrate this repulsion.
Now rub the acrylic rod with the silk, a process that will transfer negative charge to the silk and leave the acrylic positively charged. Touch the acrylic rod to the other pith ball. The pith ball will immediately repel the acrylic rod, although you may have to recharge the acrylic rod with the silk and repeat the charge transfer once or twice (acrylic doesn't work as well as Teflon). Demonstrate this repulsion, too.
Finally, shift the supports for the pith balls slowly toward one another so that the balls move closer and closer. When they are near enough, they will pull together and "kiss." Once they have touched, they will drop limply because they have little net charge left. Point out that this attraction between the pith balls is evidence that the two pith balls were oppositely charged and that there are two different charges present in our universe. Identify them as positive and negative and discuss how sliding friction tends to move them between objects (which is how you charged the rods with the silk.) Note that like charges repel but opposite charges attract.
Explanation: Sliding friction rubbed negatively charged electrons off the silk and onto the Teflon. It also rubbed negatively charged electrons off the acrylic rod and onto the silk, leaving the acrylic rod with a net positive charge.
229. Detecting Charge with an Electroscope
Description: You transfer charge from a Teflon rod to the foils of an electroscope and they repel outward to indicate the presence of charge.
Purpose: To show how a simple apparatus can detect the presence of electric charge.
Supplies:
1 electroscope
1 Teflon rod
1 silk cloth
Procedure: Rub the Teflon rod with the silk to give the rod a net negative charge. Touch the Teflon rod to the top of the electroscope so that negative charge flows onto the foils. They will repel one another and swing outward. Point out that the electroscope uses this repulsion between like charges to indicate the presence of charge on the foils.
Explanation: When you touch the Teflon rod to the electroscope, negative charges flow onto the foils. Since like charges repel one another, the two foils are swung outward by the repulsions between their charges.
230. Electric Conductors and Electric Insulators
Description: A metal rod connected to the foils of an electroscope conduct charge to the foils when you touch the rod with a charged Teflon rod. A plastic rod connected to the foils doesn't conduct charge to the foils when you touch it with the charged Teflon rod.
Purpose: To show that some materials can transport electric charge and are electric conductors, while other materials can't transport electric charge and are electric insulators.
Supplies:
1 electroscope
1 metal rod that can attach to the electroscope
1 plastic rod that can attach to the electroscope
1 Teflon rod
1 piece of silk
Procedure: Start with the electroscope uncharged and with the metal rod attached to its foils. Charge the Teflon rod by rubbing it with the silk. Now touch the Teflon rod to the metal rod so that the foils swing outward. Point out that the metal rod has transported the charge to the foils and is thus an electric conductor.
Now remove the metal rod and replace it with the plastic rod. Again start with the electroscope uncharged. Touch the charged Teflon rod to the plastic rod and show that the foils don't swing outward. Point out that the plastic rod hasn't transported the charge to the foils and is thus an electric insulator.
Explanation: The metal rod has mobile electrons (conduction level electrons or perhaps empty levels in its valence bands) that allow it to transport electric charges from one end to the other. The plastic rod has no such mobile electrons (its valence levels are completely filled and it has no conduction level electrons) and can't transport electric charges from one end to the other.
Description: You transfer electric charge to an isolated metal cup and then use an electrometer to look for that charge. You find that it's on the outside of the cup, not on the inside.
Purpose: To show that charge distributes itself relatively uniform around the outsides of conducting objects.
Supplies:
1 metal cup on an insulating stand (a cylindrical metal can with a bottom but no top)
1 metal ball on an insulating stick (for charge transfers)
1 electroscope
1 Teflon rod
1 piece of silk
Procedure: Rub the Teflon rod with the silk to give the rod a negative charge. Transfer this charge to the metal cup (Faraday's ice bucket) by rubbing the rod lightly against the cup. Now locate the charge on the cup. First look for the charge inside the cup by carefully inserting the transfer ball into the cup (don't touch the lip of the cup) and by touching the inside surface of the cup. Remove the ball from the cup and touch it to the electroscope. There will be no deflection of the foils, indicating no charge on the ball and no charge on the inside surface of the cup.
Now touch the ball to the outside surface of the cup. Again touch the ball to the electroscope. The foils will bend outward, indicating charge on the ball and charge on the outside surface of the cup.
Explanation: Like charge becomes more widely separated by spreading itself on the outside surfaces of a conducting object. No charge is found on the inside surfaces of a conducting object.
232. A Van Der Graaf Generator
Description: A van der Graaf generator operates like an automated version of Faraday's ice bucket. A belt delivers charge into a conducting ball and this charge runs quickly to the outside surfaces of the ball.
Purpose: To show how a large quantity of like charge is accumulated on the surface of a van der Graaf generator.
Supplies:
1 van der Graaf static generator
Procedure: First examine the components of the van der Graaf generator. It has a conducting metal sphere on top that will store like charge on its surface. It has an insulating rubber belt that will deliver charge to the inside of the conducting metal sphere. It has a charging system at the base of the belt that deposits charge on the belt. And finally it has a motor that turns the belt and pushes the charged belt toward the like-charged metal sphere.
Now turn on the van der Graaf generator and allow it to begin producing sparks. Point out that the motor is doing work on the charges in order to push them onto the sphere (the charges already on the sphere are repelling the newly arriving charges).
Explanation: Whenever the belt carries a charge into the sphere and allows that charge to transfer to the sphere, the charge quickly moves onto the outer surface of the sphere. Once on the outer surface of the sphere, the charge can only leave through a spark or on a passing air molecule. As more and more charges accumulate on the sphere, their potential energies increase and thus the voltage of the charges increase (voltage is energy per charge). (However, our van der Graaf generator accumulates negative charge, so it reaches a very large negative voltage.)
233. Launching a Styrofoam Cup
Description: A Styrofoam Cup placed upside down on a van der Graaf generator lifts itself into the air.
Purpose: To show the tendency for electric charges to transfer from the surface of the van der Graaf generator onto nearby objects and to show that like charges repel.
Supplies:
1 van der Graaf static generator
1 Styrofoam cup
1 grounding ball, stick, and wire
Procedure: Turn on the van der Graaf generator and ground the sphere of the van der Graaf generator (we use a metal ball on a long insulating stick, with a wire that connects the ball to earth ground) to make it safe (or less painful) to touch. Put an inverted Styrofoam cup on top of the ball and remove the grounding ball. As charge accumulates on the van der Graaf generator's sphere, some of it will transfer to the nearby cup. Soon the sphere and cup will repel one another strongly enough for the cup to lift up into the air.
Explanation: An electric charge on the surface of the van der Graaf generator can lower its total energy by moving to the Styrofoam cup. It does so with the help of passing air molecules, which serve as ferries for the charges. Once the cup and the sphere are each sufficiently charged, the upward Coulomb force on the cup exceeds its weight and the cup accelerates upward.
234. Making the Strands of a Pom-Pom Stand Up
Description: A plastic Pom-Pom is attached to the sphere of a van der Graaf Generator. As charge accumulates on its strands, they spread outward until the Pom-Pom resembles a dandelion tuft.
Purpose: To demonstrate the repulsion between like charges.
Supplies:
1 van der Graaf static generator
1 Pom-Pom (a ball of thin plastic stripes attached to a stick)
1 suction cup
1 grounding ball, stick, and wire
Procedure: Turn on the van der Graaf generator and ground its sphere to make it safe to touch. Attach the stick of the Pom-Pom to the top of the van der Graaf generator with the suction cup. Remove the grounding ball and allow charge to accumulate on the sphere and on the Pom-Pom. The plastic strands of the Pom-Pom will soon spread outward into a large uniform ball of straight plastic strips.
Explanation: Air molecules ferry electric charges from the van der Graaf generator to the plastic surfaces of the Pom-Pom. Once there are enough charges on those strands, they repel one another strongly and stand up to form a round ball.
235. Making Peoples' Hair Stand Up
Description: A person stands on a plastic stool and touches the sphere of a van der Graaf generator. As charge accumulates on the sphere and their body, their hair begins to stand up.
Purpose: To demonstrate the repulsion between like charges (and to have fun).
Supplies:
1 van der Graaf static generator
1 plastic stool (a one-step stool, about 30 cm tall)
1 grounding ball, stick, and wire
Procedure: Place the van der Graaf generator at the edge of a table and put the plastic stool a short distance away on the floor. The volunteer who will stand on the stool (for electric insulation from the ground) should be able to reach out and touch the sphere of the van der Graaf generator comfortably, but without coming too close to anything else, particularly the base of the van der Graaf generator. Before the volunteer arrives, turn on the van der Graaf generator and touch the grounding ball to the van der Graaf generator's sphere to eliminate any charge from its surface. Have the volunteer stand on the stool (it's not a matter of how tall they are—they need the electric insulation that the stool provides) and touch the sphere of the van der Graaf generator. They should feel absolutely no shock while they’re doing this because you are still grounding the sphere.
When the volunteer is ready and not near anything besides the sphere and the stool, move the grounding ball away from the van der Graaf generator's sphere. Never move the grounding ball back to the van der Graaf generator's sphere while the person is still touching the sphere because the volunteer will feel a shock. As charge accumulates on the sphere and the volunteer, that person's hair will begin to stand up. Some people's hair works better than others and there is simply no predicting whose hair will work best. It's completely trial and error! The only exception to that rule is with children—young children with fine, straight, white-blond or jet black hair always work well.
Explanation: The charge that migrates onto the volunteer's body through their conducting skin also works its way onto their hairs. When each hair is sufficiently charged, the Coulomb repulsions between the hairs lift them upward against their own weights.
236. Sharp Points and Charge - Lightning Rods
Description: When you approach the sphere of a van der Graaf generator with a smooth grounded object, sparks occur. But when you approach the sphere with a sharp grounded object, the sphere loses its charge quietly without any sparks.
Purpose: To show that sharp points are particularly good at emitting electric charges into the air.
Supplies:
1 van der Graaf static generator
1 grounding ball, stick, and wire
1 pin, needle, or sharpened metal rod
Procedure: Turn on the van der Graaf generator and allow charge to accumulate on the surface of its sphere. Approach that sphere with the grounded ball and show that sparks leap from the sphere to the ball. Now attached the pin to the surface of the grounding ball and repeat the same experiment. No sparks will occur. Moreover, you can hear the motor of the van der Graaf turning more easily—the pin is helping charge to move between the sphere and the ball so that very little charge accumulates on the sphere of the van der Graaf generator! (I do this experiment with my bare hands. I approach the charged sphere with my knuckles and it sends sparks at them—unpleasant, but not particularly painful. I then approach the charged sphere with a sharp pin in my hand and it doesn't send any sparks at all.)
Explanation: As you approach the sphere with the sharp pin, charges that are opposite to those on the sphere begin to leap off the pin's point and onto passing air molecules—a corona discharge. These charges quickly move toward the sphere and land on it, neutralizing the sphere's charge. Although the motor and belt try to recharge the sphere, the charge transfer from the pin is so effective that the sphere loses most of its net charge and can't produce any sparks.
237. An Electrostatic Bell
Description: A metal ball, hanging from a string between two oppositely charged plates, begins to move back and forth between those plates. It's ferrying charge and creating lots of noise.
Purpose: To show that opposite charges attract one another and that like charges repel.
Supplies:
2 vertical metal plates, about 10 cm square, supported on insulators
1 ball
string
1 support for ball
2 wires
Procedure: Use the string to hang the ball from the support and place it between the two plates. The two plates should be just far enough apart to give the ball a little room to move. The ball should just barely touch one of the two plates. Touch the two contacts of the Wimshurst static generator together to eliminate any charges they may have and connect the two contacts to the two plates. Now separate the two contacts and begin cranking the Wimshurst generator. When enough charge has accumulated on the two plates, the ball will be repelled by the plate that it's touching and will accelerate toward the other plate. As soon as it touches the other plate, it will reverse its charge and accelerate in the opposite direction. It will shuttle back and forth between the plates as long as you continue to turn the crank of the Wimshurst generator.
Explanation: The metal ball is repelled by the like charge of the plate that is has just touched and attracted to the opposite charge of the other plate. It accelerates back and forth between the two.
238. Putting Out a Candle with Static Electricity
Description: A candle that's placed between two oppositely charged plates is ripped apart by the Coulomb forces it experiences and extinguishes itself.
Purpose: To show that a candle flame contains some electrically charged particles and Coulomb forces acting on those charged particles can make it impossible for the flame to operate.
Supplies:
2 vertical metal plates, about 10 cm square, supported on insulators
1 candle
matches
Procedure: Space the two metal plates about 4 cm apart and put the candle between the two plates. Touch the two contacts of the Wimshurst static generator together to eliminate any charges they may have and connect the two contacts to the two plates. Light the candle. Now separate the two contacts and begin cranking the Wimshurst generator. When enough charge has accumulated on the two plates, the candle flame will become severely distorted and will probably extinguish itself.
Explanation: The charged particles in the flame are pulled toward opposite charges and the flame becomes a very horizontal, rather than vertical, structure. In its new shape, the flame has trouble sustaining itself and tends to put itself out.
239. A Simple Electrostatic Precipitator
Description: Smoke drifts upward through a metal can containing a thin metal wire. When opposite electric charges are placed on the can and the wire, the smoke suddenly disappears.
Purpose: To demonstrate the principles of electrostatic precipitation.
Supplies:
1 large coffee can, open at both ends
1 extremely thin metal wire
1 insulated support for the metal wire
1 insulated support for the coffee can
1 weight for the metal wire
1 Wimshurst static generator (or another high voltage power supply)
2 wires
1 smoke source (for example, unscented incense sticks)
matches
Procedure: Support the coffee can about 50 cm above the table and lower the metal wire through its center. Support the top of the wire and hang the weight from the bottom of the wire to pull the wire straight. Touch the two contacts of the Wimshurst static generator together to make sure that they have no charges on them and connect one contact to the coffee can and the other contact to the wire. Be careful not to break the wire. (It does matter somewhat which charge you put on the wire and which charge you put on the can, but you'll have to experiment to see which works best.)
Now light the smoke source and allow its smoke to drift upward through the coffee can. To demonstrate the electrostatic precipitator, separate the two contacts of the Wimshurst machine and turn its crank. As charge begins to accumulate on the can and wire, the smoke will abruptly disappear as it travels through the can.
Explanation: A corona discharge occurs around the electrically charged wire and this discharge transfers charge onto passing air molecules and smoke particles. These ionized particles are then repelled by the wire and are attracted to the inside surfaces of the coffee can. The missing smoke is actually coating the inside of the coffee can as a thin film of particles.
240. Deflecting a Stream of Water with a Charged Comb
Description: A thin stream of water is deflected by a nearby comb.
Purpose: To show that a charged object can electrically polarize another object and the two will attract.
Supplies:
1 hose
1 support for the hose
1 rubber or plastic comb (or a Teflon rod)
flowing water
Procedure: Connect the hose to a water faucet and support its end over the drain. Adjust the water flow so that a thin but continuous stream of water flows from the hose. Now charge a comb either by drawing it through your hair several times or by rubbing it with a piece of silk. Hold the comb near the upper part of the water stream and watch as the water stream bends toward the comb.
Explanation: The comb's electric charge attracts opposite charges onto the water stream and repels like charges out of the water stream. Since the stream is now polarized, with charges that are opposite to those on the comb closer then charges that are like those on the comb, the stream is attracted to the comb and bends toward the comb.
241. Sticking a Balloon to the Wall with Charge
Description: You rub a balloon through your hair and then stick it to the wall. Its electric charge holds it in place.
Purpose: To show that a charged particle is naturally attracted to any uncharged surface because it will polarize that surface and obtain an attractive force.
Supplies:
1 balloon (a long, thin one oriented vertically works well because it can't roll down the wall)
1 wall
Procedure: Charge the balloon by rubbing it through your hair (or rubbing it with a silk cloth). Hold it against the wall and observe that it sticks.
Explanation: The electrically charged balloon pulls opposite charges in wall toward it and repels like charges in the wall away from it. This polarization of the wall makes it possible for the balloon to stick to the wall through Coulomb forces.
Section 8.3 Magnetically Levitated Trains
244. The Forces Between Magnets
Description: A bar magnet on a horizontal pivot always turns so that its north pole faces the south pole of a magnet you're holding in your hand, or vice versa.
Purpose: To show that magnets have two different poles and that like poles repel while opposite poles attract.
Supplies:
2 bar magnets
1 horizontal swivel mount for one of the bar magnets
Procedure: Suspend one of the bar magnets on the horizontal mount. Hold the second magnet in your hand and show that its poles repel like poles of the horizontally supported magnet and that its poles attract opposite poles of that magnet.
Explanation: As with electric charges, magnetic poles come in two types: north and south. But unlike electric charges, its impossible to find an isolated north pole or an isolated south pole. Each bar magnet has a north and a south pole. Like poles on two bar magnets experience repulsive forces and opposite poles on two bar magnets experience attractive forces.
245. Visualizing a Magnetic Field
Description: A small bar magnet is inserted into a magnetic field visualizer and the magnetic flux lines become visible.
Purpose: To show how the magnetic field extends from a magnet's north pole outward and around to the magnet's south pole.
Supplies:
1 magnetic field visualizer (a clear plastic rectangle, filled with iron powder and oil, with a hollow region into which you can put a small bar magnet)
1 bar magnet
Procedure: Shake the visualizer to disperse the iron powder evenly. Insert the bar magnet into the visualizer and watch as the iron powder accumulates along the magnetic flux lines. Point out that these lines indicate the direction of the force that an isolated north pole would experience if it were at one of those locations. (The fact that isolated north poles aren't available doesn't alter the meaning of the magnetic field lines.)
Explanation: The iron powder particles are magnetized by the magnetic field and line up along the flux lines because they respond to the magnetic forces associated with those flux lines.
246. Magnetic Levitation - First Attempt
Description: You place one magnet over another so that the upper magnet is supported by repulsive forces from the lower magnet. However, you must put a stick through the two magnets to keep the upper magnet from falling off the lower magnet's magnetic cushion.
Purpose: To show that, while you can suspend one disk or ring magnet over another by magnetic repulsion, the equilibrium created by that levitation technique is unstable.
Supplies:
2 ring-shaped magnets
1 wooden dowel
Procedure: Show that when the two ring-shaped magnets are stacked so that they have like poles facing one another, they repel strongly enough to support the upper magnet. Show also that you can't balance the upper magnet above the lower magnet. Show that only when you put the dowel through the holes in the two rings can you can get a stable arrangement.
Explanation: They same repulsive force that supports the weight of the upper magnet also tends to push it to the side so that it falls off the magnetic cushion provided by the lower magnet. It’s in an unstable equilibrium.
247. Magnetic Levitation - Second Attempt
Description: You place one bar magnet above another so that their like poles are on top of one another. While the magnetic repulsion supports the upper magnet, it tends to fall of the magnetic cushion. Only when you box in the upper magnet so that it can't move horizontally will it float over the lower magnet.
Purpose: To show that, while you can suspend one bar magnet over another by magnetic repulsion, that the equilibrium created by this levitation scheme is unstable.
Supplies:
2 strong bar magnets
1 frame that prevents horizontal motion of the bar magnets
Procedure: Show that when the two bar magnets are aligned with their like poles on top of one another, that the upper magnet can be suspended by the repulsive forces. Now show that you can't balance the upper bar magnet over the lower bar magnet—the equilibrium there is unstable. Add the frame and show that only with its help to prevent horizontal motion can you suspend one bar magnet over another.
Explanation: The same repulsive forces that support the upper bar magnet also tend to push it to the side so that it falls off its magnetic cushion.
248. Magnetic Levitation - An Almost Free Bearing
Description: A magnetic toy spins above a magnetic base. While it appears that the magnetic toy is levitating, it’s actually touching the base a one point. Without that contact, it would be unstable.
Purpose: To show that magnetic suspension with permanent magnets is inherently unstable.
Supplies:
1 magnetic bearing toy (available from scientific supply companies)
Procedure: Suspend the magnet bearing toy in its base and give it a spin. Show that while the bearing remains suspended by repulsive forces above its magnetic base, it's equilibrium is unstable in one horizontal direction. It touches the base at one point in order to avoid falling off the base in the unstable direction.
Explanation: The repulsion between the floating bearing toy and its base leaves the bearing’s equilibrium stable in the up-down and back-front directions. However, that equilibrium is unstable in the left-right direction and the toy needs the contact point to avoid falling off its magnetic cushion.
249. Electronic Feedback - Newton's Folly
Description: A magnetized metal marble hangs in midair beneath an electromagnet. When you block the electric eye that senses the marble's height, it either falls or sticks to the electromagnet.
Purpose: To demonstrate that feedback can be used to make an unstable system stable.
Supplies:
1 Newton's Folly (available from Edmunds Scientific)
Procedure: Plug in Newton's Folly and carefully raise the magnetized marble toward the electromagnet from below (as per the instructions). Be careful not to block the electric eye. When the ball is in the correct position, it should become stably suspended—you can let go and lower your hands. Show that the marble is truly suspended by putting a business card between it and the electromagnet above it. But show also that it the device needs to monitor the marble's height continuously in order to avoid dropping it or attracting it all the way to the electromagnet. You can show this by blocking the electric eye (the small holes on either side of the frame). Depending on how you block the electric eye system, the marble will either fall downward or leap upward toward the electromagnet.
Explanation: The basic system uses attraction between two opposite poles to suspend the marble. This arrangement is stable in the horizontal directions but unstable in the vertical direction. Only through the use of feedback can this system be made stable.
250. AC Magnetic Levitation - Jumping Rings
Description: A small aluminum ring is placed around a group of iron rods that pass through a coil of wire connected to the AC power line. When AC current flows through the wires, the ring is repelled by the coil of wire and leaps upward.
Purpose: To show that an electromagnet that's powered by alternating current repels nearby metal.
Supplies:
1 AC electromagnet with an iron-rod pole piece that extends vertically above the wire coil
1 solid aluminum ring that fits around the iron pole pieces
1 cut aluminum ring (cut so that it isn't a complete ring and can't conduct electricity in a full circle)
Procedure: Place the aluminum ring around the pole piece and lower it onto the coil of wire. Now allow AC current to pass through the coil of wire. An AC current will begin flowing through the ring and the ring will become magnetic. The ring will experience a strong repulsion from the coil of wire and will leap up into the air.
Repeat this process with the cut aluminum ring. Because that ring can't conduct electricity, it won't become magnetic and won't be repelled by the wire coil.
Explanation: When AC current flows through the coil of wire, the electromagnet's poles reverse rapidly. The changing magnetic field induces an AC electric current in the aluminum ring and, in accordance with Lenz's law, the upward pointing pole of the coil is always the same as the downward pointing pole of the aluminum ring. The two objects repel.
251. Eddy Current Pendulum
Description: A metal pendulum swings freely through the pole pieces of an inactive electromagnet. But when the electromagnet is on, the pendulum slows to a stop as it tries to swing through the pole pieces of the electromagnet.
Purpose: To show that a conducting object that enters a magnetic field experiences a repulsive force that slows it down.
Supplies:
1 strong DC electromagnet
1 copper or aluminum pendulum with support (don’t use iron, steel, or any other ferromagnetic metal in the pendulum)
Procedure: With the DC electromagnet off, arrange the pendulum so that it swings smoothly between the electromagnet's pole pieces. Show that the inactive electromagnet has no effect on the pendulum. Now turn on the electromagnet and repeat the demonstration. The pendulum will slow dramatically as it enters the pole pieces and will probably come to a stop between them.
Explanation: As the pendulum approaches the pole pieces, the changing magnetic field it experiences induces currents in its surface. It becomes magnetic and, in accordance with Lenz's law, it repels the poles of the electromagnet. This repulsion slows its motion. The currents that gave rise to the magnetization in the pendulum quickly lose energy in the metal and the pendulum comes to rest between the pole pieces.
Follow-up: Repeat the experiment with another pendulum that can't conduct electricity (either a plastic pendulum or a metal pendulum with cuts through it that prevent currents from flowing). This modified pendulum will swing through the electromagnet even when that electromagnet is on.
252. A Magnet Falling Through A Copper Pipe
Description: A small magnet falls incredibly slowly through a copper pipe.
Purpose: To demonstrate the repulsive magnetic fields that appear when a magnet moves across a conductive surface.
Supplies:
1 small neodymium-iron-boron magnet
1 metal cylinder the same size as the magnet
1 narrow copper pipe
1 support for the copper pipe
Procedure: Support the copper pipe so that it's vertical. Drop the metal cylinder through the copper pipe and note how quickly it falls. Now drop the magnet through the pipe and watch how slowly it descends.
Explanation: As it falls, the magnet induces currents in the copper pipe and these currents exert repulsive magnetic forces on the magnet. These repulsive forces slow the magnet’s descent.
253. A Magnet Sliding Through a Half-Copper, Half-Plexiglas Track
Description: A small disk magnet rolls through a narrow track that's made of Plexiglas at one end and copper at the other. The magnet rolls quickly through the Plexiglas portion of the track but slows dramatically when it enters the copper portion of the track.
Purpose: To demonstrate the repulsive forces that occur when a magnet moves past a conducting surface.
Supplies:
1 small disk neodymium-iron-boron magnet
1 track for the magnet, cut from a square copper bar at one end and from a square Plexiglas bar at the other end. The two bars are joined and framed in Plexiglas to keep them together and to keep the magnet in the track.
Procedure: Tilt the track so that the magnetic disk rolls along the track. Show that the disk rolls quickly through the Plexiglas portion of the track but slows when it rolls through the copper portion of the track.
Explanation: The moving magnet induces currents in the conducting copper and experiences repulsive magnetic forces from the currents it induces. The magnet rolls freely through the Plexiglas because currents can't flow in the Plexiglas.
254. Electrodynamic Magnetic Levitation of Magnet on a Spinning Metal Disk
Description: A large disk magnet floats above a spinning aluminum disk.
Purpose: To demonstrate electrodynamic levitation.
Supplies:
1 large neodymium-iron-boron disk magnet (the larger and thinner, the better)
1 sturdy aluminum disk about 40 cm in diameter, with a spindle attached
1 variable-speed motor for spinning the aluminum disk
1 sturdy mount for the motor
Procedure: Mount the aluminum disk on the motor and attach the motor to a sturdy table so that the aluminum disk spins in a horizontal plane. Be sure that everything is well balanced and strong enough to tolerate high rotational speeds. The disk's surface should be able to reach speeds of 200 km/h without any damage! If you are concerned about the disk coming apart at these high speeds, build a safety fence around the spinning disk. Support the magnet on a flexible strap that will keep it horizontal but will allow it to rise or fall vertically.
Turn on the motor and bring the aluminum rotor to a relatively high surface speed of at least 100 km/h. Use the strap to lower the magnet carefully toward the outer surface of this disk. The strap should be oriented tangent to the disk's edge, with the disk turning in the direction that leads from your hand toward the magnet. The magnet will be pulled in the direction of the disk's rotation by magnetic drag forces and you should hold the strap tightly so that it isn't pulled out of your hand. Before the magnet touches the aluminum disk, it will experience a strong magnetic repulsion and it will begin to hover a few centimeters above the spinning aluminum disk. The faster the aluminum disk turns, the higher the magnet will hover and the less magnetic drag force it will experience. Be carefully not to spin the disk so fast that it flies apart. Safety first!
Explanation: The magnet induces currents in the aluminum disk and the disk becomes magnetic. It repels the magnet, suspending the magnet in the air and giving rise to the magnetic drag force that tends to pull the magnet along with the disk. The magnetic drag force diminishes with higher speeds because the currents in the aluminum have less time to waste energy.
255. Superconductors and Magnetic Levitation
Description: A small permanent magnet hovers above the surface of a high temperature superconductor.
Purpose: To demonstrate the perpetual current flow and magnetization of a superconductor when approached by a magnet.
Supplies:
1 high-Tc superconductor disk
1 small neodymium-iron-boron magnet
1 Styrofoam cup
1 thin foam rubber or sponge pad
liquid nitrogen
Procedure: Cut the Styrofoam cup to form a shallow tub and place the superconductor disk on the foam rubber pad in the middle of this tub. Fill this tub with liquid nitrogen and allow the disk to cool until the liquid nitrogen is barely boiling. Now lower the permanent magnet onto the disk and watch as it floats above the disk.
Explanation: The approaching magnet induces currents in the superconductor disk and the two repel one another. This repulsion suspends the magnet in midair. Because the currents in the superconductor don't decay away or lose energy, the suspension continues indefinitely.
Follow-up: Even if you leave the magnet on the superconducting disk while it's cooling down, the magnet will lift up off the surface of the superconductor as soon as the superconductor becomes cool enough to superconduct. This behavior, in which magnetic fields are excluded from a superconductor, is called the Meissner effect and is something not seen in normal electrodynamic levitation. It's unique to certain types of superconductors.
Section 9.3 Tape Recorders
265. Magnetic Domains
Description: An array of magnetic arrows (tiny compasses) forms aligned domains.
Purpose: To show that magnetic domains tend to form in any extended ferromagnetic system.
Supplies:
1 array of magnetic arrows (available from a scientific supply company)
1 bar magnet
Procedure: Set the array of magnetic arrows on the table and inspect its arrows. You'll find groups of nearby arrows that are aligned with one another, but overall they will have little or no average alignment. These local regions of alignment are analogous to the domains in a ferromagnetic solid.
Now bring one pole of the bar magnet near the edge of the array. The array will change so that virtually all of the arrows will be aligned. They will all point either toward or away from the pole of the bar magnet. You have magnetized the array—its domains have changed so that they have a net magnetic alignment.
Take the bar magnet away from the array and show that much of its magnetic alignment remains. The array is permanently magnetized.
Finally, wave the bar magnet across the array carefully and gradually move it farther and farther away until it has no more effect. The array will once more consist of small aligned domains that have no average overall alignment. You have demagnetized the array.
Explanation: If all the magnetic arrows were to point in the same direction, the array would be a large magnet and would have considerable magnetic potential energy. The array normally lowers its energy by breaking up into domains and allowing the magnetizations of these domains to cancel one another. But when you bring the strong external magnetic field near the array, you force it into alignment. Even when you take away the external magnet, the array remains aligned—it needs a disturbance to break up into domains once again. When you jiggle the magnet nearby, you create this disturbance and the array breaks up into domains.
266. A Magnet and Steel Nails
Description: Steel nails normally don't stick to one another. But when you touch the pole of a permanent magnet to one of the nails, the nail becomes a magnet. When this nail touches another nail, that nail becomes magnetic, and so on. When you remove the permanent magnet, the nails slowly lose most of their magnetizations.
Purpose: To show how the presence of a strong magnetic pole magnetizes steel or iron.
Supplies:
1 bar magnet
3 or more steel nails
Procedure: First show that the nails don't normally stick to one another. Then touch the north pole of the bar magnet to a nail. The nail will stick to the bar magnet because it will become magnetized. The presence of the nearby north pole rearranges the magnetic domains inside the steel so that their south poles all point toward the north pole of the permanent magnet. As a result, the other end of the nail becomes a north pole. Show that this nail can magnetize another nail it touches in a similar manner. Form a chain of nails dangling from the bar magnet.
Finally, remove the bar magnet from the first nail. The chain of nails will slowly fall apart as the domains in the nails gradually return to their original random orientations. A few domains won't return to normal, so the nails will remain slightly magnetized as a result of their exposure to the bar magnet.
Explanation: Iron and most steels contain magnetic domains. Until these materials are exposed to magnetic fields, the domains are randomly aligned and their magnetization cancel one another. However, when these materials are exposed to magnetic fields, the domains grow or shrink until the materials exhibit substantial overall magnetizations. These magnetizations only remain while the external magnetic fields persist. The domains in very pure iron rearrange easily when the external fields vanish, so that very pure iron completely loses its magnetization. But in steels, the impurities in the crystals prevent the domains from rearranging so easily. Steel is a little harder to magnetize when an external magnetic pole approaches it and it doesn't demagnetize completely when the external magnet is taken away.
267. Aluminum and Copper are Non-Magnetic
Description: While steel sticks to a bar magnet, aluminum and copper do not.
Purpose: To show that most metals are non-magnetic (they are not ferromagnetic).
Supplies:
1 strip of steel (not stainless steel!)
1 strip of copper
1 strip of aluminum
1 bar magnet
Procedure: Show that steel sticks to the bar magnet while copper and aluminum do not.
Explanation: The steel contains magnetic domains that can be aligned by the proximity of a strong magnetic pole. The copper and aluminum have no magnetic domain structure at all, so a nearby magnetic pole has no effect on their internal magnetic structures.
268. Domain Flipping in a Piece of Soft Iron
Description: An iron rod sits in a coil of wire that's attached to a sensitive audio amplifier. As a bar magnet is brought up to the iron, the domains inside the iron flip into alignment with the magnet. These flipping domains induce currents in the coil of wire and create a "shoop" sound from the amplifier's speakers.
Purpose: To show that the domains in iron flip when the iron is magnetized.
Supplies:
1 iron rod
1 coil of wire
1 preamplifier, amplifier, and speaker
1 bar magnet
Procedure: Connect the coil of wire to the preamplifier, amplifier, and speaker. Insert the iron rod inside the coil. Turn on the amplifiers and slowly bring one pole of the magnet up to the iron rod. You will hear a "shoop" sound emerge from the speaker. Each component of the "shoop" corresponds to a domain flipping in the iron rod. Since there are so many domains and they flip at random moments between the start to the finish, their overall sound is the "shoop" sound. If you reverse the bar magnet, you can repeat the experiment and hear the "shoop" again.
Explanation: Each time you magnetize the iron, the domains in the iron rod align with the bar magnet. Their rearrangement creates a changing magnetic field through the coil and induces a current in its wire.
269. Reversing the Magnetization of a "Permanent" Magnet
Description: A bar magnet is inserted in a magnetizer and its poles are permanently reversed. A second trip through the magnetizer flips its poles back to normal.
Purpose: To show that the poles of a "permanent" magnet can be reversed during the magnetization process.
Supplies:
2 bar magnets, with their ends clearly labeled north and south (or red and white)
1 horizontal swivel mount for one of the bar magnets
1 bar magnet magnetizer (available from a scientific supply company)
Procedure: Support one of the bar magnets on the swivel mount. Hold the other magnet in your hand and show that the opposite poles of the two magnets attract and the like poles repel. Now insert the magnet that you have in your hand into the magnetizer and magnetize it backwards! When you again hold it in your hand, its "north" pole will attract the north pole of the magnet in the swivel. Show that the poles of the hand-held magnet are completely reversed. Finally, reinsert the magnet into the magnetizer and magnetize it properly. Show that its poles are back to normal.
Explanation: A permanent magnet is a material that, once magnetized in a certain direction, remains magnetized in that direction. While the factory may have magnetized the bar magnet in a particular direction, you can reverse that direction if you have the right equipment (typically a coil of wire and a highly charged capacitor).
270. Sprinkling Iron Fillings on a Credit Card
Description: You sprinkle iron filings on the magnetic strip of a credit card. The filings align in patterns, indicating that there is a pattern to the magnetization of the permanent magnet particles in the magnetic strip.
Purpose: To show how the magnetization of a credit card strip contains information.
Supplies:
1 credit card (this test is non-destructive; you can clean off the credit card and it will still work)
1 shaker of iron filings (finely ground)
Procedure: Sprinkle iron filings on the magnetic strip of a credit card and gently tap the card to allow the loose filings to slip away. You'll see a pattern to the filings that shows that there is a pattern to the magnetization of the magnetic strip.
Explanation: The magnetic strip of a credit card is like a very coarse magnetic tape. The magnetic patterns on the credit card strip are so huge that you can see them with your eye, or at least with a magnifying glass.
271. A Simple Tape Player
Description: You construct of simple tape player by inserting an iron rod in a coil of wire that's attached to an amplifier and speaker. You then pull a long refrigerator magnet strip across the iron "playback head" and hear a humming sound from the speaker. The faster you pull the strip across the iron rod, the higher the pitch of the hum.
Purpose: To demonstrate how a tape recorder plays back a tape.
Supplies:
1 long magnetic strip (a long refrigerator magnet or a magnetic strip for a office organizational bulletin board)
1 iron rod
1 coil of wire
1 preamplifier, amplifier, and speaker
Procedure: Connect the coil of wire to the preamplifier, amplifier, and speaker. Insert the iron rod into the coil. Turn on the amplifiers and draw the long magnetic strip across the iron rod. A humming sound will emerge from the speaker. The faster you move the magnetic strip, the higher the pitch of the hum. Point out that the magnetic strip has many poles on it and that they reverse every few millimeters (you can show this with iron fillings if you like). As you pull the strip across the iron rod, the iron's magnetization reverses periodically and it induces fluctuating currents in the coil of wire. The amplifiers and speaker use this fluctuating current to produce the humming sound.
Explanation: Just as in a magnetic tape that has recorded sound on it, the magnetic strip has a fluctuating magnetization on its surface. As you draw it across the "playback head," the amplifiers and speaker produce a fluctuating air pressure that is the humming sound.
272. A Reconstructed Tape Recorder
Description: A piece of magnetic tape slides across the playback head of a tape recorder. The amplifier and speaker of the tape recorder reproduce the sound.
Purpose: To show how the parts of a tape recorder work.
Supplies:
1 cassette tape recorder (to be disassembled)
1 cassette tape
parts, time, and perseverance
Procedure: Extract the playback head of the tape recorder (or a tape player) and mount it and the preamplifier on a board that allows them to be inserted into the center of a cassette cartridge so that the head touches the tape. Position a variable speed motor so that it will pull the tape through the cassette tape cartridge at a steady, slow speed. Connect the playback head and preamplifier to an amplifier and speaker.
Now start the tape moving through the cartridge and bring the playback head into contact with the tape as the tape moves through the middle of the tape cartridge. The speaker will reproduce the sound recorded on the tape. Getting all of this working correctly takes a little time and energy, but it's pretty satisfying when it works. It really helps demystify tape recorders.
Explanation: The moving tape induces currents in the playback head and these currents are amplified and delivered to the speaker to reproduce the sound.
Follow-up: Tape player kits exist and can be modified to make it easy to see how the tape recorder works.
Section 9.4 Electric Power Generation
273. Generating Electricity - A Coil and a Magnet
Description: When a magnet moves past a coil of wire, a current flows through the wire.
Purpose: To show that changing or moving magnetic fields can induce currents in electric conductors.
Supplies:
1 coil of wire
1 bar magnet
1 current meter (one that reads both positive and negative currents)
Procedure: Connect the two ends of the coil of wire to the two terminals of the current meter. Now move one pole of the bar magnet past the coil. You'll observe that the meter needle moves first one way and then the other. Show that as the pole approaches the coil, the needle moves one way and as the pole moves away from the coil, the needle moves the other way. Try reversing the magnet (use its other pole)—the effect will reverse.
Explanation: The changing magnetic field through the coil produces an electric field around it and this electric field pushes charges through the coil's windings. The meter registers this flowing current. The direction of current flow is determined by the direction in which the electric field points and that direction depends on how the magnetic field is changing.
274. Generating Electricity - A Coil and a Two-Color LED
Description: When a magnet moves past a large coil of wire, a current flows through it and illuminates an LED. The LED's color depends on which way the magnet moves and on which of its poles is being used.
Purpose: To show that moving a magnet past a conductor can cause a current to flow through that conductor.
Supplies:
1 large coil of wire (several hundred or even a thousand turns)
1 two-color LED (actually two different LEDs connected in parallel in the same package. One LED glows when the current flows one direction and the other LED glows when the current flows in the opposite direction. Alternatively, use two LEDs connected in parallel but in the opposite directions)
1 strong bar magnet
Procedure: Connect the LED to the two ends of the wire coil. Now hold the bar magnet in your hand and bring one of its poles toward the wire coil. The faster you move the magnet, the more effective it will be. The LED should light with one of its colors. Now pull the magnet out of the coil quickly. The LED should light with its other color. Repeat this process rapidly several times and point out that you are generating alternating current. If you were to attach the magnet to a spinning rotor, the LED would blink back and forth rapidly as the magnet swept by. Show that reversing the pole of the magnet reverses its effects.
Explanation: The changing magnetic field in the coil of wire induces currents in the coil. The coil is large enough (has enough turns) that these induced currents reach the high voltages (about 3 to 5 V) needed to power an LED.
275. An AC Generator
Description: You turn the crank of an AC generator and illuminate a light bulb. You show that it's much harder to turn the crank of the generator when current is flowing through the light bulb than when the circuit to the light bulb is open.
Purpose: To show how an AC generator works.
Supplies:
1 AC generator
1 suitable light bulb for the generator
1 light bulb holder
2 wires
Procedure: Connect the two terminals of the generator to the two terminals of the light bulb holder and light bulb. Turn the generator and show that the light bulb lights up. Allow a student to turn the generator and open and close the circuit to show that it's much harder to turn the generator while current is flowing and the generator is producing electric power.
Explanation: The generator moves a magnet past a coil (or a coil past a magnet) and generates an alternating electric current in the coil and the circuit to which that coil is attached. In this case, the current flows back and forth through the light bulb and its filament becomes hot.
276. Two DC motors Connected in Parallel
Description: Two DC motors (with permanent magnets) are connected to one another by wires. When you turn one of the motors, the other motor also turns. Reversing the direction in which you turn the first motor reverses the direction in which the other motor turns.
Purpose: To show how a DC generator works.
Supplies:
2 DC motors (good bearings and permanent magnets are essential—we use two 12 V motors that are large and powerful; probably about 1/10 hp or so)
2 wires
Procedure: Connect the two DC motors together with the two wires so that you form one large circuit. Now spin the rotor of one of the motors and observe that the other motor spins. That's because you're generating electricity with the first motor (effectively a generator) and that electricity is powering the second motor. Do the same with the second motor and show that the two motors are interchangeable. Now show that reversing the direction in which you spin the first motor causes the second motor to reverse its direction of rotation. That's because the motors are acting as DC generators—they contain switching systems that ensure that the current flows in one direction that's determined only by the direction in which you spin the rotor. Similarly, the direction of current flow through a DC motor determines its direction of rotation.
Explanation: When you spin the rotor of the DC motor, you are moving a permanent magnet past a coil (or vice versa) and generating a current in that coil. A switching system inside the motor/generator changes the connections regularly so that current always flows in the same direction through the external portions of the circuit (as long as you don't reverse the direction in which the motor/generator's rotor is spinning). The DC electricity that you generate with the first motor/generator powers the second motor/generator, which turns in a direction determined by the direction of current flow through the circuit.
277. Hero's Engine
Description: Steam produced by water boiling in a spherical vessel emerges from that vessel through two arms that are arranged in a Z shape. As the arms push the steam in one direction, the steam pushes back and the vessel experiences a torque. It begins to spin rapidly.
Purpose: To show how steam can be used to create rotational motion (a primitive turbine-like heat engine).
Supplies:
1 Hero's engine (available from a scientific supply company)
1 suspension for the Hero's engine (preferably with a swivel clip)
1 gas burner
matches
water
Procedure: Partly fill the Hero's engine with water and install the cap. Suspend the Hero's engine from its support and place the burner beneath it. Ignite the burner and allow the water to boil. When steam begins to emerge from the arms of the Hero's engine, the reaction forces on the arms will produce a torque on the engine and it will begin to spin rapidly. Turn off the burner so that it doesn't get out of control.
Explanation: The ejected steam exerts a torque on the engine, which undergoes angular acceleration. The steam is doing work on the engine, converting a small amount of its thermal energy into work as heat flows from the hot steam to the colder room air. The Hero's engine is a simple heat engine.
278. An Air Turbine or Windmill
Description: You blow air from a compressed air line or tank at a turbine or fan and it begins to spin. With the turbine or fan attached to a generator, it produces electric power.
Purpose: To show that a high-pressure (or high-speed) fluid can be used to generate electricity.
Supplies:
1 turbine or fan assembly, attached to a generator
1 light bulb
1 light bulb holder
2 wires
1 hose
compressed air or a tank of high pressure gas
Procedure: Insert the bulb in the holder and use the two wires to connect it to the generator. Allow the air or gas to flow through hose and direct the stream of air or gas toward the turbine blades. The blades will begin to spin, turning the generator and generating electricity. The light bulb will illuminate.
Explanation: As the air or gas flows through the turbine blades, they experience lift forces. These lift forces produce torques on the blades about their central pivot and the blades begin to turn. They turn the generator, which produces electricity.
279. Diodes - One Way Devices for Current
Description: A battery and light bulb are connected in a circuit so that the bulb lights up. When a diode is inserted into the circuit in one direction, it has essentially no effect and the bulb remains bright. But when the diode is reversed, no current flows through the circuit and the bulb is dark.
Purpose: To show that a diode only carries current in one direction.
Supplies:
1 12 V battery
1 12 V light bulb
1 light bulb holder
1 power diode
3 wires
Procedure: Insert the light bulb in the holder and use two of the wires to connect the battery to the bulb. The bulb will glow brightly. Now insert the diode into the circuit so that the battery's positive terminal connects to the anode of the diode and the diode's cathode connects to the light bulb. The light bulb will continue to glow. Finally, reverse the diode's connection, so that its anode is connected to the light bulb and its cathode is connected to the positive terminal of the battery. The light bulb will be dark because no current will flow. Discuss the fact that the diode only permits current (positive charges) to flow from its anode to its cathode.
Explanation: When the diode is forward biased (its anode is positively charged and its cathode is negatively charged), conduction level electrons in the cathode's n-type semiconductor can approach the diode's p-n junction and leap across the junction into empty conduction levels in the anode's p-type semiconductor. The anode's positive charges can then meet the oncoming electrons so that there is a net flow of charge and current through the diode. But when the diode is reverse biased (its anode is negatively charged and its cathode is positively charged), the depletion region near the p-n junction widens and no charges cross the junction.
280. A Solar Cell
Description: A solar cell is connected to a small motor. When the cell is exposed to light, the motor turns.
Purpose: To show that a solar cell can produce electricity directly from light.
Supplies:
1 solar cell
1 ultra-low friction motor (specially designed for solar cell operation—available from a scientific supply company)
2 wires
1 100 W (or more) incandescent spot light
Procedure: Use the two wires to connect the solar cell to the motor. Now expose the solar cell to the bright light from the spot light. Current will begin flowing through the solar cell and receiving power. This power will be delivered to the motor and will cause it to turn.
Explanation: The solar cell is a specially designed diode. Light energy transfers electrons from the n-type semiconductor of the cathode to the p-type semiconductor of the anode. The anode becomes negatively charged and the cathode becomes positively charged. Since the electrons can't return through the diode's p-n junction, they flow through the circuit (including the motor). The light energy is causing this current flow and is powering the motor.
Section 9.5 Electric Motors
281. Hanging from an Electromagnet
Description: A strong electromagnet hangs for the ceiling. A steel surface is touched to it and it's turned on. The forces between the electromagnet and the steel are so strong that you can hang from the steel without pulling it away from the electromagnet.
Purpose: To demonstrate the tremendous forces that are possible with electromagnets.
Supplies:
1 strong, battery-powered electromagnet (available from scientific supply companies)
1 thick steel plate, the same diameter as the electromagnet
2 strong steel eyelets with threaded shafts
2 ropes
Procedure: Use a drill and tap to attach one of the eyelets to the back of the electromagnet and the other to the back of the steel plate. Attach the ropes to the eyelets and hang the electromagnet from the ceiling. Form a loop in the rope attached to the steel plate so that you can hold onto the rope tightly. Now touch the steel plate to the electromagnet and turn the electromagnet on. The plate will bind very strongly to the electromagnet. Pull downward on the steel plate to show that it can't be pulled away easily. Try hanging on the plate (though be prepared for it to pull away from the electromagnet). If the electromagnet is sufficiently strong, the plate will remain attached.
Explanation: Steel is a ferromagnetic metal, meaning that it contains magnetically ordered domains. When you bring the steel near the electromagnet, the steel's domains change size and reorient to give the steel its own magnetic poles. The steel's poles are opposite to those of the electromagnet and the two bind together strongly.
282. A Galvanometer
Description: When you send current through the coil of a galvanometer, the coil moves. It experiences a torque in the presence of a magnetic field.
Purpose: To show that the torque between a current-carrying coil and a fixed permanent magnet can cause that coil to turn.
Supplies:
1 galvanometer (or a coil of wire that's supported in a low-friction bearing and surrounded by permanent magnets)
1 battery
2 wires
1 resistor (to limit the current through the galvanometer, if necessary)
Procedure: Use the two wires to connect the battery to the galvanometer. If the galvanometer involves thin wires, you should include a current-limiting resistor in the circuit. As soon as you complete the circuit and current begins to flow through the galvanometer, its coil will become magnetic and will experience a torque due to its interactions with the surrounding magnets. However, it will turn only once and then settle down. Unlike a motor, the galvanometer coil has an equilibrium orientation into which it's able to settle.
Explanation: The galvanometer coil will turn to bring its magnetic poles as close as possible to the opposite poles of the surrounding permanent magnets.
283. A DC Motor
Description: A DC motor with a visible commutator turns rapidly as current passes through it from a battery.
Purpose: To show how a DC motor works.
Supplies:
1 DC motor demonstration, with a visible commutator
1 battery
2 wires
Procedure: Use the two wires to connect the DC motor to the battery. The motor will begin spinning. Reverse the battery and show that the motor turns the other way. Point out that the motor reverses because all the poles of the coil reverse but the permanent magnets that surround the coil remain unchanged. As a result, the torques on the rotor reverse and the motor spins backward. Stop the motor and discuss how the commutator reverses the flow of current through the coil just before the coil reaches its equilibrium orientation. This current reversal ensures that the coil keeps turning because the coil can never actually reach its equilibrium orientation.
Explanation: The battery provides power to the current that then flows through the coil of the motor. This current magnetizes the coil and causes the coil to experience a torque in the presence of the surrounding permanent magnets. The coil rotates so as to approach its equilibrium orientation within the permanent magnets, but before it arrives, the commutator causes the current through the coil to reverse and it must turn further. The coil never reaches an equilibrium orientation and continues to turn indefinitely.
284. A Very Simple DC Motor
Description: A tiny motor built right on top of a battery turns for hours without stopping.
Purpose: To illustrate just how easy it is to build an electric motor.
Supplies:
1 "D" battery
1 strong rubber band
2 large paper clips
1 square magnet, about 2 cm on a side and about 0.3 cm thick, with a north pole on one side and a south pole on the other.
enamel-coated copper wire, about #24 gauge
fine sandpaper
pliers
tape
1 small base
Procedure: One end of a paper clip has two metal loops. Locate this end of each clip and bend the outer loop over the inner loop so that you form an oval opening at that end of the paper clip. Place one of the modified paper clips at each end of the battery so that the two oval openings project outward from the same side of the battery. Hold the two paper clips in place with the rubber band. Lie the battery on its side so that the paper clips point directly upward and tape the battery to the base so that the battery won't roll. Use tape to attach the square magnet to the top of the battery, between the two paper clips.
Now wind a circular coil from the enamel-coated copper wire. You should form a coil about 2 cm in diameter that contains about 10 turns of wire. One end of the wire in the coil should extend about 3 cm to the left from the coil and the other end should extend about 3 cm to the right. Wrap the wire ends once or twice around the other 10 turns of wire before extending them outward, to help hold the coil together. You should end up with a wire ring that has an end wire extending leftward at 9 O'clock and another end wire extending rightward at 3 O'clock.
Sand away the insulation from one end wire but be careful with the other end wire. Hold the coil of wire so that the coil is in a vertical plane with the untouched end wire oriented horizontally. Lower that end wire onto a firm horizontal surface and sand away only the enamel that's on the upper half of the end wire. Leave the lower half enamel-coated.
Carefully insert the coil's end wires into the two oval loops of the two paper clips—one end wire into each oval—and let the end wires touch the paper clips. If the paper clips are touching the battery terminals and if the end wires of the wire coil are making contact with the paper clips, the coil should begin to move. You may have to spin the coil to get it started. Note that it will only spin properly in one direction, determined by the direction of current flow through the coil and the orientation of the magnet. The coil will spin as long as the electric connections are good and will operate for hours before depleting the battery's energy.
Explanation: Because of the partial insulation on the enamel wire, the coil is an electromagnet only for half its orientations. It is attracted or repelled by the magnet beneath it during half its rotation, but just as it gets to its equilibrium orientation, the current flow vanishes and it continues on for half a turn because of its rotational inertia. It continues to turn indefinitely.
285. Sophisticated DC Motors
Description: A DC motor that's attached to a variable-current power supply turns more rapidly as the current passing through it is increased. When the current passing through it is reversed, its direction of rotation reverses.
Purpose: To show that a DC motor's rotational speed increases as the current passing through it increases (assuming that its only load is friction) and that its direction of rotation reverses as the current through it reverses.
Supplies:
1 good quality DC motor
1 variable-current DC power supply
2 wires
Procedure: Use the two wires to connect the power supply to the motor. Show that as the current through the motor increases, so does its rotational speed. Show also that when you reverse the current passing through the motor, that its direction of rotation reverses.
Explanation: The rotational speed of the unloaded motor is limited by its ability to do work against sliding friction. The faster it turns, the more work it does each second and the more electric power it requires. Thus increasing the current passing through the motor and voltage drop of that current increases the power the motor receives and allows it to turn faster. Since reversing the current through the motor interchanges all the north and south poles of the motor's electromagnets, the torques in the motor reverse and it turns backwards.
286. A Simple Induction Motor
Description: An aluminum pizza platter or pie dish floats on water. When you move a strong magnet around in a circle above the platter, the platter begins to rotate with the magnet, even though the two aren't touching.
Purpose: To show how magnetic drag forces allow a magnet that's circling a conducting wheel to pull that wheel around with it.
Supplies:
1 aluminum pizza platter or pie dish
1 large, shallow container of water (large enough to float the aluminum dish in)
1 strong magnet
Procedure: Float the platter or dish in the water and stop it from turning. Now hold one pole of the magnet a few centimeters above the platter and begin to circle the outer edge of the platter with the magnet. The platter will experience angular acceleration and will begin to turn with the circling magnet.
Explanation: The moving magnet induces currents in the platter and makes that platter magnetic. The repulsive forces between the magnet and platter tend to push the platter out in front of the magnet. If you could continue this motion steadily enough, the platter would end up turning just a little more slowly than the magnet.
287. A Large Single-Phase Induction Motor
Description: A capacitor-start motor leaps into action when you turn it on and rotates steadily there after.
Purpose: To demonstrate the operation of a powerful induction motor.
Supplies:
1 large induction motor with a starting capacitor (1/2 hp or whatever you can find)
Procedure: Hold the induction motor in place (I use my foot) and plug it in. It will jump as its rotor begins to spin. Point out the raised ridge on its side. This ridge contains a capacitor that helps to create a magnetic pole in the stator that circles the rotor in a particular direction as the motor starts up. During its operation, the rotor turns almost as fast as the circling pole of the stator. Since the rate at which the stator's pole circles the rotor depends on the cycling of the power line, the motor's rotational speed is determined by the power line frequency. Many induction motors complete one full turn for every two cycles of the power line. These motors turn at almost 1800 rpm (almost 30 turns per second) in the United States or almost 1500 rpm in many other countries.
Explanation: The stator of the induction motor is built from electromagnets. The starting capacitor provides a delayed phase to some of the electromagnets during the starting process so that the magnetic poles of the stator circle the rotor in a particular direction. (Note for the experts: Once the rotor is turning properly, the delayed phase isn't needed any more. The poles of the stator are then driven directly from the single phase power and these poles oscillate back and forth rather than circling the rotor. However, these oscillating poles can be decomposed into pair of poles that circle with and against the direction in which the rotor is spinning. It turns out that the torque exerted on the rotor by the poles that are circling with the rotor are strongest and they keep the rotor turning steadily and powerfully forward.)
288. An Electric Fan
Description: The induction motor of an electric fan turns at 2 or 3 different speeds, as determined by the rotation rates of the poles on its stator.
Purpose: To show how varying the rotation speeds of the stator poles can change the rotation speed of an induction motor's rotor.
Supplies:
1 2- or 3-speed fan
Procedure: Show that the fan has two or three different speeds of rotation. These speeds are determined by how rapidly the poles of the stator circle the rotor.
Explanation: The faster the poles of the stator circle the rotor, the faster the rotor must turn to keep up with the circling poles.
Description: A copper disk that can turn on a bearing is held horizontally above the pole piece of an AC electromagnet. When the electromagnet is operating and another piece of copper shades half the pole piece from the copper disk, the disk begins to turn.
Purpose: To demonstrate another type of induction motor—a shaded pole motor.
Supplies:
1 copper disk, about 10 cm in diameter that turns about a central bearing
1 support for the copper disk and its bearing
1 AC electromagnet with a vertical pole piece that extends upward above the electromagnet
1 thick piece of highly conductive copper sheet (about 3 or 4 mm thick)
Procedure: Mount the copper disk horizontally above the pole piece of the AC electromagnet. The pole piece should end about 1 cm below one edge of the disk. Turn on the AC electromagnet and gradually slide the copper sheet on top of the pole piece until it covers half the pole piece. The edge of the strip should be aligned with the radius of the disk. The disk will begin turning so that its surface moves from above the uncovered portion of the pole piece to above the covered portion. If you shift the copper sheet to the other side of the pole piece, the disk will begin to turn the other way.
Explanation: The presence of the copper sheet above the pole piece delays the formation of a magnetic pole on the copper-shaded side of the pole piece. This delay occurs because the induced currents in the copper sheet temporarily shield the area above the sheet from the magnetic field of the pole piece. In effect, the pole moves from the unshaded portion of the pole piece to the shaded portion. The copper disk moves with this moving pole and it turns.
Follow-up: You can replace the disk and bearing with a copper ball that floats in water. The ball will begin to rotate when you cover half the pole piece with copper.
Section 10.1 Audio Amplifiers
You may wish to repeat the Ohm's law demonstration from Section 12.2 to show how a resistor impedes the flow of electric current and the capacitor demonstration from Section 1.4 to show how a capacitor stores separated electric charge.
290. The Current from a Microphone
Description: The current from a microphone is displayed on an oscilloscope while you make various sounds.
Purpose: To show how the air pressure fluctuations at the microphone are represented by current fluctuations in the circuit to which the microphone is attached.
Supplies:
1 microphone (with power supply, if necessary)
1 oscilloscope
wires
Procedure: Connect the microphone to the input of the oscilloscope and turn both on. Set the oscilloscope trigger so that a clear trace appears on the screen when you make a single-pitch sound (a whistle, for example). Point out that the oscilloscope displays the current in the circuit on the vertical axis (with zero appearing at the center of the screen, so that excursions below the center of the screen represent reversals of the current) and that time is the horizontal axis. Note that broad fluctuations in the trace represent low frequency sounds and low frequency alternating currents. Note also that narrow (rapid) fluctuations in the trace represent high frequency sounds and high frequency alternating currents. Show that larger volumes produce larger amplitude alternating currents.
Explanation: The microphone produces currents that are proportional to changes in air pressure. As sound reaches the microphone, the rising and falling air pressures are represented by the microphone as forward and backward currents through the circuit connected to the microphone.
291. A Speaker
Description: A variable-amplitude 60 Hz current flows into a large speaker that rest horizontally on the table. Several marbles in the cone of that speaker begin to leap up and down.
Purpose: To show how a speaker uses an alternating current to produce sound.
Supplies:
1 large (woofer) speaker, without a cabinet
1 low-voltage transformer (12 VAC, 5 A or so)
1 variable-voltage autotransformer (a Variac)
3 or more marbles
wires
Procedure: Connect the primary of the low-voltage transformer to the output of the variable-voltage autotransformer. Connect the secondary of the low-voltage transformer to the speaker. Plug in the autotransformer and slowly turn up its voltage. The speaker should begin to hum more and more loudly. Put the marbles in the speaker and allow them to bounce up and down. Discuss the motion of the speaker cone as the alternating current in its coil flows back and forth. Discuss how this motion produces compressions and rarefactions of the air; thus producing sound.
Explanation: The AC current flowing through the secondary coil of the low-voltage transformer and the coil of the speaker magnetizes the coil of the speaker and causes it to be alternately attracted and repelled by the speaker's permanent magnet. The speaker's paper cone is connected to its coil and both move toward and away from the speaker's permanent magnet. This motion causes the marbles to jump about.
292. A MOSFET
Description: You show that a tiny amount of electric charge (delivered with your finger) on the gate of a MOSFET can control the flow of a large amount of electric current between its source and drain. The MOSFET controls a light bulb.
Purpose: To show how charge affects the conductivity of a MOSFET and allows it to control the current flowing in a circuit.
Supplies:
1 n-channel enhancement-mode MOSFET with a suitable current and voltage rating (I have usually used Motorola MTP1N50 MOSFETs, which are rated at 1 A (hence the "1") and 500 V (hence the "50"). However, if you want to use a high current bulb, an MTP10N40E would be appropriate—10 A at 400 V. In any case, be prepared to replace the MOSFET once in a while when you damage it with static electricity. It just happens.
1 12 V light bulb (less than 1 A if you use a 1 A MOSFET, but can be higher current if you use a more powerful MOSFET)
1 light bulb holder
1 12 V battery
wires
Procedure: Before handling the MOSFET, always touch an earth ground to remove any charge you may have accumulated! Insert the light bulb in the holder. Connect the positive terminal of the battery to one terminal of the light bulb holder. Connect the other terminal of the light bulb holder to the drain of the MOSFET. Connect the source of the MOSFET to the negative terminal of the battery. Now you're ready to begin switching the light on and off.
To turn the light on, touch one hand to the positive terminal of the battery and then touch your other hand to the gate of the MOSFET (in that order! If you touch the MOSFET first, you may have excess charge on you and may destroy the MOSFET!). Positive charge will flow onto the gate of the MOSFET and it will conduct current. The light bulb will turn on.
To turn the light off, touch one hand to the negative terminal of the battery and then touch your other hand to the gate of the MOSFET (again battery first!). Positive charge will flow off the gate of the MOSFET and it will stop conducting current. The light bulb will turn off.
Since the charge (or lack of charge) will remain on the gate while you are not touching it, the light will remain on or off indefinitely while you leave the gate alone.
Explanation: When positive charge is present on the gate of the MOSFET, electrons are attracted into the normally p-type semiconductor of the channel and the channel becomes effectively n-type semiconductor. Because both the source and drain are already n-type semiconductor, the p-n junction between the source and channel and between the channel and drain vanish and the entire MOSFET acts like a piece of n-type semiconductor. Current can flow through it from the source to the drain. However, when the positive charge is removed from the gate, the channel becomes p-type again, the p-n junctions reappear and current can't flow through the MOSFET anymore.
293. An Audio Amplifier
Description: You build a simple audio amplifier and use it to amplifier sound from a small tape or CD player so that it can be reproduced by a reasonably large speaker. The amplifier is so sensitive that you can act as part of the wiring connecting the tape or CD player to the input portion of the amplifier.
Purpose: To show how an audio amplifier works.
Supplies:
1 n-channel enhancement-mode MOSFET with a suitable current and voltage rating (I have usually used Motorola MTP1N50 MOSFETs, which are rated at 1 A (hence the "1") and 500 V (hence the "50"). However, an MOSFET that's capable of handling more current would also be fine. Be prepared to replace the MOSFET if you burn it out.
1 1 mF capacitor (20 V or higher)
1 100 mF capacitor (20 V or higher)
1 100 KW resistor
1 50 W resistor (2 Watt)
1 9 V battery or an equivalent power supply
1 speaker (8 W or 4 W)
1 small tape or CD player
wires
Procedure: Construct the amplifier shown in the figure below (also Fig. 13.1.9 in the book). I do it on a giant, homemade bread board with the components already mounted on cards with pins that plug into the breadboard. Each component is labeled with its symbol so that when the amplifier is complete, it looks like the figure below.
Be careful as you assemble the amplifier not to burn out the MOSFET. It should be inserted last and you should touch earth ground (and ground the rest of the amplifier, at least briefly) before you touch the MOSFET.
When the amplifier is complete, connect the speaker to its output wires (on the right) and the tape player or CD player to the input wires (on the left). If you now turn on the tape player or CD player, sound will come out of the speaker. Discuss how alternating currents in the input circuit cause charge to flow on and off the gate of the MOSFET. Discuss how charge on the gate of the MOSFET controls the current flowing between its source and drain. Discuss how the MOSFET diverts current that flows down from the battery's positive terminal through the 50 W resistor and keeps that current from flowing to the speaker. By alternately diverting and not-diverting this current from the 50 W resistor, the MOSFET produces an fluctuating current in its output circuit and through the speaker. The speaker produces sound.
For a display of how sensitive the MOSFET is to charge, disconnect one of the input wires from the tape or CD player and use your hands to remake the connection. Enough current will flow through you to allow the amplifier to play the music.
Explanation: Current in the input circuit controls the charge on the MOSFET's gate and the MOSFET controls the current flowing through the speaker.
Section 10.2 Computers
294. Series and Parallel Circuits
Description: You create a circuit with a battery and bulb, in which two switches are in series. Both switches must be closed simultaneously before current will flow and the lamp will light. You then arrange the switches in parallel and either switch can close the circuit.
Purpose: To show the differences between series and parallel arrangements for switches.
Supplies:
2 switches (knife switches, if possible)
1 12 V battery
1 12 V bulb
1 bulb holder
wires
Procedure: Connect the battery and bulb in a complete circuit and show that the bulb lights up. Now insert one switch into the circuit and show that it must be closed in order for the bulb to light. Add a second switch in series with the first switch and show that both switches must be closed for the bulb to light.
Now disconnect the second switch and reinsert it in parallel with the first switch. Show that closing either switch causes the bulb to light. Discuss how in a series arrangement, the same current must flow through both devices to reach its destination. Discuss how in a parallel arrangement, current can flow through either device to reach its destination.
Explanation: In general, two devices in series experience the same current but their overall voltage drop is the sum of their individual voltage drops. Two devices in parallel experience the same voltage drop, but their overall current is the sum of their individual currents.
295. A Simple CMOS Inverter
Description: You build a simple CMOS inverter. You then show that when you deliver positive charge to its input, it delivers negative charge to its output and vice versa.
Purpose: To show how an inverter works.
Supplies:
1 n-channel enhancement-mode MOSFET with a suitable current and voltage rating (I have usually used Motorola MTP1N50 MOSFETs, which are rated at 1 A (hence the "1") and 500 V (hence the "50"). Be prepared to replace the MOSFET if you burn it out.
1 p-channel enhancement-mode MOSFET with a suitable current and voltage rating (I have usually used Motorola MTP2P50 MOSFETs, which are rated at 2 A (hence the "2") and 500 V (hence the "50"). Be prepared to replace the MOSFET if you burn it out.
1 9 V battery
1 voltmeter or equivalent
wire
Procedure: Connect the two MOSFETs according to the figure below (also Fig. 13.2.6 of the book), but use the 9 V battery as the supply, rather than the 3 V shown (the power MOSFETs need 9 V rather than 3 V). The upper MOSFET is the p-channel MOSFET and its source is connected to the positive terminal of the 9 V battery. Be careful to ground yourself and the components before working with them. Attach the voltmeter to the output to monitor its voltage (and charge).
To deliver positive charge to the input of this inverter, touch one hand to the positive terminal of the battery and then touch your other hand to the input wire. The output will go to 0 V (negative charge).
To deliver negative charge to the input of this inverter, touch one hand to the negative terminal of the battery and then touch your other hand to the input wire. The output will go to 9 V (positive charge).
Explanation: This CMOS inverter is using the charge delivered to its input to control two MOSFETs. The MOSFETs are arranged so that positive charges on their gates turns on the n-channel MOSFET and it delivers negative charge to the output. Negative charges on their gates turns on the p-channel MOSFET and it delivers positive charge to the output.
296. A Simple CMOS NAND Gate
Description: You build a simple CMOS NAND gate. You then show that when you deliver positive charge to both of its inputs, it delivers negative charge to its output. If either input has negative charge on it, it delivers positive charge to its output.
Purpose: To show how a computer gate works.
Supplies:
2 n-channel enhancement-mode MOSFETs with a suitable current and voltage rating (I have usually used Motorola MTP1N50 MOSFETs, which are rated at 1 A (hence the "1") and 500 V (hence the "50"). Be prepared to replace the MOSFET if you burn it out.
2 p-channel enhancement-mode MOSFETs with a suitable current and voltage rating (I have usually used Motorola MTP2P50 MOSFETs, which are rated at 2 A (hence the "2") and 500 V (hence the "50"). Be prepared to replace the MOSFET if you burn it out.
1 9 V battery
1 voltmeter or equivalent
wire
Procedure: Connect the four MOSFETs according to the figure below (also Fig. 13.2.8 of the book), but use the 9 V battery as the supply, rather than the 3 V shown (the power MOSFETs need 9 V rather than 3 V). The upper MOSFETs are the p-channel MOSFETs and their sources are connected to the positive terminal of the 9 V battery. Be careful to ground yourself and the components before working with them. Attach the voltmeter to the output to monitor its voltage (and charge).
To deliver positive charge to an input of this gate, touch one hand to the positive terminal of the battery and then touch your other hand to the input wire. To deliver negative charge to an input of this gate, touch one hand to the negative terminal of the battery and then touch your other hand to the input wire. Don't reverse the touch order or you will zap the MOSFETs!
When both inputs are positively charged, the output will be negative (0 V). When either input is negatively charged, the output will be positive (9 V).
Explanation: This CMOS NAND gate is using the charge delivered to its inputs to control four MOSFETs. The two p-channel MOSFETs are arranged in parallel and deliver positive charge to the output when either input is negatively charged. The two n-channel MOSFETs are arranged in series and deliver negative charge to the output when both inputs are positively charged. This arrangement gives the output a NAND relationship to the inputs.
Section 11.2 Television
302. Fluorescence
Description: Various materials are exposed to ultraviolet light and glow different colors.
Purpose: To demonstrate fluorescence.
Supplies:
1 ultraviolet lamp
fluorescent dyes and materials of various colors
Procedure: Turn on the ultraviolet lamp and show that you can't see its light. Point out that normal materials remain dark when exposed to only ultraviolet light. Now put the various fluorescent materials in the ultraviolet light and observe that they begin to emit visible light of various colors. Discuss the fact that this light is new light, radiated by the dyes and materials using energy they obtained from the ultraviolet light.
Explanation: A fluorescent material absorbs a photon of ultraviolet light and emits a new photon of light. While the new photon can have all of the energy of the original photon, so that it's just a new version of the original photon, the fluorescence that we observe most often involves the emission of a lower-energy photon—usually a visible photon. The missing energy usually becomes thermal energy.
303. Fluorescence Caused by Electron Impact
Description: A beam of electrons in a simple cathode ray tube causes the phosphor coating on the inside of the tube to glow (probably green).
Purpose: To show that energy from a beam of electrons can cause fluorescence.
Supplies:
1 simple cathode ray tube and its power supply
Procedure: Turn on the cathode ray tube and show that the impact of electrons on its phosphor screen causes that screen to emit light. The electrons are providing energy to the phosphors and they turn that energy into visible light.
Explanation: Phosphors can produce light whenever they are shifted to electronically excited states. Whether that excitation is the result of exposure to high energy light photons or the result of collisions with particles, the phosphors produce light.
304. Deflecting a Beam of Electrons with Electric Fields
Description: An electrostatic field created by a static generator deflects a beam of electrons in a cathode ray tube.
Purpose: To show that a beam of electrons accelerates in response to electric fields.
Supplies:
1 simple cathode ray tube and its power supply
2 metal plates with insulating supports
2 wires
Procedure: Touch the two contacts of the Wimshurst generator together to be sure that it doesn't have any stored charge. Use the wires to connect its two contacts to the two plates, being sure that the wires aren't near anything conductive or near one another. Position the plates at the sides of the cathode ray tube. Turn on the cathode ray tube. Separate the two contacts of the Wimshurst generator and turn its crank to generate static electricity. As charge builds up on the plates, the beam of electrons in the cathode ray tube will steer toward the positively charged plate.
Explanation: The beam of negatively charged electrons is attracted toward the positively charged plate and repelled by the negatively charged plate. The electrons accelerate toward the positive plate and the beam is deflected.
305. Deflecting a Beam of Electrons with Magnetic Fields
Description: A magnetic field created by a hand-held magnet deflects a beam of electrons in a cathode ray tube.
Purpose: To show that a beam of moving electrons accelerates in the presence of magnetic fields.
Supplies:
1 simple cathode ray tube and its power supply
1 strong bar magnet
Procedure: Turn on the cathode ray tube and then hold the bar magnet near its face. The spot formed when the electrons hit the phosphors will move, indicating that the magnetic field has deflected the electron beam.
Explanation: Moving electrons experience a transverse force when they move through a magnetic field. While this force is at right angles to their velocities and does no work on the electrons, it does alter their trajectories.
306. Deflecting a Beam of Electrons with a Magnetic Field - in a Black and White Television Set
Description: You hold a strong magnet up to a black and white television set and the picture distorts.
Purpose: To show that the television set is using a beam of electrons to form its image and to show that this beam of electrons can be steered by a magnetic field.
Supplies:
1 black and white television set (or an old color television set, if you don't mind spoiling it or are willing and able to demagnetize its shadow mask after the demonstration)
1 strong bar magnet
Procedure: Turn on the television and obtain a clear picture. Now bring the bar magnet up to the surface of the screen and show that the image distorts. If you're using a color television, the colors will also shift because the electrons no longer travel in their usual paths through the shadow mask. The effect will vanish when you remove the bar magnet from a black and white set, but the image may remain distorted or color-shifted on a color set. To "repair" a color television set, you need to demagnetize its shadow mask with a large AC demagnetizing coil.
Explanation: Moving electrons inside the television's picture tube are deflected by their passage through the extra magnetic field and they hit the screen at unintended positions.
307. Mixing the Primary Colors of Light
Description: By mixing various amounts of red, green, and blue light, you can make people perceive any possible color.
Purpose: To show how the primary colors of light can be mixed (as they are in a television) to make us see any possible color.
Supplies:
3 light sources of variable brightness
1 red filter
1 blue filter
1 green filter
Procedure: Place the three filters over the three light sources and partially overlap their beams on a white screen. Show that by adjusting their relative intensities, you can form various colors in their overlapping regions. When red and green are mixed evenly, you see yellow. When green and blue are mixed evenly, you see cyan. When red and blue are mixed evenly, you see magenta. And when all three are mixed evenly, you see white.
Explanation: Our eyes are really only sensitive to three types of light: red, green, and blue. While wavelengths of naturally occurring light that fall in between the wavelengths of pure red, pure green, and pure blue light cause us to see intermediate colors, we can be tricked into seeing those colors by the proper mixture of these primary colors of light.
Section 13.3 Telescopes and Microscopes
Before the demonstration about virtual images, I repeat the demonstration about real images from Section 16.1. Then I can combine the two demonstrations to form the Keplerian telescope that follows.
336. Forming a Virtual Image
Description: You hold a magnifying glass in front of a picture and see an enlarged virtual image of that picture. This image appears behind the lens, so you can't put your fingers in it the way you can with a real image.
Purpose: To show how a converging lens forms a virtual image of a very nearby object.
Supplies:
1 picture
1 magnifying glass (or another converging lens)
Procedure: Hold the magnifying glass a short distance in front of the picture and show that a virtual image of the picture appears. Point out that the image is larger than the picture and that it's located on the same side of the lens as the picture. You can't touch the virtual image. The virtual image is also upright, in contrast to a real image, which is inverted.
Explanation: The converging lens takes the diverging light rays that emerge from a particular point on the picture and bends them so that they don't diverge quite as fast. You see them as coming from a more distant but much larger virtual image.
337. A Keplerian Telescope
Description: You use a magnifying glass to allow close inspection of the real image formed by a converging lens, thus producing a Keplerian telescope.
Purpose: To show how two converging lenses can form a simple telescope.
Supplies:
1 light bulb (or another bright, identifiable object)
1 converging lens (about 50 mm in diameter, with a focal length of about 250 mm or so)
1 magnifying glass (or another converging lens)
1 optics bench (optional—otherwise just use lens and component holders)
Procedure: Place the light bulb several meters from the first converging lens and locate the real image of this light bulb that the first lens forms. You can use your hand to find the pattern of light in space because you can touch a real image.
Now take the magnifying glass and use it to produce an enlarged virtual image of that real image. You may want to observe this real image with a television camera and monitor so that everyone can see it.
Explanation: The first lens forms a real image of the light bulb and the second lens allows you to make a close (magnified) inspection of that image. The final virtual image is inverted because it's an upright virtual image of an inverted real image.
338. Forming a Virtual Image with a Mirror
Description: You look at an object in a mirror and notice that what you see is a virtual image.
Purpose: To show that mirrors can form virtual images.
Supplies:
1 mirror
1 light bulb (or another object)
Procedure: Place the light bulb a short distance in front of the mirror and observe the image that the mirror forms. This image is located on the other side of the mirror from the object, where you can't touch it. It's thus a virtual image.
Explanation: The mirror bends the light rays so that they appear to come from an object that's located behind the mirror, the same distance behind the mirror as the object is in front of the mirror.
339. Forming a Virtual Image with a Curved Mirror
Description: You look at an object in a curved mirror and notice that what you see is an enlarged virtual image.
Purpose: To show that curved mirrors can form enlarged virtual images.
Supplies:
1 concave mirror
1 light bulb (or another object)
Procedure: Place the light bulb a short distance in front of the mirror and observe the enlarged virtual image that the mirror forms.
Explanation: The mirror bends the light rays so that they appear to come from an object that's located behind the mirror. This virtual image is located at a greater distance behind the mirror than the object is in front of the mirror. This virtual image is also greatly enlarged relative to the object.
340. Forming a Real Image with a Curved Mirror
Description: Light from a distant light bulb reflects from a curved mirror and forms an inverted real image.
Purpose: To show that curved mirrors can form real images.
Supplies:
1 concave mirror
1 light bulb (or another object)
1 ground-glass screen
Procedure: Place the light bulb a long distance in front of the mirror (perhaps in the back of the darkened room) and observe the real image that the mirror forms on a nearby ground glass screen.
Explanation: The mirror bends the mildly diverging light rays from the distant bulb so that they converge together on the screen. The real image that forms on the screen is inverted in this process.
Follow-up: Use a magnifying glass to inspect the real image, thereby creating a reflecting telescope. Also, insert apertures in front of the mirror so that you use less of the mirror. Show that the real image darkens but remains complete. Point out that one of the values of a large aperture is light-gathering ability. Large mirrors collect more light and do their jobs faster.
341. The Importance of Large Apertures - Diffraction
Description: A laser beam is sent through a series of progressively smaller pinholes. With each smaller size, the resulting beam spreads outward more strongly.
Purpose: To show that sending light through an aperture causes it to spread outward (and that this spreading limits the resolution of a telescope).
Supplies:
1 laser beam with a good quality beam (a helium-neon laser is probably better than a solid state laser pointer. In principle, you want a clean TEM00 mode from the laser.)
1 set of pinholes
1 screen
Procedure: Direct the beam from the laser at the screen and notice how small the beam spot is. Now insert the pinholes into the beam, one at a time. As these holes get smaller, not only does the light spot get dimmer—it also gets wider.
Explanation: The light propagates as a wave. When the wave is force to go through a narrow aperture, it naturally spreads out in its subsequent travels. The narrower the aperture, the more rapidly the wave spreads. In a telescope, making the light go through the aperture defined by the mirror's diameter causes the light spread and limits the telescope's ability to resolve nearby stars.
Section 14.3 Nuclear Reactors
Description: You hold a Geiger counter near various radioactive sources and listen to their decays. When you insert certain materials between the sources and the Geiger counter, the counts diminish, indicating that the particles released by the decays are being blocked by the materials.
Purpose: To show that certain materials block the fragments of radioactive decays and can thus be used to control radiation and induced nuclear reactions.
Supplies:
1 or more radioactive sources (appropriate licensing, training, and safety precautions must be followed)
1 Geiger counter
shielding materials, ranging from cardboard for beta decays to lead sheets for more energetic particles
Procedure: Use the Geiger counter to monitor the decays of the various radioactive sources. Then insert the shielding materials between the sources and the Geiger counter to show that the decay fragments can be block (absorbed or reflected) by these materials.
Explanation: The electrons from beta decays are easy to block because the low-mass electrons are easily deflected. But more massive alpha particles must encounter the nuclei of massive atoms such as lead to deflect them from their paths. Gamma rays are also stopped only by large atoms because they interact most strongly with the tightly bound inner electrons of those giant atoms.
Section 15.1 Water, Steam, and Ice
346. Melting Ice
Description: A thermometer inserted in a container filled with a mixture of water and ice reads 0° C, even when the container is heated by a flame or cooled by dry ice.
Purpose: To show that the phase transition between water and ice occurs at 0° C, and that adding or removing heat from a mixture of the two causes one phase to transform into the other and doesn't change the temperature of the mixture.
Supplies:
1 Pyrex or Kimax beaker
water and ice mixture
1 thermometer
1 support for the beaker
1 support for the thermometer
1 gas burner
matches
1 cube of dry ice
Procedure: Place the beaker on the support and fill it with a mixture of ice and water. Insert the thermometer in the beaker and support the thermometer so that it doesn't touch the sides of the beaker. In a few seconds, the thermometer will read 0° C. To show that adding or removing heat from the mixture of water and ice won't change its temperature, first add heat to the mixture by heating it gently with the gas burner (don't heat too aggressively, or you'll break the beaker). The thermometer will still read 0° C. Finally, put away the burner and put the beaker on the cube of dry ice. Make sure that the thermometer doesn't touch the sides of the beaker. The thermometer will still read 0° C.
Explanation: While water and ice are in equilibrium with one another, the temperature must be 0° C. If you add heat to this mixture, some of the ice will transform into water but the mixture's temperature will remain at 0° C. If you remove heat from this mixture, some of the water will transform into ice but the mixture's temperature will remain at 0° C.
347. Boiling Water with Heat
Description: A beaker of water is heated with a burner. Although water will be seen to evaporate once the water is hot, it will only begin to boil when the water's temperature approaches 100° C. Once the water is boiling, additional heat will not cause its temperature to rise.
Purpose: To show that while evaporation can proceed at any temperature, boiling appears when evaporation becomes rapid enough to occur within the body of the liquid. Also to show that during boiling, adding heat to the water causes it to transform into steam rather than to become hotter.
Supplies:
1 Pyrex or Kimax beaker
water
1 support for the beaker
1 gas burner
matches
1 thermometer
1 support for the thermometer
Procedure: Place the beaker on the support and fill it half way full of water. Insert the thermometer into it and support the thermometer so that it doesn't touch the sides of the beaker. Light the burner and put it under the beaker. Heat the beaker gently so that it doesn't break. As the water becomes warmer, mist will appear above the water. A short while later, gas bubbles will appear on the walls of the beaker. And finally, bubbles of steam will appear within the water and the water will begin to boil. At that point, the temperature of the water will be approximately 100° C and this temperature will remain constant, despite the continued input of heat by the burner.
Explanation: As the water warms up, evaporation from its surface will become faster and faster. A mist will appear above the water when the evaporation becomes fast enough to send hot, water-saturated air upward into the cooler air above the beaker—as this hot, water-saturated air cools, water droplets form in it and create the mist that you see. The gas bubbles that appear on the walls of the beaker are dissolved gases that comes out of solution as the water nears its boiling temperature—most gases are less soluble in hot water than in cold water. Finally, boiling occurs when evaporation is so rapid that it begins to occur within the body of the liquid. For these evaporation bubbles to form and grow, they must be able to withstand the crushing effects of atmospheric pressure. By the time the water reaches 100° C, the bubbles of steam inside the water are so dense with water molecules that they have a pressure equal to atmospheric pressure and can't be crushed by atmospheric pressure.
348. Boiling Water in a Vacuum
Description: A glass of room temperature water is put in a glass bell jar and the air is removed from that bell jar by a vacuum pump. The water begins to boil. Moments later, air is admitted to the bell jar and it's removed. The water is still cool.
Purpose: To show that water's boiling temperature depends on the ambient pressure.
Supplies:
1 glass
water
1 bell jar and vacuum pump system
Procedure: Fill the glass half way full of water and insert your finger in it to show that it's cool. Put the glass in the bell jar and turn on the vacuum pump. When enough air has left the bell jar, the water will begin to boil. Stop the vacuum pump and allow air to reenter the bell jar. Open the jar and insert your finger into the water to show that its still cool.
Explanation: While evaporation is always occurring at the surface of cold water, it can't normally occur in the body of cold water because any evaporation bubble that appears inside the water will have too low a density and pressure to withstand the crushing effects of atmospheric pressure. But when a vacuum system has removed most of the air and air pressure from around a glass of water, evaporation bubbles that appear inside the water will be able to grow and expand. The water will boil even at low temperatures.
Follow-up: Try to soft boil an egg in a glass of water that's boiling in a bell jar. The egg won't cook at all. That's because boiling a three minute egg really means exposing that egg to water at 100° C for three minutes. In the vacuum chamber, you're exposing an egg to water at room temperature for three minutes and that has no effect on the egg at all. Why does it take longer to boil an egg at high altitude than it does at sea-level?
Another Follow-up: Try putting ice water in the vacuum. It will also boil if you're patient enough.
349. Condensing Steam - Crushing a Beverage Can
Description: You heat a small amount of water in an open beverage can until the can fills with steam. You then quickly invert the can and plunge it into a pan of cold water. The can is immediately crushed by atmospheric pressure.
Purpose: To show that removing heat from steam causes it to condense into water and that water occupies a much smaller volume than steam.
Supplies:
1 empty aluminum beverage can
1 ring stand
1 gas burner
1 cooking pan
tongs
matches
water
Procedure: Fill the cooking pan with about 3 cm of cold water. Pour about 2 ml of water into the beverage can and place it on the ring stand. Light the burner and heat the bottom of the can until the water boils. After the can has completely filled with steam and the steam has completely displaced any air the can contained (about 20 seconds of boiling), use the tongs to pick the can up, invert it, and plunge it into the pan of cold water. The can will collapse with a crunching sound.
Explanation: Boiling water in the can fills it with steam rather than air. When the steam is immersed in cold water, it gives up heat to the cold water and undergoes a phase change back into water. Water occupies much less volume than steam and the can is left virtually empty. With nothing inside it to support its walls, the can is crushed by the surrounding air pressure.
350. Dissolving Salt, Sugar, and Carbon Dioxide in Water
Description: You mix sugar, then salt, then carbon dioxide into water. All three dissolve easily.
Purpose: To discuss the mechanisms whereby added materials dissolve in water.
Supplies:
3 glasses
water
salt
sugar
1 soda siphon
1 carbon dioxide cylinder
1 spoon
Procedure: First add a spoonful of salt to a glass of water and stir. In a few seconds, the salt will have disappeared. Point out that the salt is still there, it has just decomposed into individual sodium positive ions and chlorine negative ions, each of which is now wrapped in an entourage of water molecules.
Now add a spoonful of sugar to the glass of water and stir. Again, it will dissolve. Point out that the sugar molecules are separated from one another and surrounded by shells of water molecules.
Finally, fill the soda siphon with water, put the top on, and charge the siphon with carbon dioxide according to its instructions. Shake the siphon to disperse the carbon dioxide and wait a few seconds. Then serve the carbonated water into a glass. It will bubble merrily. Note that the carbon dioxide molecules have attached themselves to water molecules to form a weak acid known as carbonic acid.
Explanation: Salt dissolves well in water because water molecules are strongly attracted to sodium and chlorine ions. They wrap those ions in solvation shells of water molecules. The negative ends of the water molecules (their oxygen atoms) turn toward a positive sodium ion and the positive ends of the water molecules (their hydrogen atoms) turn toward a negative chlorine ion. Sugar dissolves well in water because water molecules bond relatively well to sugar molecules. Water molecules form hydrogen bonds with the oxygen-hydrogen groups on a sugar molecule and construct a solvation shell around the sugar molecule. Finally, carbon dioxide dissolves well in water because water molecules combine with carbon dioxide molecules to form a new molecule—carbonic acid. The binding between these two molecules is modest but it's enough to make it easy for carbon dioxide to dissolve in water.
351. Depressing the Melting Point of Ice with Salt or Sugar
Description: A beaker of melting ice initially has a temperature of 0° C. When salt or sugar is added to the ice, the temperature drops well below 0° C.
Purpose: To show that adding a water-soluble solid to ice depresses its melting temperature.
Supplies:
1 beaker
ice
salt or sugar
1 thermometer
1 support for the thermometer
1 spoon
Procedure: Fill the beaker with ice and carefully insert the thermometer in it. Support the thermometer so that it doesn't touch the walls of the beaker. After a few seconds, the thermometer will read 0° C. Now remove the thermometer and add a large spoonful of salt or sugar to the ice. Stir the mixture and reinsert the thermometer. After a few seconds the thermometer will read below 0° C.
Explanation: Adding a water soluble solid to ice destabilizes the solid phase at 0° C. The ice begins to melt to form salty or sugary water at 0° C, but this melting requires heat. The ice that does melt extracts heat from the ice that doesn't melt and the remaining ice becomes colder and colder. Soon the entire mixture, including the salty or sugary water, is at a temperature well below 0° C. The addition of the salt or sugar has caused more of the ice to become water and, because melting the ice has used some of the mixture's thermal energy, the mixture is now colder than it was before.
352. Raising the Boiling Point of Water with Salt or Sugar
Description: A beaker of boiling water initially has a temperature of 100° C. When salt or sugar is added to the water, the temperature rises well above 100° C.
Purpose: To show that adding a water-soluble solid to water raises its boiling temperature.
Supplies:
1 beaker
water
salt or sugar
1 thermometer
1 support for the thermometer
1 spoon
1 support for the beaker
1 gas burner
matches
Procedure: Place the beaker on the support and fill it with water. Carefully insert the thermometer in it and support the thermometer so that it doesn't touch the walls of the beaker. Light the burner and gently heat the beaker. Be careful not to heat the beaker too quickly or it may break. Soon the water will boil and the thermometer will read about 100° C. Now add a large spoonful of salt or sugar to the boiling water. Stir the mixture. When the mixture again begins to boil, the thermometer will read well above 100° C.
Explanation: Adding a water soluble solid to water interferes with its ability to evaporate. With many of the water molecules involved in stabilizing the dissolved solid, there are fewer water molecules evaporating at any given temperature. The water temperature must exceed 100° C before evaporation is fast enough for evaporation bubbles to become stable within the body of the water so that boiling can occur.
353. Regelation of Ice
Description: A heavily weighted wire is draped over a melting ice cube. The wire slowly descends into the ice cube, leaving a healed scare of solid ice above it.
Purpose: To show that pressure depresses ice's melting temperature.
Supplies:
1 large ice cube (frozen in a rectangular muffin tin)
1 board to support the ice cube
1 clamp
1 piece of piano wire
1 heavy weight
Procedure: Clamp the support board to a sturdy table so that it extends out over the floor. Place the ice cube on the support. Tie loops at the two ends of the piano wire, drape the wire over the ice cube, and hang the heavy weight from the two loops so that the wire is pulled tightly against the ice cube. When the ice cube warms to 0° C and begins to melt, the wire will begin to cut into the ice cube and will soon disappear below its surface. The ice will reform above it, so that the wire will soon be trapped in solid ice.
Explanation: This whole process takes place while the ice cube is at almost exactly 0° C. The elevated pressure below the piano wire depresses the ice's melting temperature so that water's liquid phase is more stable below the wire than is water's solid phase. The ice there melts and the wire descends into the liquid water. Relieved of the pressure, the water returns to its solid phase. Ice thus melts below the wire and reforms above the wire. In fact, there is a continual heat transfer from the freezing water above the wire to the melting ice below the wire. In this manner, the wire drifts right through the solid ice cube.
Section 16.1 Knives and Steel
You might want to repeat the breaking a penny demonstration from Section 6.1 to show how reduced temperature prevents dislocations from moving and makes some metals hard and brittle.
354. Hardening and Annealing a Steel Nail
Description: You try to bend a hardened steel nail and it breaks. You take an identical nail, heat it red hot, and let it cool slowly. It then bends rather than breaking. You straighten this nail and reheat it. However, this time you plunge the red hot nail into water to harden it. Now it breaks rather than bending.
Purpose: To show how heat treatment hardens carbon steels.
Supplies:
2 high carbon nails (masonry nails—we have found a supply of flat-sided masonry nails that work very well. They are very hard and very brittle. That's what you want.)
1 propane torch
matches
1 container of water
1 vise
1 pliers
1 tongs
safety glasses
Procedure: Clamp one of the nails in the vise and try to bend it with the pliers. Instead of bending, it will break. Now take the second nail in the tongs and heat it red hot with the torch. Allow it to cool gradually until it's at room temperature (a minute or two). Now clamp it in the vise and try to bend it with the pliers. It will bend without breaking. Straighten it back out and hold it in the tongs. Reheat it red hot and this time plunge it into the water. This rapid cooling will harden the steel. When you return it to the vise and try to bend it, it will break.
Explanation: The steel is hard because it contains tiny particles of hard cementite (iron carbide) scattered throughout its ferrite crystals (iron). When you try to bend this hard steel, the cementite particles keep the ferrite crystals from undergoing plastic deformation (slip) and the nail breaks. But when you heat and slowly cool the steel, the carbon that dissolves in the hot steel has time to migrate out of the ferrite crystals. The steel is then a soft mixture of large ferrite crystals and a few large cementite crystals. It bends easily because the ferrite crystals have no tiny cementite particles in them to prevent them from undergoing plastic deformation.
355. Different Steels
Description: You compare the properties of several different steel alloys.
Purpose: To show how small compositional changes and changes in processing can have substantial effects on the characteristics of steels.
Supplies:
1 piece of low-carbon steel (common steel)
1 piece of high-carbon steel (tool steel)
1 piece of 18-8 stainless steel
1 magnet
1 plastic container of hydrochloric acid
Procedure: Discuss the compositional differences between the steels. Show that the high-carbon steel can cut the low-carbon steel because the former is much harder than the latter. Show that both the carbon steels are magnetic while the stainless steel is not. Show that the carbon steels react with hydrochloric acid while the stainless steel does not.
Explanation: The high-carbon steel is harder than the low-carbon steel because it contains a large proportion of iron carbide particles (and perhaps other slip-inhibiting inclusions). The stainless steel is chemically inert because of its high content of chromium and nickel atoms.
Section 16.2 Windows and Glass
356. Melting Glass - Quartz vs. Soda-Lime Glass
Description: You try to melt quartz glass tubing unsuccessfully while soda-lime glass tubing melts easily.
Purpose: To show that the addition of soda and lime to quartz glass dramatically reduces its melting and softening temperatures.
Supplies:
1 piece of quartz glass (or Vycor glass) tubing or rod
1 piece of soda-lime glass tubing or rod
1 gas burner or propane torch
matches
Procedure: Try to melt the quartz glass tube with the burner or torch. You will be unable to do so. Now try to melt the soda-lime glass tube. It will melt and flow easily.
Explanation: Adding the soda and lime to the quartz makes it much easier to work with. The sodium ions terminate the covalent networks that are the basis for quartz glasses and weaken those networks. As a result, soda-lime glasses are softer and melt more easily than pure quartz glass. In fact, soda-lime glasses are eutectics—they melt at temperatures below the melting points of the chemicals from which they are made.
357. Thermal Shock and Glass
Description: You show that heating soda-lime glass rapidly causes it to crack from the stresses of uneven thermal expansion. Borosilicate glass doesn't suffer such problems. Quartz glass can handle rapid heating well, too. Upon rapid cooling in cold water, even the borosilicate glass may break. But quartz glass is still unaffected.
Purpose: To show that thermal expansion and contraction can cause glasses to tear apart during uneven heating and cooling.
Supplies:
1 glass slide (soda-lime glass)
1 pyrex tube or dish
1 quartz glass tube (or Vycor glass)
1 propane torch
matches
1 container of water
safety glasses
Procedure: Heat the glass slide rapidly with the torch. It will crack or shatter. Now heat the pyrex tube or dish. Unless you heat it particularly quickly in one spot, it should survive. Now heat the quartz tube. You can't damage it with heat.
Next reheat the pyrex tube or dish and plunge it into cold water. It will almost certainly crack or shatter. Try the same with the quartz tube. It will survive without injury.
Explanation: Soda-lime glass is soft and has a large coefficient of volume expansion. When you heat part of it rapidly, the heated part expands. The heated and unheated parts of the glass exert tremendous stresses on one another and they tear the weak glass apart. Borosilicate glasses are still structurally weak, but they have much smaller coefficients of volume expansion. The heated and unheated parts are less able to tear one another apart. However, very rapid temperature changes (as occur when hot glass is plunged into water) still cause the glass to tear itself apart. Quartz glass is so strong and has such a small coefficient of volume expansion that it's very hard to injure with thermal shock.
358. The Disappearing Glass Container
Description: You pour salad oil into a clear container that has a Pyrex or Kimax item inside it. The item appears to vanish.
Purpose: To show that there are no reflections when light moves between two materials with the same index of refraction.
Supplies:
1 bottle of salad oil (Wesson works well)
1 Pyrex or Kimax flask or beaker
1 clear container
Procedure: Put the flask or beaker in the container and observe that it's plainly visible. Now pour the salad oil into the container and into the flask or beaker. The flask or beaker will become essentially invisible.
Explanation: The indices of refraction of the salad oil and borosilicate glasses are almost identical. With no change of speed upon entry or exit from the flask or beaker, light doesn't refract or reflect, and you can't tell that the flask or beaker is there.
359. Tempered Glass - A Bologna Bottle
Description: You use a peculiar glass bottle to pound in a nail. You then drop a tiny chip of sharp crystal into the bottle and it falls apart.
Purpose: To show that the surface stresses experienced by glass determine its resistance to tearing and breakage.
Supplies:
1 bologna bottle (available from a scientific supply company, at non-negligible expense. Sargent-Welch charged \$41 for them recently. Still, they are remarkable.)
1 piece of wood
1 nail with a large head (just to be safe)
safety glasses
Procedure: Hold the neck of the bologna bottle and tap the nail into the wood with the side of the round bottle. Having demonstrated that the outside of the bottle is extremely tough, hold the bottle upright over a garbage can and drop the crystal chip that came with the bottle into the neck of the bottle. When this chip hits the inside bottom of the bottle, the bottle will tear itself apart and its pieces will drop into the garbage can.
Explanation: The bottle is tempered in such a way that the outside surface is experiencing compression and the inside surface is experiencing tensile stress. Since it's very hard to start a tear in a layer that is being compressed, it's hard to tear the outside of the bologna bottle. But the inside is under tension and the slightest injury to it will cause the surface to tear itself to shreds.
360. Tempered Glass - Rupert Drops
Description: When you break the tail of a small glass drop, the drop crumbles into dust.
Purpose: To show that tempered glass exhibits dicing fracture when its compressed outer skin is broken.
Supplies:
2 or 3 Rupert drops (available from a scientific supply company)
1 needle-nosed pliers
cloth gloves
safety glasses
Procedure: Hold a Rupert drop in your gloved hand and break off its tail with the pliers. If the drop has been properly tempered (I've had mixed luck), it will tear itself to powder. You may have to try more than one to observe this self-destruction.
Explanation: The Rupert drops are tempered glass—their outer surfaces are under compression while their insides are under tension. When you break through the compressed surface layer and expose the tense inner portion of the drop, it tears itself apart.
361. Glass Fibers
Description: You heat the middle of a glass rod until it softens and then pull its ends away from one another. A glass fiber forms in between the ends. This fiber is relatively flexible and extremely strong for its size.
Purpose: To show how glass fibers are formed.
Supplies:
1 glass rod
1 gas burner
matches
safety glasses
Procedure: Light the burner and hold the middle of the glass rod over the flame. When the glass has softened significantly, pull the two ends of the rod away from one another in a smooth and steady motion. Stop when you have stretched the rod to about 1 m long. Allow the pieces to cool briefly. Show that the glass fiber is flexible (don't bend it too far or it will break!). Be careful with the hot ends of the glass until they've had enough time to cool completely. Be careful with eyes.
Explanation: The glass fiber's strength comes in part because of its relative lack of defects on its surface. With so little surface on any given length of fiber, there are only a couple of sites for a tear to begin as you bend the fiber.
Section 16.3 Plastics
Many of the demonstrations listed in this section are standard experiments done by students of organic chemistry. You may be able to obtain the materials for these experiments from your local chemistry department already prepared and ready to go.
362. Natural Polymers
Description: You display several natural polymers.
Purpose: To show that polymers (plastics) are common in nature.
Supplies:
1 sheet of paper (cellulose)
1 rubber band (rubber)
1 piece of wool
1 piece of silk
1 box of cornstarch
Procedure: Simply point out that each of these materials consists of extremely long molecules that are used to give structure and function to biological systems.
Explanation: Cellulose and starch are both sugar polymers. Rubber is a polymer of isoprene monomers. Wool and silk are both protein polymers.
363. Cellulose Derivatives
Description: You show that a piece of nitrocellulose (celluloid) is quite clear and tough, but that it burns nicely. A piece of cellulose acetate (acetate plastic) is much more practical.
Purpose: To show some of the early synthetic plastics.
Supplies:
1 piece of clear nitrocellulose sheet (can be made by allowing collodion to dry on a sheet of shiny aluminum foil. Because the ether solvent in collodion is dangerously flammable, you should only do this drying in a fume hood or outdoors. Be careful!)
1 piece of cellulose acetate plastic
tongs
matches
water (in case of fire)
Procedure: Show that the nitrocellulose (celluloid) sheet is clear and flexible. But then hold it in the tongs and light it with a match. The nitrocellulose will burn rapidly and leave no ash. Note that relatively non-flammable cellulose acetate replaced nitrocellulose.
Explanation: Both nitrocellulose and cellulose acetate can be reshaped in ways that cellulose itself cannot. However, nitrocellulose is extremely flammable (in its highly nitrated form it's a high explosive and the principle component of smokeless powder), so cellulose acetate is a safer choice. It also ages less and is less susceptible to light damage.
364. Reptation in Wet Cornstarch
Description: A mixture of cornstarch and water appears liquid-like when you stir it slowly but feels hard when you poke it suddenly or try to throw it abruptly out of its container.
Purpose: To show that the long molecules of cornstarch moves slowly past one another (reptation) in a solution. If you try to deform the solution quickly, the cornstarch molecules won't permit it to flow. Only if you're patient will it behave as a liquid.
Supplies:
2 plastic cups
1 stirring stick
cornstarch
water
Procedure: Half fill one of the cups with cornstarch and gradually add water to it, stirring carefully with each addition. After you have added a modest amount of water, the entire powder will be wet and it will begin to flow as you stir slowly. (Don't add too much water—be patient and stir carefully.) When the whole mixture behaves like a very thick liquid when you stir slowly, it's ready.
First show that you can pour the "liquid" from one cup to the other. You'll see that it doesn't quite pour normally…it tends to crack as it pours. Next show that if you poke it quickly with your finger, it feels hard and doesn't get your finger wet. Finally, hold the cup by its bottom and try to throw its contents at someone. The mixture will remain in the cup as long as your motion is very rapid.
Explanation: The cornstarch mixture flows slowly because it must wait for the long molecules to disentangle themselves in order to change its shape. This disentanglement is done through reptation of the molecules and depends on their thermal energies and thermal motions. When presented with large, sudden stresses, the mixture resists deformation. But with time, it flows to relieve those stress.
365. Glue Putty
Description: You mix white glue, water, and borax to create a soft putty that flows slowly like a liquid but that tears when exposed to sudden large stresses.
Purpose: To show that the long molecules in glue take time to disconnect from one another and to disentangle themselves. With patience, the material will flow but when stressed suddenly, it tears.
Supplies:
1 large mixing container
1 smaller container
1 measuring cup
1 stirring stick
white glue
borax
water
Procedure: In the large container, mix about 125 ml of glue and 125 ml of water. In the small container, dissolve 5 ml of borax powder in about 125 ml of water. Slowly add the borax solution to the glue, stirring as you do. The glue will congeal into a blob of glue putty. If you knead this material carefully and add the right amount of the borax solution, it will be soft and relatively non-sticky. Show that with time the putty will drip from your hands or flatten itself into a puddle, but that if you pull on it suddenly, it will tear into pieces. Is this material solid or liquid? How does time enter into the answer to that question?
Explanation: The borax molecules form hydrogen bonded bridges between the long molecules of the glue and effectively tie the whole mass of molecules together into one big molecule—like weak vulcanization. The water molecules that originally plasticized the glue are caught up in this network of molecules. Because the hydrogen bonds are relatively easy to break, the mass can rearrange and flow if you wait.
366. Slime
Description: You mix solutions of poly(vinyl alcohol) and sodium borate together to get a gooey glob of slimy plastic. Like glue putty, this material flows slowly like a liquid but tears like a solid when exposed to sudden large stresses.
Purpose: To show that the long molecules in poly(vinyl alcohol) take time to disconnect from one another and to disentangle themselves. With patience, the material will flow but when stressed suddenly, it tears.
Supplies:
poly(vinyl alcohol) (a white powdery substance available from a chemical supply company)
sodium borate
water
1 heated magnetic stirrer
several containers
1 stirring stick
Procedure: Dissolve 4 grams of poly(vinyl alcohol) in 100 ml of water (this recipe can be scaled up). You will have to heat the water to about 70° C and stir it with a magnetic stirrer for an hour or two. You may want to filter the resulting solution through a strainer because some of the material just won't dissolve, no matter how long you wait. In a second container, dissolve 4 grams of sodium borate in 100 ml of water.
To form the slime, slowly stir some of the sodium borate solution into the poly(vinyl alcohol) solution—about 5 to 10 ml of the sodium borate solution will be enough. The mixture will form a gooey elastic material, commonly called "slime."
Explanation: The borate ions in the sodium borate solution form hydrogen bonded bridges between the long poly(vinyl alcohol) molecules—like weak vulcanization. A vast network of molecules forms and the water is caught up on that network. Because the hydrogen bonds are relatively easy to break, the mass can rearrange and flow if you wait.
367. Making Plexiglas
Description: You add a tiny amount of catalyst to a test tube of methyl methacrylate and heat it to about 90° C. About 20 minutes later, the test tube is full of solid poly(methyl methacrylate) or Plexiglas.
Purpose: To demonstrate a common polymerization process.
Supplies:
methyl methacrylate (an irritating chemical that makes your eyes tear. Use only in good ventilation.)
benzoyl peroxide (a contact explosive—never keep more than a tiny bit around and never let it come in contact with metals. Use only ceramic or glass containers or scoops. This stuff is potentially bad news. It's used frequently in chemistry departments for this very reaction. It's also used in acne medications.)
1 large test tube
1 glass stirring rod
1 hot water bath (at about 90° C—don't let it boil because the methyl methacrylate will also boil and pop out of the test tube…I spoiled a good jacket with this stuff several years back)
safety glasses
Procedure: Half-fill the test tube with methyl methacrylate and add a pea-sized amount of benzoyl peroxide (which acts as a catalyst for the polymerization). Stir. Place the test tube in the hot water bath and cook the mixture for about 20 minutes at about 90° C. The test tube will then contain a nearly solid, clear mass that will harden completely when you allow it to cool. You have made a glassy plastic called poly(methyl methacrylate), Plexiglas, or Lucite.
Explanation: The benzoyl peroxide forms free radicals that initiate the polymerization of the methyl methacrylate molecules. With the help of thermal energy, the monomers are consumed and long molecules are formed.
368. Epoxy
Description: You mix two liquids and stir them together. About 5 minutes later, you have a solid material.
Purpose: To demonstrate that polymerizations are common in high performance adhesives. (Students are remarkably unaware of any glues besides superglue and white glue. They don't understand that superglue polymerizes and doesn't simply dry the way white glue does.)
Supplies:
1 single-use pouch of 5 minute epoxy
1 stirring stick
1 piece of cardboard
Procedure: Tear off the end of the 5 minute epoxy pouch and squeeze both liquids onto the cardboard. Stir the mixture until it's uniform and leave the stick in it. About 5 minutes later, it will be completely hard. Discuss the fact that the glue has not "dried," that it has polymerized into a clear plastic. All of the atoms that were in the package are still present, but the molecules have joined together into giant chains that are no longer mobile. The plastic is in the glassy regime.
Explanation: During polymerization the epoxy rings that are present in the resin molecules open and link together, forming long chain molecules that are a glassy solid at room temperature.
369. Superglue
Description: You squeeze a few drops of superglue (cyanoacrylate monomer) onto a smooth metal surface and press a second smooth metal surface against it. About 1 minute later, it's difficult to separate those surfaces—they are joined by long polymer molecules.
Purpose: To demonstrate a polymerization that proceeds simply in the presence of moisture.
Supplies:
2 pieces of smooth, flat metal
1 tube of cyanoacrylate glue (superglue)
Procedure: Squeeze a few drops of the glue onto one of the metal pieces and press the second metal piece on top. Rub the pieces against one another to distribute the glue. Leave them pressed against one another for about a minute and then show that they have bonded together.
Explanation: The cyanoacrylate monomer in this polymerization is quite similar to the methyl methacrylate monomer used to form Plexiglas. However, cyanoacrylate will polymerize just in the presence of moisture. Since moisture is everywhere, all you need to do is squeeze it out onto a surface and it will begin to polymerize. Like Plexiglas, this cyanoacrylate plastic is glassy at room temperature.
370. Plastics Fail by Tearing - Piercing a Balloon
Description: To show that plastics fail when a tear propagates through them, you carefully insert a sharpened knitting needle all the way through a balloon. You carefully work the needle between the molecules of rubber, so that you don't start a tear, and the needle don't pop the balloon.
Purpose: To show that polymers break by tearing.
Supplies:
1 good-quality latex rubber balloon
1 sharpened knitting needle (or another needle-sharp thin rod with smooth polished edges)
oil
Procedure: Inflate the balloon and tie it off. Place a drop of oil near the nipple portion of the balloon (where the stresses on its surface aren't as high as elsewhere and the rubber is relatively thick). Carefully insert the needle through the oil drop and into the rubber. Twisting the needle helps it find its way between the rubber molecules. Once you have the needle inside the balloon, aim it at the bump at the other end (another region of relatively low stress). Carefully push the needle through that area of the balloon so that it comes out the other side. You will then have a balloon with a knitting needle passing all the way through it.
Explanation: As long as you don't start a tear, the rubber will tolerate the insertion of the needle between its molecules.
371. Nylon
Description: You pour a solution of adipic chloride in cylcohexane onto a solution of 1,6-hexanediamine in water. A film forms at the interface between the two and you catch this film with a copper wire. You then pull out the film as a long, continuous piece of Nylon–6,6.
Purpose: To show how nylon is made from two monomers that form a copolymer.
Supplies:
5% solution of 1,6-hexanediamine in water
20% sodium hydroxide in water
1 clean 100 ml beaker
1 clean copper wire
rubber gloves
safety glasses
Procedure: Put about 20 ml of the 1,6-hexanediamine solution in the beaker and add about 20 drops of the sodium hydroxide solution. Now carefully pour about 20 ml of the adipic chloride solution down the inside of the beaker so that it floats neatly on top of the other liquid. A film of nylon will appear at the interface between the two layers. Bend the copper wire into a hook and lift that film out of the liquid. You'll be able to pull out a continuous strand of nylon almost indefinitely. Having fresh solutions helps because pure chemicals gives the longest and strongest nylon molecules.
Explanation: 1,6-hexanediamine is a two-ended base while adipic chloride is essentially a two-ended acid. The base and acid ends join in a nearly endless chain molecule when the two chemical are brought together.
372. Polyurethane Foam
Description: You mix two liquids together in a cup and then wait. In a few moments, a dark foam rises up in the cup and pours over its sides. In a minute, you have a hard mushroom of polyurethane foam.
Purpose: To show how polyurethane foam is made.
Supplies:
polyurethane foam kit (two chemicals that mix to form polyurethane foam—from a hardware or hobby store)
1 paper or plastic cup
1 stirring stick
Procedure: Pour equal quantities of the two chemicals into the paper cup (or follow the directions on the kit). The paper cup should be about ¼ full when you're done. Stir. In about 20 seconds, the mixture will foam up and begin to overflow the cup. A minute later, it will be hard to the touch.
Explanation: During its polymerization reaction, the chemicals release carbon dioxide gas. This gas inflates the hardening plastic and turns it into a foam. Polyurethane is glassy at room temperature, giving the foam a firm character.
373. High-Strength Polymers
Description: You step into a loop at the end of normal plastic rope that's attached to the ceiling and the rope stretches considerably as it begins to support your weight. You then step into a loop at the end of a high-strength rope that's also attached to the ceiling. It doesn't stretch noticeably.
Purpose: To show how straight-chain polymers (either liquid crystal materials like Kevlar or artificially oriented materials like Spectra polyethylene) have enormous tensile strengths and barely stretch at all.
Supplies:
1 polypropylene rope (about a quarter inch diameter)
1 Spectra or Kevlar rope of the same diameter
Procedure: Suspend both ropes from the ceiling and tie loops in them near the ground. When you step into the loop of the polypropylene rope, the rope will stretch considerably. But when you step in the loop of the Spectra or Kevlar rope, it won't stretch. You'll feel like you just stepped onto a steel cable. You can bounce all you like, but nothing will happen.
Explanation: The molecules in Spectra and Kevlar are all aligned straight so they all work together to support your weight. Moreover, because they are already aligned straight, the rope can't stretch without actually stretching or breaking the molecules. Normal ropes stretch because the molecules are bent or coiled and they can unwind to give the rope some additional length.