How Things Work - Chapter 7 Demonstrations
Section 7.1 Woodstoves
Demonstration 7.1.1: Burning a Candle to Create Thermal Energy from Chemical Potential Energy
Description: A burning candle creates thermal energy before it's extinguished.
Purpose: To show that a chemical reaction between wax molecules and oxygen molecules converts stored energy (chemical potential energy) into thermal energy.
1 short candle
1 base for candle
1 large glass jar or beaker that's tall enough to smoother the candle without approaching the candle flame too closely.
Procedure: Mount the candle on the base and light the candle with the match. Discuss how the chemical reactions between the wax molecules and the oxygen molecules in the air are converting their stored energy (chemical potential energy) into thermal energy. Discuss the need for the initial heat (the lighted match) to provide the activation energy that weakens the chemical bonds in the starting materials so that the reactions can proceed. Point out that the thermal energy that this system can provide is limited to the stored chemical potential energy and that when either the candle or the oxygen runs out, the production of thermal energy will cease. Then smoother the candle by placing the inverted jar or beaker over it so that no new oxygen can get to it.
Explanation: With the aid of the heat from the match, the bonds between the hydrogen and carbon atoms in the wax molecules begin to weaken and they become attracted to the oxygen atoms of passing oxygen molecules. New bonds form between the hydrogen atoms and oxygen atoms, and between the carbon atoms and oxygen atoms. Water molecules and carbon dioxide molecules are created. Since the chemical potential energies of these new molecules are lower than those of the starting molecules, some chemical potential energy has been converted into thermal energy.
Follow-up: Discuss how sliding friction provides the activation energy needed to start the chemical reactions in the match that you used to light the candle.
Demonstration 7.1.2: Plunging a Hot Metal Rod into Water to Show That Heat Flows from Hot to Cold
Description: A red hot metal rod is immersed in cold water and heat flows from the hot rod to the colder water. A room temperature metal rod is immersed in liquid nitrogen and heat flows from the warm rod to the very cold liquid nitrogen.
Purpose: To illustrate that heat naturally flows from a hotter object to a colder object.
1 metal rod
1 container of water
1 container of liquid nitrogen
1 gas burner
Procedure: Start the burner and heat the end of the metal rod until it glows red hot. Now immerse it in the cold water. Discuss the fact that heat flowed from the hotter rod to the colder water, not the other way around (what would have happened if it had gone the other way?). Point out that the total amount of thermal energy in the system remained constant (neglecting fine details like the formation of steam). Now immerse the cool metal rod in liquid nitrogen. Again heat flows from the hotter rod to the colder liquid nitrogen.
Explanation: While the flow of heat from a colder object to a hotter object wouldn't violate any of the basic laws of mechanics, it's simply not observed. Such a reverse flow of heat is so unlikely that it never happens. It's as unlikely as having a vase, shattered in a fall to the floor, reconstruct itself during a subsequent fall.
Demonstration 7.1.3: Pouring Liquid Nitrogen into Water to Show that Heat Flows from Hot to Cold
Description: Liquid nitrogen is poured into a cup of water and a cloud of mist appears.
Purpose: To show that heat flows from hotter objects to colder objects.
1 Styrofoam cup, half full of water
Procedure: Pour a modest amount of liquid nitrogen onto the water in the cup and observe the mist flowing out over the edges of the cup. Discuss how heat is flowing from the warmer water to the colder liquid nitrogen and how this is causing the nitrogen to boil violently and atomize the droplets of water. With time, the cold liquid nitrogen will all turn to warmer nitrogen gas and some of the warmer water will turn to colder ice. You can also discuss why the liquid nitrogen floats on the water (its density is much lower than that of water) and why the misty flow of cold nitrogen gas flows downward around the sides of the cup (its density is much higher than that of room temperature air).
Explanation: The boiling nitrogen breaks the water into tiny droplets that float around in the air as mist. Because the mist is chilled by the liquid nitrogen, the water droplets don't evaporate and the mist flows over the edges of the cup.
Demonstration 7.1.4: Freezing Objects in Liquid Nitrogen
Description: Various objects are immersed in liquid nitrogen and become hard and fragile.
Purpose: To show that heat flows from hotter objects to colder objects and that the materials properties of common objects can change substantially when they are taken to extreme temperatures.
1 container for liquid nitrogen (a dewar or a Styrofoam container. We use a wonderfully made wide, shallow dewar that was made for us by William Shoup of the University of Virginia glass shop—(804) 924-3967)
1 rubber racket ball
1 nail with a large head
1 piece of wood
1 lead bell (a bell made out of lead)
Procedure: Pour liquid nitrogen into the container and allow it to stop boiling violently. Then immerse each of the objects into the liquid nitrogen, one at a time, and allow them to freeze. The flower will freeze almost instantly and will become as brittle as glass. If you hold a microphone near it as you strike it on the table, you will hear it shatter as though it were made of paper-thin glass. The racket ball will become extremely hard and will sound like a rigid plastic ball when you bounce it gently on the table. If you throw it against a solid floor, it will shatter. However, be careful of the flying fragments because they're as hard and sharp as pieces from a broken bottle. Freezing the banana will take several minutes. Don't let it freeze too long, or it will shatter spontaneously because of the internal stresses its experiences during the freezing process. Use the frozen banana to pound the nail into wood. While an over-frozen banana will tend to break and the nail may punch holes in the surface of an under-frozen banana, the banana hammer is still impressive. The lead bell will emit a dull thud when warm, but will tinkle brightly when chilled to liquid nitrogen temperature.
Explanation: Heat flows out of room temperature objects when they're immersed in liquid nitrogen. While objects that are already solid at room temperature change relatively little when they're cooled to liquid nitrogen temperature (77° K, -195° C, or –319° F), objects that have relatively mobile molecules (liquids, gases, and elastic materials) change dramatically. The lead bell is a particularly interesting case: chilling it doesn't cause a change in phase (it remains a solid). However, near room temperature defects in the lead known as dislocations are extremely mobile and they move about through the lead crystals and allow those crystals to deform easily -- the warm lead is extremely soft and pliable, so the lead bell dents instead of chiming. But dislocations are immobile in the cold lead and the lead becomes hard and brittle. The lead bell chimes when struck.
Demonstration 7.1.5: Breaking a Frozen Penny
Description: A recent United States Penny (1983 or later) is cooled in liquid nitrogen, placed on a hard surface, and struck with a hammer. It shatters into fragments.
Purpose: To show that cooling some metals makes them brittle.
1 recent penny (1983 or later, because they are mostly zinc with a thin copper coating. Very recent pennies seem to have the thinnest copper coatings and probably work best.)
1 anvil or another hard, sturdy surface
Procedure: First place the penny on the hard surface and hit it with the hammer. It may dent, but it will not break. Then chill it to liquid nitrogen temperature and repeat the experiment. It will shatter into pieces. This demonstration works best if you support one edge of the penny with the tongs while you hit the penny with the hammer. That way, the hammer will bend the penny rather than simply compressing it. The cold penny will break rather than bend and will crumble into small fragments.
Explanation: Even some solids become more brittle when they are cooled to very low temperatures.
Demonstration 7.1.6: Making Ice Cream with Liquid Nitrogen
Description: You first mix cream, milk, sugar, and vanilla in a large metal bowl and them begin stirring in liquid nitrogen. In about 5 minutes, you have a bowl of ice cream.
Purpose: To show that heat flows from a hotter object to a colder object (and to make dessert in a hurry).
4 liters of cream
750 grams (1.5 pounds) of sugar (roughly)
15 ml (1 tbsp.) of vanilla
1 very large, shallow metal mixing bowl (the bigger, the better!)
1 metal mixing spoon
liquid nitrogen (at least 4 liters, perhaps more)
Procedure: Combine the cream, sugar, and vanilla in the mixing bowl and stir thoroughly. Slowly add the liquid nitrogen to the mixture and stir. Don't add to much liquid nitrogen or stir too quickly at first, or the boiling mixture will overflow the bowl. Keep adding liquid nitrogen, about 0.5 liters at a time, and then stir until it has mostly boiled away. By the time you have added about 4 liters of liquid nitrogen, the mixture should have thickened into ice cream. You can use milk in place of some of the cream, but the freezing process becomes trickier: the milk-cream mixture takes longer to freeze and it tends to bubble over. Using pure cream makes the freezing particularly easy and mess-free.
Explanation: Heat flows from the warmer ice cream mixture to the much colder liquid nitrogen. Most of the gaseous nitrogen escapes, but some is trapped in the ice cream and improves its taste.
Demonstration 7.1.7: Thermal Conductivities of Metals
Description: A thick metal disk has three metal spokes extending from it at equally spaced angles. One spoke is aluminum, one is copper, and the third is stainless steel. Metal marbles hang by wax from these spokes at equal distances from the central disk. When the disk is heated by a gas burner, the spokes begin to warm up and the marbles begin to drop. They leave the copper spoke first, then the aluminum spoke, and lastly the stainless steel spoke.
Purpose: To show that different metals have different thermal conductivities.
1 thick metal disk (copper, aluminum, or brass)
3 metal spokes that fit tightly into the metal disk (one copper, one aluminum, and one stainless steel). They should all have equal dimensions.
1 ring stand for the disk and spoke assembly
12 metal marbles
hard wax (sealing wax)
1 gas burner
Procedure: Assemble the spokes on the metal disk so that they all are in good thermal contact with the disk. Screw-in attachment is ideal, if you can thread the metal components. Now mark each metal spoke at 4 evenly space intervals and use the wax (melting it with a match or burner) to attach a metal marble at each mark on the three spokes. Hot glue (glue gun glue) may work instead of the wax—I haven't tried it. Now suspend the disk and spoke assembly on the ring stand. When ready, place the burner under the central disk and ignite it. As the disk warms up, heat will begin to flow out the three metal spokes and will eventually melt the wax. Since copper is the best conductor of heat, the marbles will begin to fall from the copper spoke first. Aluminum will be next, followed by stainless steel. Point out that copper is also the best electric conductor, followed by aluminum, followed by stainless steel.
Explanation: The relationship between thermal conductivity and electric conductivity isn't a coincidence. The mobile electrons in these metals dominate their thermal conductivities.
Demonstration 7.1.8: Holding Red Hot Thin-Walled Stainless Steel Tubing
Description: You hold one end of a piece of thin-walled stainless steel tubing in your hand and heat the other end red hot.
Purpose: To show that some metal objects have such poor thermal conductivities that two regions of very different temperatures can exist very near one another.
1 piece of thin-walled stainless steel tubing (about 30 cm of 1 cm tubing, with as thin a wall as you can find)
1 gas burner
Procedure: Ignite the burner and hold one end of the thin-walled tube in your hand. Hold the other end of the tube in the flame and allow it to begin glowing red hot. Be prepared to drop the tube if for some reason it becomes uncomfortably hot.
Explanation: Stainless steel is a poor conductor of both electricity and heat. When reduced to a thin cylindrical surface (a thin-walled tube), stainless steel loses heat so quickly to the air that even regions only a few centimeters from a burner remain relatively cool.
Simple Alternative: You can also hold one end of a piece of aluminum foil and heat the other end of the foil until it melts or burns. This works because the foil is so thin that air is able to carry away its heat before it reaches your fingers.
Demonstration 7.1.9: Visualizing Convection in Water
Description: Light is projected through a clear glass cell containing water and an electrically heated metal wire. When electricity heats the wire, swirls of rising water can be seen leaving the filament.
Purpose: To show how convection carries heat upward from a hot object.
1 glass or plastic box, or a miniature aquarium
1 nichrome wire or filament
2 heavy-gauge wires
1 powerful battery or power supply
1 slide projector or other bright light source
1 large converging lens with holder (about 5 cm in diameter and roughly 20 cm focal length)
2 large flat mirrors and supports (optional)
Procedure: Form a small coil from the nichrome wire. Attach the heavy wires to the nichrome wire and place the nichrome wire at the bottom of the glass or plastic box. Be sure that the nichrome itself doesn't touch the sides of the box (use the heavy-gauge wires to anchor the nichrome wire in place). Fill the box almost full of water. Direct the light from the slide projector through the water-filled box and place the converging lens on the far side of the box. Move that lens back and forth until it projects a clear image of the nichrome wire on the wall or a screen. This image will be inverted, so you should point this out to the observers. (If you add two mirrors to this set up, one at 45° to bend the horizontal light so that it travels straight upward and the other at 45° to bend the upward light so that it travels horizontally but in the reverse direction from its starting direction, you can cast an upright image onto the wall.) Now connect the wires to the battery and begin heating the nichrome filament. Swirls of hot water will appear in the image projected on the wall and will move toward the top of the box (they'll move downward in an inverted image).
Explanation: When the filament heats the water, that water becomes less dense than the surrounding water and it floats upward, lifted by the buoyant force.
Demonstration 7.1.10: Putting a Candle Out by Eliminating Convection
Description: A candle burning inside a jar extinguishes when you drop it.
Purpose: To demonstrate the importance of convection in sustaining a candle flame.
1 small candle
1 plastic jar that is large enough to hold the burning candle and let it burn for a considerable time
Procedure: Attach the candle to the lid of the jar (probably by melting the base of the candle onto the jar lid). Light the candle and seal the burning candle inside the jar. Now drop the jar and candle. As they fall, the candle will go out.
Explanation: The combustion process in the candle flame requires convection to lift burned gases away from the wick and to bring fresh oxygen in from below the wick. When you drop the jar, it and its contents experience weightlessness and it is as though there were no gravity and therefore no convection. Without anything to drive the burned gases away and bring in new oxygen, the flame goes out.
Demonstration 7.1.11: Boiling Water in a Hand-held Test Tube
Description: You hold the bottom of a water-filled test tube in your hand and heat the top of that test tube with a gas burner. The water at the top of the test tube begins to boil.
Purpose: To show that convection only works when the heat source is at the bottom of a fluid.
1 large Pyrex or Kimax test tube
1 gas burner
Procedure: Almost fill the test tube with water (don't overfill it, because boiling water will then pour down its outside surfaces and reach your hand). Ignite the gas burner. Hold the bottom of the test tube in your hand and tip the top of the test tube into the flame. After a few seconds, the water at the top of the test tube will begin to boil but the water near your hand will remain cool. (Keep an eye on the swirling water below the boiling area—the stirring effect of the bubbles is mixing the hot and cold regions together. Don't let the hot water drift downward to your hand.)
Explanation: Heating the water near the top of the test tube causes it to expand and become less dense. It floats easily atop the cooler, more dense water at the bottom of the test tube. Convection never starts and your hand remains cool.
Demonstration 7.1.12: Boiling Water in a Paper Cup
Description: A water-filled paper cup is carefully suspended above the flame of a gas burner. While the free edges of the cup soon burn away, most of the cup remains intact and the water in it eventually begins to boil.
Purpose: To show that water can be so effective at removing thermal energy from a thin paper surface that that surface won't burn even when exposed to an open flame.
1 wax-coated paper cup (a Dixie cup works well)
1 ring-shaped metal support for the cup (this ring must catch the cup uniformly around its middle, within the water-filled portion of the cup)
1 gas burner
Procedure: Partially fill the paper cup with water and insert it into the metal support. The cup must be filled at least to the level at which it's supported, otherwise the cup will burn near the support and will fall. Place the burner underneath the cup and ignite it. While the exposed edges of the cup will soon burn, the portions of the cup that are touching the water will do little more than scorch. The water will gradually warm up and will eventually boil.
Explanation: Although the burner transfers considerable heat to the paper surfaces of the cup, the paper surfaces quickly transfer that heat to the water. Since the temperatures of the paper surfaces never greatly exceed the boiling temperature of water, the paper doesn't burn and the water's temperature rises to its boiling point.
Demonstration 7.1.13: Cooking Matches and Marshmallows with Thermal Radiation
Description: A bright light bulb in the focus of a large parabolic reflector projects a dazzling beam of light across the room. A second reflector collects this light and concentrates it on various objects in its focus.
Purpose: To show that thermal radiation and light are the same things and that thermal radiation can carry heat from a hotter object to a colder object.
2 large parabolic metal reflectors, with supports
1 small, clear, high wattage light bulb (we use a projector bulb, but a 500 W halogen lamp bulb should also work—it's a little long, but very bright.)
1 power source for the light bulb
black spray paint
1 support for the match
1 stick for the marshmallows
Procedure: Carefully align the light bulb in the focus of the first reflector. When you have it properly aligned, the reflector should project an intense beam of light across the room. Align the second reflector so that it catches this beam of light and concentrates the light at its focus. Be careful not to injure your eyes or to burn yourself. Dimming the lamp makes alignment easier. Now turn off the light bulb, place the match in the support, and position the match head at the focus of the second reflector. When everything is well aligned, turn up on the lamp. The match will promptly ignite. Remove the match and its support and begin toasting the marshmallow in the focus of the second reflector. To speed the cooking, spray paint the second marshmallow black and place it in the focus of the second reflector. This black marshmallow will cook extremely quickly and may even ignite.
Explanation: The filament of the light bulb is the hottest object in the room and it transfers heat via radiation to everything around it. The reflectors simply improve the coupling between the filament and whatever is in the focus of the second reflector. Spray painting the marshmallow black increases its emissivity, making it better at both absorbing and emitting thermal radiation.
Demonstration 7.1.14: Using a Thermopile to "See" Thermal Radiation
Description: A thermopile is pointed at a number of objects to see which are emitting the most thermal radiation.
Purpose: To show that even room temperature objects emit thermal radiation.
1 thermopile, a device that detects the infrared radiation emitted by relatively low temperature objects
1 moderately heated cube with different surfaces (black, white, shiny, and dull metallic gray)
1 container of liquid nitrogen
Procedure: Point the thermopile at various surfaces to show that they emit different amounts of thermal radiation. The heated black surface will emit quite a bit while the colder, less black room will emit substantially less. Compare the emission by the black surface to that emitted by the shiny and gray surfaces (which are at the same temperature). The latter surfaces should emit less thermal radiation. Examine the thermal radiation from the white surface. In the apparatus I normally use, the white surface emits considerable thermal radiation, an indication that it's not "white" in the infrared. Examine the thermal radiation from your hand. Finally, examine the thermal radiation from the container of liquid nitrogen (don't look at the liquid nitrogen through any room temperature glass walls—you must point the thermopile directly at the liquid from above). There will be very, very little thermal radiation emerging from the liquid nitrogen.
Explanation: The thermal radiation emitted by a surface depends on that surface's temperature and on its emissivity (its ability to emit and absorb light; in short, its blackness).
Demonstration 7.1.15: Creating Thermal Energy from Electric Energy
Description: Current flowing through a wire causes that wire to glow red hot.
Purpose: To show that friction-like effects inside a wire can convert electric energy (a mixture of kinetic and potential energies in moving electric charges) into thermal energy.
1 large (car) battery or a high-current power supply
1 high-current switch
1 segment of nichrome wire (heating wire)
3 large-gauge wires
1 support base for the nichrome wire
Procedure: Connect one terminal of the battery to the switch, the switch to the nichrome wire (mounted on the base), and the nichrome wire to the other terminal of the battery. Make sure that the switch is open while you're working. Then close the switch and allow current to flow through the circuit. The nichrome wire should become extremely hot. Point out that electricity is flowing through the circuit in an endless loop and that the current of electric charges is obtaining energy from the battery and delivering that energy to the nichrome wire. Through friction-like processes, the electric charges in that current are converting their kinetic and potential energies into thermal energy and this thermal energy is causing the wire to become extremely hot.
Explanation: Collisions between the moving electric charges in the current and the atoms in the nichrome wire transfer energy from the charges to the atoms. The atoms become hotter, vibrating more and more vigorously with their increasing thermal energies.
Simple Alternative: Turn on an incandescent lamp.
Demonstration 7.1.16: Creating Thermal Energy from a Phase Transition
Description: You trigger the crystallization of the liquid in a plastic heat pack. The pack becomes warm as its clear liquid converts to a stiff, white solid.
Purpose: To show that still another form of potential energy (also a chemical potential energy) can become thermal energy.
1 heat pack (sodium acetate solution in a plastic envelope, with a metal trigger capsule)
Procedure: Show that the heat pack contains a clear liquid. Explain that this liquid is a supersaturated solution of a chemical that has difficulty crystallizing into a solid—its molecules can't find the proper arrangement for a crystal to begin growing. As a result, the chemical is trapped in its liquid phase despite being at a temperature well below its freezing temperature. Now trigger the crystallization by clinking the trigger capsule. White streams of crystallizing material will emerge from the trigger and spread throughout the heat pack. The heat pack will become hot. Discuss how the molecules release chemical potential energy as they stick to one another to form crystals. This released energy becomes thermal energy and you feel the heat pack become hotter. Explain that the heat pack can be reused by immersing it in boiling water and allowing all the crystals to melt. The boiling water's thermal energy then provides the chemical potential energy needed to separate the molecules and convert the material back into a liquid.
Explanation: The heat released when sodium acetate crystallizes from the supersaturated solution is similar to the heat released when water freezes into ice. In this case, the crystallization occurs well below the sodium acetate's freezing temperature, so that freezing is sudden and the large amount of thermal energy released heats the system up dramatically.
Demonstration 7.1.17: How Temperature Affects a Gas
Description: When a sealed sphere containing helium gas is heated or cooled, the pressure of the gas inside that sphere is seen to increase or decrease.
Purpose: To define temperature in terms of the average thermal kinetic energies of the particles in a material.
1 helium-filled sphere with a pressure gauge attached to it
1 gas burner or other heating device (a heat gun or hairdryer)
1 cold bath (liquid nitrogen or ice water)
Procedure: Observe the pressure of the gas inside the sphere while it's at room temperature. Point out that this pressure reflects the thermal motions of the helium atoms—they are hitting the surfaces of the pressure gauge and the pressure that gauge is reading depends on the number of particles hitting each second and on how hard they hit on average. Now heat the sphere up and show that the pressure inside the sphere increases. Because the helium atoms now have more thermal kinetic energy on average than they had at room temperature, they are moving faster, hitting the surfaces in the pressure gauge more often, and hitting those surfaces harder than before. Finally, cool the sphere by immersing it in liquid nitrogen (or ice water). The pressure inside the sphere will drop. Discuss how the average thermal kinetic energy of the atoms has decreased. Discuss how this average thermal kinetic energy can serve as the basis for a temperature scale and that there will be a bottom to this temperature scale—absolute zero—at which point the average thermal kinetic energies of the atoms is zero.
Explanation: Helium is almost an ideal gas—its atoms interact so weakly that they're almost perfectly independent. Virtually all of its thermal energy is in the form of thermal kinetic energy—the motion of its atoms. When you raise or lower its temperature, the average thermal kinetic energies of the atoms increase or decrease in proportion to the temperature change, with the zero of thermal kinetic energy corresponding to the zero of the absolute temperature (assuming that the helium didn't liquefy at about 4.5 K and stop behaving as an ideal gas).
Section 7.2 Water, Steam, and Ice
Demonstration 7.2.1: Transforming Ice into Water
Description: A thermometer embedded in an ice cube shows a rising temperature until the ice begins to melt, holds steady at 0 °C while the ice cube is melting, and then rises when there is only water left.
Purpose: To show that ice is stable below 0 °C, water is stable above 0 °C, and that the two can coexist at 0 °C.
1 ice cube with an electronic thermometer embedded in it
Procedure: The ice cube should start very cold, probably just out of the freezer. When you place it in a container, it will begin to warm up and the thermometer will read progressively higher temperatures. But once the cube begins to melt, which it will do at almost exactly 0 °C, the temperature will stop rising. Only when the ice has transformed into water (or the thermometer falls out of the remaining ice) will the temperature resume its rise.
Explanation: While there is only ice present, heat flowing into the ice causes its temperature to rise. But once the ice has warmed up to 0 °C, it begins to melt and its temperature stops rising. Added heat then goes into melting the ice rather than increasing the temperature. Finally, when there is no ice surrounding the thermometer, added heat again causes the temperature of the water to rise.
Demonstration 7.2.2: The Temperature of Ice Water
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.
1 Pyrex or Kimax beaker
water and ice mixture
1 support for the beaker
1 support for the thermometer
1 gas burner
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.
Demonstration 7.2.3: 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.
1 large ice cube (frozen in a rectangular muffin tin)
1 board to support the ice cube
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.
Demonstration 7.2.4: 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.
1 Pyrex or Kimax beaker
1 support for the beaker
1 gas burner
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.
Demonstration 7.2.5: 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.
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.
Demonstration 7.2.6: 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.
1 empty aluminum beverage can
1 ring stand
1 gas burner
1 cooking pan
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.
Demonstration 7.2.7: 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.
1 soda siphon
1 carbon dioxide cylinder
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.
Demonstration 7.2.8: 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.
salt or sugar
1 support for the thermometer
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.
Demonstration 7.2.9: 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.
salt or sugar
1 support for the thermometer
1 support for the beaker
1 gas burner
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.
Section 7.3 Incandescent Light Bulbs
Demonstration 7.3.1: Clear-But-Dim Incandescent Light Bulb
Description: You turn on a clear incandescent lightbulb and its filament lights up.
Purpose: To show that the source of the incandescent lightbulb's light is actually its electrically heated filament.
1 low-wattage, clear incandescent lightbulb
Procedure: Turn the lightbulb on and examine the source of light. Note that all of the visible light originates from the filament.
Explanation: The light emerging from an incandescent light bulb originates in its filament. The glass bulb serves only to protect the filament and to diffuse the light from the filament.
Demonstration 7.3.2: Opening an Incandescent Light Bulb
Description: You place an incandescent light bulb in a paper bag and tap it with a hammer until the glass envelope breaks. The filament and its supporting structure are then visible.
Purpose: To show the active structures inside the incandescent bulb, particularly the filament.
1 inexpensive incandescent light bulb
1 small, sturdy paper or cloth bag
1 magnifying glass or low-magnification microscope
Procedure: Insert the light bulb in the bag and close the bag to prevent glass from escaping. Place the wrapped bulb on a hard surface and tap it carefully with the hammer until the glass envelope breaks (squeezing it in a vise also works well). Carefully extract only the inner portion of the bulb and discard the broken envelope. Point out the filament and the two wires that carry current to and from it. Use a magnifying glass or microscope to study the double spiral structure of the filament—it's a very thin wire that has been wound into a spiral and then wound into a spiral again so that the long filament wire will fit in a small space.
Explanation: The light emerging from an incandescent light bulb originates in its filament. The glass bulb serves only to protect the filament and to diffuse the light from the filament.
Demonstration 7.3.3: Blackbodies are Really Black
Description: A piece of burning carbon looks cherry red in the dark but pitch black when brightly illuminated.
Purpose: To demonstrate that black objects are both excellent absorbers of light and excellent emitters of light.
1 piece of burning carbon (no ash content)
1 high-intensity lamp
Procedure: Place the burning carbon where it can be easily seen and turn out the room lights. It will appear to be glowing red-hot. Now turn on the lights and illuminate the hot carbon with bright light from the high-intensity lamp. It will appear jet black. Repeat.
Explanation: When there is no light illuminating it, the only light coming from the carbon is its own blackbody spectrum, giving a cherry-red glow. But when it is brightly illuminated, the carbon's thermal radiation is insignificant and we see only that it absorbs all of the light that falls on it. I've tried this with ordinary charcoal, but it contains too much ash to be truly convincing. It looks pretty white under bright illumination. Pure carbon is much better.
Demonstration 7.3.4: An Incandescent Light Bulb at Various Temperatures
Description: An incandescent light bulb is connected to a variable voltage transformer. As the current passing through the bulb's filament increases, so does its temperature. The bulb's brightness increases and the color of its light shifts from reddish, to orangish, to yellowish as it heats up.
Purpose: To show that both the brightness and spectrum of thermal radiation depend on the temperature of the emitting surface.
1 incandescent light bulb
1 lamp or bulb holder
1 variable voltage transformer (a Variac auto-transformer is ideal)
Procedure: Insert the incandescent bulb in the lamp or bulb holder and plug the lamp into the variable voltage transformer. Slowly turn up the voltage of the transformer until the bulb glows a dim red. You may want to turn out the room lights. As you turn up the voltage still further, the temperature of the bulb's filament will increase and its brightness will also increase. Point out that the color of the light emitted by the bulb also changes—it becomes less red and more yellow. That's because an object's spectrum of thermal radiation shifts toward shorter wavelengths as that object becomes hotter.
Explanation: The bulb's filament is essentially a black body with a temperature that's determined by the amount of electric power it receives. As you turn up the voltage of the transformer, the filament receives more and more power and becomes hotter and hotter. (The filament's temperature is determined by its need to get rid of energy as heat just as quickly as that energy arrives as electric power. As more power arrives at the filament, its temperature must rise higher in order for more heat to leave each second.) With its increasing temperature, the filament emits both more light and shorter wavelength light.
Follow-up: If you put a transmission diffraction grating in front of a color CCD camera and point the camera at the proper angle with respect to the glowing light bulb, you will see the spectrum of light emitted by the bulb as a rainbow smear of color on the color monitor. Placing a black surface strategically in front of the camera helps clarify the spectrum, and using a tall, thin, clear incandescent bulb helps even more. As you turn up the temperature of the filament, the smear of color will shift toward short wavelengths to include more and more green and blue light. Use an auto-iris camera and/or crossed polarizers, so that you don't saturate the camera as the brightness of the bulb increases.
Demonstration 7.3.5: Different Wattage Bulbs - More of the Same Light
Description: Several bulbs of different wattages are illuminated at once. While they have different brightnesses, their colors are the same.
Purpose: To show that the filaments of different wattage bulbs all operate at essentially the same temperature.
3 normal bulbs (made by the same manufacturer and not extended life) of different wattages (such as 25 W, 60 W, and 100 W)
3 bulb holders
Procedure: Insert the three bulbs in the bulb holders and turn them all on. Note that while the 25 W bulb is much dimmer than the 100 W bulb, it has essentially the same color (spectrum) of light.
Explanation: The low wattage bulb's filament operates at the same temperature as that of the high wattage bulb. However, the low wattage bulb's filament is smaller and thus less bright than that of the high wattage bulb.
Demonstration 7.3.6: A Three-Way Bulb
Description: A three-way bulb emits three different light levels as you turn its switch.
Purpose: To show that the light emitted by a three way bulb doesn't change in color—it changes only in brightness.
1 three-way bulb
1 lamp for the three-way bulb
1 paper or cloth bag
Procedure: Cycle the bulb several times through its three different light levels. Point out that while its brightness is changing, the color of the light it emits isn't changing. This means that the temperature of the filament(s) inside isn't changing with the light level. The only way that this can occur is if the filament(s)'s surface area is changing. That's exactly what's happening. The bulb contains 2 separate filaments. At the lowest light level, only the smaller filament is operating. At the medium light level, only the larger filament is operating. And at the highest light level, both the filaments are operating.
Finally, break open the bulb—insert it in the bag and tap it with the hammer until the glass shatters. Carefully remove the exposed bulb from the bag and discard the glass fragments. Examine the two different-sized filaments.
Explanation: To maintain a constant filament temperature, light spectrum, and energy efficiency while varying its brightness, the bulb must change the size of its filament. It does this discretely by using one or both of its two different filaments.
Demonstration 7.3.7: An Unprotected Filament Burns Up
Description: You turn on a light bulb that has had its outer glass envelope removed. The filament burns with a cloud of white smoke.
Purpose: To show that hot tungsten burns and must be protected from oxygen.
1 incandescent light bulb without its glass envelope (Remove the glass envelope by wrapping the bulb in a paper or cloth bag and tapping it with a hammer until the glass shatters.)
1 lamp holder
Procedure: Unplug the lamp holder. Carefully insert the exposed light bulb in the lamp holder (don't cut your fingers or break the filament). Now plug in the lamp holder and turn on the bulb. The filament will burn.
Explanation: Tungsten, like most metals, can oxidize and it burns readily at high temperatures. In a normal incandescent bulb, the tungsten is protected from oxygen by the glass envelope.
Demonstration 7.3.8: A Halogen Bulb
Description: The active portion of a halogen bulb is a small, clear envelope with a tungsten filament inside. When it's turned on, the filament emits a brilliant yellow-white light and the small envelope gets rather hot.
Purpose: To show that the structure of a halogen bulb isn't quite the same as that of a normal incandescent bulb.
1 screw-in halogen bulb (a replacement for a normal incandescent bulb—this type of halogen lamp has a heavy protective envelope)
1 small halogen lamp (any halogen lamp that doesn't have a large protective envelope)
1 normal incandescent lamp of roughly the same wattage as the screw-in halogen bulb
2 bulb holders for the screw-in bulbs
Procedure: Show that the active component of a halogen lamp is quite small. It has to be small because it must operate at high temperatures. Insert both of the screw-in bulbs into the lamp holders and turn them on. Show that the halogen lamp is somewhat brighter and emits a whiter (less yellow) light than the normal incandescent bulb.
Explanation: The halogen lamp recycles the tungsten atoms that sublime from the tungsten filament during operation. For this halogen-mediated recycling to work, the entire bulb (including the clear envelope around the filament) must operate well above room temperature. That's why the envelope is small and close to the filament. Because the filament is continuously rebuilt, it can and does operate at higher temperatures than the filament of a normal bulb without exhibiting a short operating life. The halogen bulb thus emits a larger fraction of its thermal radiation as visible light, making it brighter. Its thermal radiation includes a larger fraction of green and blue wavelengths, making that radiation less yellow and more white than the light of a normal bulb.
Demonstration 7.3.9: The Diffusing Effect of the Glass Envelope
Description: You turn on two different light bulbs of equal wattage: one has a normal, clouded envelope and the other has a clear envelope. The clear bulb is much less pleasant to look at.
Purpose: To show that the clouded surface of a normal bulb diffuses the light so that it appears to originate from a larger, dimmer surface.
1 normal incandescent bulb (60 W)
1 clear incandescent bulb (60 W)
2 bulb holders
Procedure: Insert both bulbs in lamp holders and turn them on. The normal bulb will emit light from its entire surface, so that the surface appears relatively dim. The clear bulb emits light only from its filament, which appears dazzlingly bright. The colors of the two bulbs are identical—the white coating only redirects the light from the filament.
Explanation: The white particles on the inside surface of the normal incandescent bulb scatter and redirect light from the filament. The result is light that emerges from a larger surface and thus appears less dazzling and casts more diffuse shadows.
Demonstration 7.3.10: Long Life Bulbs - Less Light for the Money
Description: You compare the light produced by a normal incandescent bulb to the light produced by a long-life bulb of an equivalent wattage. The long life bulb emits redder, dimmer light.
Purpose: To show that the filament of the long life bulb operates at a lower temperature than that of a normal bulb.
1 normal incandescent bulb (60 W)
1 long life incandescent bulb (approximately 60 W)
2 bulb holders
Procedure: Insert the two bulbs in the bulb holders and turn them on. Note that the long life bulb is dimmer and redder than the normal incandescent bulb, even though both are using essentially the same amount of electric power. Point out that while the long life bulb may be more convenient because it doesn't require changing as often, it produces less light for each kilowatt hour of electricity.
Explanation: The filament of the long life bulb last so long because it operates at a lower temperature than the filament of a normal incandescent bulb. Its long life isn't free—you pay for it with decreased light efficiency over the whole operating life of the bulb.
Demonstration 7.3.11: Heat Lamps
Description: You turn on a heat lamp and observe that it barely produces any visible light at all. Its dim, red glow is just the tip of the iceberg—most of its thermal radiation is invisible infrared light.
Purpose: To show that a low temperature filament produces very little visible light but lots of invisible infrared light.
1 heat lamp
Procedure: Turn on the heat lamp and examine its light. All that you can see is a dim red glow. However, if you put your hand near it, you can feel the warmth transferred to you via invisible infrared light. Because its filament operates at a relatively low temperature, most of the heat lamp's thermal radiation is this infrared light.
Explanation: Below about 1500°C, a hot filament emits relatively little visible light and what little it does emit is red light. Most of the filament's thermal radiation is at much longer wavelengths and is invisible.