How Things Work - Chapter 10 Demonstrations

# Section 10.1 Static Electricity

Demonstration 10.1.1:  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 contact 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: Contact transferred negatively charged electrons off the silk and onto the Teflon. It also transferred negatively charged electrons off the acrylic rod and onto the silk, leaving the acrylic rod with a net positive charge.
Demonstration 10.1.2:  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.
Demonstration 10.1.3:  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.
Demonstration 10.1.4:  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.
Demonstration 10.1.5:  Raising the Voltage of a Capacitor by Separating its Plates
Description: You put opposite charges on the two plates of a capacitor. As you separate the two plates, an electroscope attached to one of the plates shows a rise in its charge, indicating a rise in the voltage of the capacitor plate.
Purpose: To show that separating opposite charges increases the voltage of the positive charges and lowers the voltage of the negative charges.
Supplies:
1 capacitor with plates that can be separated safely while they are charged.
1 electroscope
1 wire to connect capacitor plate to electroscope
1 source of negative charge (e.g., a teflon rod and silk)
1 source of positive charge (e.g., an acrylic rod and silk)
Procedure: Connect one plate of the capacitor to the electroscope. Remove all charge from the capacitor's plates and move them close together. Put negative charge on one plate and positive charge on the other plate. The electroscope should not deflect very much. Now gradually move the capacitor plates apart. The electroscope should indicate an increase in charge.
Explanation: While the two oppositely charged plates were close together, their opposite charges had little electrostatic potential energy. The positively charge plate had a modest positive voltage and the negatively charged plate had a modest negative voltage. But as you separated the plates and did work on the charges, the positively charged plate's voltage rose dramatically and the negatively charged plate's voltage dropped dramatically. The high voltage (whether positive or negative) pushed additional charge (whether positive or negative) onto the electroscope and it indicated that charge increase.
Demonstration 10.1.6:  Faraday's Ice Bucket
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.
Demonstration 10.1.7:  A Van De Graaff Generator
Description: A van de Graaff 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 de Graaff generator.
Supplies:
1 van der Graaf static generator
Procedure: First examine the components of the van de Graaff 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 de Graaff 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 de Graaff generator accumulates negative charge, so it reaches a very large negative voltage.)
Demonstration 10.1.8:  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.
Demonstration 10.1.9:  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.
Demonstration 10.1.10:  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.
Demonstration 10.1.11:  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.

# Section 10.2 Xerographic Copiers

Demonstration 10.2.1:  Pith Balls, Charged Rods, and an Electric Field
Description: You put positive charge on both a pith ball and a rod, and the two objects repel one another. You then consider whether the rod is repelling the pith ball directly or whether there is something around the rod that is acting to push the pith ball away from the rod -- an electric field.
Purpose: To examine the concept of an electric field.
Supplies:
1 metal-coated pitch ball, hanging from a thread.
1 acrylic rod
1 piece of silk
1 large piece of cardboard or another surface to block the view.
Procedure: Rub the silk on the acrylic rod and it becomes positively charged. Touch the acrylic rod to the pith ball to put positive charge on that ball. Recharge the acrylic rod. Now bring the rod near the pith ball and they'll repel one another. Discuss the grade-school explanation for this behavior--that the positive charge on the rod is repelling the positive charge on the pith ball. Now block everyone's view of the rod with the piece of cardboard, but all them to see the pith ball. You can still make the pith ball move, even though they can't see what's making it move. Discuss the idea that there is something in space that is pushing on the pith ball, something that may or may not be created by charge and that influences charges that are immersed in it. That something is an electric field.
Explanation: The positive charge on the acrylic rod produces an electric field and that field, in turn, pushes on the pith ball's positive charges.
Demonstration 10.2.2:  Visualizing an Electric Field with Felt and Oil
Description: Felt sprinkled onto a bath of oil forms lines along the electric field direction created by probes from a static generator.
Purpose: To show that electric fields are real.
Supplies:
1 static generator, such as a Wimshurst Machine
1 sprinkle container of felt powder
1 glass container with a shallow pool of oil in it
2 electrodes from the static generator to the oil bath
Procedure: Insert the electrodes into the oil bath, separated by sufficient distance that no sparks will occur when you operate the static generator. Sprinkle a light dusting of felt dust onto the oil. Then operate the static generator. The felt should move to form lines of electric flux between the two electrodes.
Explanation: The electric field polarizes the felt particles so that they cling together positive end to negative end in long, visible chains. Since the polarization is along the field direction, the chains are also along the field direction.
Demonstration 10.2.3:  Ringing a Bell with Charge and an Electric Field
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:
1 Wimshurst static generator
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. Viewed in terms of an electric field, the ball acquires a charge from the plate it touched last and then the electric field (a voltage gradient) pushes it toward the other plate.
Demonstration 10.2.4:  Putting Out a Candle with an Electric Field
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:
1 Wimshurst static generator
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 and the electric field is strong enough, the candle flame will become severely distorted and will probably extinguish itself.
Explanation: The charged particles in the flame are pushed by the electric field 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.
Demonstration 10.2.5:  A Corona Discharge near a High-Voltage Needle
Description: A sharp needle attached to one side of a static generator exhibits a white-blue glow in a dark room.
Purpose: To show that a corona discharge occurs when a sharp object is maintained at high voltage.
Supplies:
1 Wimshurst static generator
1 sharp pin
tape
1 sensitive video camera (optional)
Procedure: Tape the pin to one ball of the Wimshurst static generator so that the tip of that needle points toward the other ball, a couple of centimeters away. Darken the lights and crank the static generator. A faint glow will develop around the needle tip as charge passes from it to the air as a corona discharge.
Explanation: The high-voltage needle initiates a corona discharge and sprays charges into the air. The charges acquire enough energy as they move through the high electric field around the tip to cause the air molecules to glow.
Demonstration 10.2.6:  Sharp Points and Corona Discharges - 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.
Demonstration 10.2.7:  Photoconductors - A CdS Cell
Description: A cadmium-sulfide photoconductive cell measures the amount of light reaching its surface.
Purpose: To show how light can convert a photoconductor from an insulator to a conductor.
Supplies:
1 CdS (Cadmium-Sulfide) photoconductive cell, or an equivalent photoconductive cell
1 ohm meter (or a display that shows the electric resistance of the CdS cell
1 flashlight (or another light source)
wires
Procedure: Connect the CdS cell to the ohm meter. Show that the darkened CdS cell is basically an insulator. Now expose the CdS cell to light and show that it becomes electrically conducting.
Explanation: Light promotes electrons from the filled valence levels in the CdS cell to the empty conduction levels in the CdS cell. This shifting of charges allows the CdS to transport electric charge. The ohm meter applies a modest electric field across the CdS cell and, when light is present, electric charge begins to flow through the CdS cell.
Demonstration 10.2.8:  Forming a Charge Image and Developing it with Felt Toner
Description: Charge is deposited on an insulating surface with an array of sharp, electrically charged points. The charge in some areas of the surface is erased with your finger. Finally, felt dust is sprinkled on the surface and it sticks to those areas that are still charged.
Purpose: To illustrate the xerographic process, although without the photoconductive aspect.
Supplies:
1 metal sheet (about 30 cm on a side)
1 clear plastic sheet (self-adhesive laminate plastic works well)
1 support for the metal sheet
1 van der Graaf static generator
1 strip of metal screening, cut to reveal a row of sharp metal points
1 wooden stick handle for the metal screening
2 wires
1 shaker container of felt dust (or another fine, non-conductive powder)
Procedure: Attach or glue the clear plastic sheet to the surface of the metal sheet. Make sure that the entire surface of the metal sheet is covered by plastic. Mount the sandwich on the support and ground the metal sheet with one wire. Attach the metal screening to the stick and connect it to sphere of the van der Graaf generator with the other wire. Turn on the van der Graaf generator and brush the metal screening across the plastic coated surface. This action will cover the plastic with charges. Turn off the van der Graaf generator. With you finger, rub an identifiable mark on the plastic surface. While you won't see anything, you will have removed charge from part of the plastic surface. Now sprinkle the felt dust on the plastic surface and blow away any excess. You should see the mark you made as a light region in an otherwise relatively dark background on the plastic sheet.
Explanation: The charged metal screening deposited electric charge on the plastic surface. When you touched parts of the plastic surface, you provided a way for the charge to escape from the surface and erased those parts of the surface. In a real xerographic copier this erasure is done by light, which turns the photoconductor (here the plastic layer) into a conductor so that the charge can escape into the metal sheet. When you then sprinkle dust onto the sheet, the dust is attracted to any charged portions of the sheet. (In a real xerographic copier, the toner particles are charged by their carrier system. Here, the felt dust isn't explicitly charged and is held in place largely by polarization effects.)

Follow-up: I plan to build a metal sheet with a real photoconductor surface—one that will be sensitive to blue light but insensitive to red light. I will be able to work with it in class under red illumination and expose it to a pattern of blue light to form a charge image. When I have a version that works, I will post information about it on the demonstration web site.

Demonstration 10.2.9:  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.

# Section 10.3 Flashlights

Some of the demonstrations from Section 7.3 would also be valuable as an introduction (or reintroduction) to light bulbs.
Demonstration 10.3.1:  A Simple Circuit with a Battery and Light Bulb
Description: You connect a battery and a light bulb with wires and create a circuit. The light bulb begins to emit light.
Purpose: To show how a circuit works.
Supplies:
3 fresh 1.5 V batteries (flashlight batteries)
3 battery holders
1 1.5 V light bulb
1 3.0 V light bulb
1 4.5 V light bulb
1 light bulb holder
1 current visualizer (available from a scientific supply company—an electronic device with a row of LEDs that create a moving light pattern to illustrate the direction and amount of current flow)
1 switch
6 wires
Procedure: Start with one battery and the 1.5 V light bulb. Connect one wire between the battery's positive terminal and one terminal of the light bulb. Discuss that while this single connection is enough to allow positive charge to flow briefly from the battery to the light bulb, that this flow quickly stops. Now connect a second wire from the battery's negative terminal to the other terminal of the light bulb. The light bulb begins to glow. Discuss why the second wire is so important.
Now insert the current visualizer into the circuit between the battery's positive terminal and the light bulb. The visualizer will show that current (the flow of positive charge) always flows through the circuit in one direction, from the battery's positive terminal to the light bulb and back to the battery's negative terminal. Point out that the same electric charge is being used over and over—that it's flowing in a loop, picking up energy from the battery (at a rate of 1.5 J for each coulomb that passes through the battery; hence the label "1.5 V"), releasing that energy in the light bulb, and returning to the battery to make another trip.
Now reverse the battery connections. The flow of current through the rest of the circuit will reverse. This demonstration shows that the battery is responsible for determining the direction of current flow in the circuit. The battery creates the initial charge imbalance that pushes charge through the circuit.
Once it's clear how the circuit works, add the switch to the circuit. Show how opening the switch stops the flow of current through the circuit and turns off the light. Discuss the fact that you can insert the switch at any point in the circuit because any break in the circuit stops the current flow.
Now replace the 1.5 V light bulb with the 3.0 V light bulb. It will glow dimly. Discuss the fact that this bulb needs a current of more energetic charges to glow properly. Add a second battery in series with the first battery (connect the negative terminal of one battery to the positive terminal of the other to produce a battery chain with a total voltage of 3.0 V) and note that the light bulb glows much more brightly. Discuss the fact that the current now passes through both batteries and thus receives more energy per charge (3.0 J per coulomb, in accordance with the total battery voltage of 3.0 V).
Replace the 3.0 V light bulb with a 4.5 V light bulb and then add a third battery to the chain of batteries. Once again, the bulb will glow brightly. However, now reverse one of the batteries in the chain. The 4.5 V light bulb will become very dim. Replace it temporarily with the 1.5 V light bulb to show that the chain of batteries is now giving the current flowing through it only 1.5 J per coulomb (1.5 V). The reversed battery is actually taking away energy from the charges passing through it! As a result, the reversed battery is "recharging," although probably not very well because it's not designed to be recharged.
Replace the 1.5 V light bulb with the 4.5 V light bulb and return the battery to its proper situation in the chain. Again, the light bulb will glow brightly.
Explanation: Electric charge mustn't accumulate at any point in this arrangement of components. If it did, it would repel any additional charge and the current would stop flowing. By arranging the components in a circuit, the charge can flow continuously through it without accumulating anywhere. The charge simply shuttles energy from the battery to the light bulb's filament. Reversing one battery in the chain causes that battery to extract energy from the current passing through it, rather than adding energy to that current.
Demonstration 10.3.2:  A Short Circuit
Description: A circuit consisting of 3 batteries and a light bulb is working properly and the light bulb is emitting light. When a wire is connected directly from one terminal of the light bulb to the other, the light bulb dims and the wire begins to glow red hot—a short circuit.
Purpose: To show that current can take alternative paths through a circuit, some of which can be dangerous.
Supplies:
3 fresh 1.5 V batteries
3 battery holders
1 4.5 V light bulb
1 light bulb holder (with extra terminals for the nichrome wire below)
4 not-too-heavy gauge wires
1 piece of nichrome wire with terminals at its ends that connect easily and safely to the terminals of the light bulb holder
1 piece of paper
water to extinguish the burning paper if necessary
Procedure: Connect the 3 batteries in series and then connect the chain of batteries to the light bulb. The light bulb should glow brightly. Discuss the operation of the circuit. Now insert the nichrome wire across the terminals of the light bulb, so that current can flow through the nichrome wire rather than through the light bulb (the two should be wired in parallel to one another). The light bulb will dim and the nichrome wire will begin to glow red hot (adjust the length and thickness of the nichrome wire so that it glows nicely). For illustrative purposes, you can light a small piece of paper on fire with the hot wire. Discuss how the short circuit that the nichrome wire provides diverts current from the light bulb and why the nichrome wire becomes so hot.
Explanation: The nichrome wire presents a low resistance path for the current to take through the circuit. Since various resistances within the rest of the circuit limit the amount of current that can flow, the amount of current available for the light bulb decreases significantly and it dims. The nichrome wire converts much of the current's electrostatic potential energy and kinetic energy into thermal energy and it becomes very hot.
Demonstration 10.3.3:  Ohm's law
Description: A simple arrangement of a variable DC power supply, a resistor, a voltmeter, and an ammeter demonstrate that the current passing through the resistor is proportional to the voltage drop across it—Ohm's law.
Purpose: To show the relationship between current and voltage in an object that obeys Ohm's law.
Supplies:
1 variable-voltage DC power supply (for example, 0–10 V)
1 resistor (for example, 1000 ohms)
1 voltmeter (for example 0–10 V full scale)
1 ammeter (for example 0–10 mA full scale)
wires
Procedure: Form a circuit by connecting the positive terminal of the power supply to the positive terminal of the ammeter, the negative terminal of the ammeter to one end of the resistor, and the other end of the resistor to the negative terminal of the power supply. Also connect the positive terminal of the voltmeter to the ammeter-side of the resistor and the negative terminal of the voltmeter to the power supply side of the resistor.
Now show that as you slowly turn up the voltage of the power supply, the voltage drop across the resistor (as measured by the voltmeter) increases and the current through the resistor (as measured by the ammeter) increases in equal proportion. Point out that this perfect proportionality between the voltage drop across the resistor and the current that passes through the resistor is true of almost any conducting object, including electric wires—the more voltage drop that they experience, the more current that flows through them, or the more current that flows through them, the more voltage drop that they experience! Ohm's law. Point out that a wire that’s carrying lots of current and that’s thus experiencing a large voltage drop is also consuming lots of power. The power that it's consuming is the product of its current times its voltage drop.
Explanation: An ohmic device draws a current that's proportional to the voltage drop across it (or equivalently, it experiences a voltage drop that's proportional to the current passing through it). Since most conductors behave in an ohmic fashion, this relationship between current and voltage is almost universal. Because the power consumed by a device (energy per second) is the product of the current passing through it (charges per second) times the voltage drop across it (energy consumed per charge), the power consumed by an ohmic device is proportional to the square of its voltage drop or, equivalently, to the square of the current passing through it. The other factor that figures into this power consumption is the resistance of the ohmic device. For a set current flow, the power consumption of an ohmic device decreases as its resistance decreases.