The Video Encyclopedia of Physics Demonstrations ™

The Video Encyclopedia of Physics Demonstrations™ Explanatory Material By: Dr. Richard E. Berg University of Maryland

Scripts By: Brett Carroll University of Washington

Equipment List By: John A. Davis University of Washington

Editor: Rosemary Wellner

Graphic Design: Wade Lageose/Art Hotel

Typography: Malcolm Kirton

Our special thanks to Jearl Walker for his assistance during the production of this series; to Gerhard Salinger for his support and encouragement during the production of this series; and to Joan Abend, without whom all this would not have been possible.

We also wish to acknowledge the hard work of Laura Cepio, David DeSalvo, Michael Glotzer, Elizabeth Prescott and Maria Ysmael.

This material is based upon work supported by The National Science Foundation under Grant Number MDR-9150092.

© The Education Group & Associates, 1992.

ISBN 1-881389-00-6

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Requests for permission to make copies of any part of the work should be mailed to: The Education Group, 1235 Sunset Plaza Drive, Los Angeles, CA 90069. DISC SEVENTEEN

Chapter 40 Electrostatic Induction

Demo 17-01 Electrostatic Induction ...... 6 Demo 17-02 Metal Rod Attraction ...... 8 Demo 17-03 Electrophorus ...... 10 Demo 17-04 Induction Generator...... 12 Demo 17-05 Kelvin Water Dropper...... 14 Demo 17-06 Wooden Needle...... 16

Chapter 41 Electric Fields

Demo 17-07 Van de Graaff Generator...... 20 Demo 17-08 Van de Graaff with Streamers...... 22 Demo 17-09 Van de Graaff and Wand ...... 24 Demo 17-10 Electric Field...... 26 Demo 17-11 Lightning Rod ...... 28 Demo 17-12 Pinwheel...... 30 Demo 17-13 Point and Candle...... 32 Demo 17-14 Faraday Cage...... 34 Demo 17-15 Faraday Ice Pail...... 36 Demo 17-16 Smoke Precipitation ...... 38 Demo 17-17 Electron Discharge Tube with Wheel ...... 40

Chapter 42 Resistance and DC Circuits

Demo 17-18 Resistance Wires...... 44 Demo 17-19 Ohm’s Law...... 46 Demo 17-20 Heated Wire ...... 48 Demo 17-21 Cooled Wire...... 50 Demo 17-22 Electron Motion Model ...... 52 Demo 17-23 Series/Parallel Resistors...... 54 Demo 17-24 Series/Parallel Light Bulbs ...... 56 Demo 17-25 Wheatstone Bridge...... 58 Demo 17-26 Galvanometer as Voltmeter and Ammeter...... 60 Demo 17-27 Conservation of Current...... 62

C HAPTER 40

ELECTROSTATIC INDUCTION

5 Demo 17-01 Electrostatic Induction

Two electrically isolated metal spheres are placed in contact, and a charged rod is held adjacent to one of the spheres as shown in Figure 1. When the spheres are separated, keeping the charged rod in its position as shown, they will carry equal and opposite charges, as indicated.† This is verified by charg- ing and discharging an electrometer by contacting it with each of the balls suc- cessively.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstrations E-8, Electrostatic Induction, E-9, and E-23, Charging Electroscope by Induction. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ea-11, Induction Charging.

6 C HAPTER 40: ELECTROSTATIC I NDUCTION Electrostatic Induction / Script Demo 17-01

An electroscope and a pair of metal spheres will be used to demonstrate elec- trostatic induction. If one of the spheres is charged, the sphere can cause the electroscope to deflect.

Now we’ll start with both spheres neutral. The spheres are placed in contact with one another. A negatively charged rod is brought near, but not touching, one of the spheres. The spheres are separated and the rod is removed. When one of the spheres is touched to the electroscope, the scope deflects.

What will happen if we touch the other sphere to the electroscope as well?

The electroscope goes back to zero. The charges induced on the two spheres are equal and opposite.

This animation shows how the charges develop on the two spheres as the charged rod is brought near.

Equipment

1. Electroscope. 2. Pair of metal spheres on insulated stands. 3. Plastic rod. 4. Wool cloth.

C HAPTER 40: ELECTROSTATIC I NDUCTION 7 Demo 17-02 Metal Rod Attraction

A charged rod is held close to a metal rod which can rotate on a bearing stand, as shown in Figure 1. The charged rod induces the opposite charge in the closest part of the metal rod, thereby introducing Coulomb attraction between the two rods, as demonstrated on the video.

Figure 1

8 C HAPTER 40: ELECTROSTATIC I NDUCTION Metal Rod Attraction / Script Demo 17-02

This aluminum rod is placed on a bearing stand so that it is free to rotate.

A plastic rod is positively charged by rubbing it with a piece of wool. When the plastic rod is held near the neutral metal rod, the rod is attracted and swings toward the plastic.

We’ll repeat that with a rubber rod, which develops a negative charge when rubbed with wool.

The negatively charged rubber rod also attracts the neutral metal rod.

This animation shows how the free charges in the metal rod are attracted or repelled by the two rods.

This separation of charge inside the metal causes the aluminum rod to be attracted to a charged rod of either polarity.

Equipment

1. Length of aluminum rod. 2. Low friction-bearing pivot on a stand. 3. Two plastic rods capable of opposite charges. 4. Wool cloth.

C HAPTER 40: ELECTROSTATIC I NDUCTION 9 Demo 17-03 Electrophorus

A electrophorus is a device that can be charged once and used many times to charge a conducting plate. An acrylic electrophorus sheet is charged negatively by rubbing it with fur. A metal plate is then placed on the acrylic sheet and grounded by touching it with a grounded wire. This process charges the metal plate with the opposite charge to that on the acrylic sheet, as illustrated by Figure 1, which is taken from the video graphics. The metal plate can be dis- charged in performing a demonstration and charged many more times using the same procedure, with no need to recharge the acrylic sheet because it retains its original charge.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-10, Electrophorus. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ea-19, Electrophorus.

10 C HAPTER 40: ELECTROSTATIC I NDUCTION Electrophorus / Script Demo 17-03

Here is a device called an electrophorus, which generates electric charge in an interesting way. It consists of an acrylic sheet and a round metal plate attached to an insulating arm. If we rub the acrylic with a wool cloth, the acrylic acquires a positive charge.

The metal plate is then placed on the acrylic.

A grounded wire is then touched to the metal plate. If the plate is now lifted off the acrylic and brought up to a second metal plate that is grounded, a spark jumps between the plates. If we repeat the procedure without recharging the acrylic sheet, we can get another spark.

And another.

How can we continuously generate charge without recharging the acrylic?

The electrophorus works by electrostatic induction; when the neutral metal plate is lowered onto the positively charged acrylic, the negative charges in the metal are pulled to the bottom of the plate. That leaves a net positive charge on the top surface of the plate. When the top surface is connected to ground, electrons conducted from the ground neutralize the upper surface. The plate now has a net negative charge. The positive charge is still on the acrylic, so the cycle can be repeated.

Equipment

1. Sheet of plastic. 2. Similarly sized aluminum disc attached to a non-conductive handle. 3. Wool cloth. 4. Grounded aluminum plate. 5. Grounding wire to use with the electrophorus.

C HAPTER 40: ELECTROSTATIC I NDUCTION 11 Demo 17-04 Induction Generator

The generator shown in this demonstration is a standard Wimshurst machine, which generates a high voltage by the process of induction with positive feed- back.† The mechanism by which a similar machine, called a “voltage doubler,” works is explained in detail on the video, using a sequence of animated graph- ics, one of which is shown in Figure 1. This generator and a similar larger one are used in several of the following demonstrations.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-26, Toepler-Holtz and Wimshurst Machines. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ea-22, Wimshurst Machine.

12 C HAPTER 40: ELECTROSTATIC I NDUCTION Induction Generator / Script Demo 17-04

Electrostatic generators such as this Wimshurst machine use electrostatic induc- tion to separate positive and negative charges, and can create very high volt- ages.

Here is an animation of a very simple type of electrostatic generator known as a voltage doubler. An acrylic disc with small pieces of metal foil near its outer edge is mounted so that it can rotate when turned by a crank.

Two metal shells are wrapped around the edges of the disc. A metal bar with conductive brushes at either end contacts two of the foil pieces at the positions shown.

Assume the two metal shells start out with a small imbalance of charge, so that one is more positive than the other. That charge imbalance induces charges on the two foil pieces which are in contact with the brushes. When the disc rotates, these foil pieces break contact with the brushes, each coming away with a small charge that is opposite that of the metal shell on that side. Each foil piece that follows comes away with the same charge. When these charged foil pieces reach the center of the opposite shell, they contact a conductive brush inside the shell and surrender their charges to that shell.

As charge builds up on the shells, the process of induction becomes stronger, leaving more charge on each of the foil pieces and accelerating the charging process.

If discharge rods are attached to the shells, a spark will pass between them, reducing their charges and starting the cycle again.

Equipment

Wimshurst machine.

C HAPTER 40: ELECTROSTATIC I NDUCTION 13 Demo 17-05 Kelvin Water Dropper

The Kelvin water dropper generates electrical charge in water drops through a combination of tribeoelectric effects at the start of the charging process, and positive feedback, as demonstrated in the video.† A graphics sequence follow- ing the demonstration of the actual water drop generator explains in detail how the device functions, part of which is shown in Figure 1.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-25, Kelvin Water Dropper. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ea-14, Kelvin Water Dropper.

14 C HAPTER 40: ELECTROSTATIC I NDUCTION Kelvin Water Dropper / Script Demo 17-05

Here is an interesting device known as a Kelvin water dropper. That generates static electricity from the energy in falling drops of water.

Water is allowed to fall from these two droppers, through metal rings, and into the metal cans below.

After a few seconds, enough voltage develops between the cans to flash a small neon bulb.

This animation shows how the generator works.

Assume a small charge imbalance initially exists between the cans. The can which is more positive is connected to this ring and the positive charge on the ring attracts negative charges into the droplet at the end of the dropper.

The droplet breaks away, carrying the negative charge down into the nega- tively charged can below, and increasing the charge on the can.

On the other side, positively charged droplets fall into the positively charged can. The charges on the two cans increase until the voltage is high enough to flash the neon lamp.

Equipment

1. Kelvin water dropper electrostatic generator built by soldering a brass rod up and away from the upper rim of two tin cans at a 45° angle, whose end is soldered to a brass ring. One has a small neon lamp, socket, and short piece of wire soldered near its midpoint. 2. Reservoir that feeds two glass nozzles. 3. Support system to hold the reservoir and position the two nozzles just above the center of the brass rings. 4. Two thumb screws, a “T” coupler, and the necessary rubber tubing to complete the water delivery to the needles. 5. The cans are set on slabs of paraffin to electrically insulate them from the base and arranged so the two brass rods cross over without touching, with the short wire from the lamp being brought near the other rod, leaving an appropriate spark gap. 6. Supply of water.

C HAPTER 40: ELECTROSTATIC I NDUCTION 15 Demo 17-06 Wooden Needle

This experiment demonstrates one effect of polarization of water molecules in a wooden beam.† A charged rod is held near a wooden beam that is mounted on a pivot, as shown in Figure 1. The electric field of the charged rod polar- izes the charge in the water molecules in the beam. Because the electric field is non-uniform, a net attractive force is exerted between the rod and the polar- ized molecules. This attractive force rotates the wooden beam on its pivot.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-1, Electric Charges on Solids Separated after Contact. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ea-17, Conductivity of a “Two by Four.”

16 C HAPTER 40: ELECTROSTATIC I NDUCTION Wooden Needle / Script Demo 17-06

This wood bar is placed on a stand so that it is free to rotate. When a posi- tively charged plastic rod is brought near, the wood is attracted and follows the rod.

Repeating this demonstration with a negatively charged rubber rod shows the same effect.

Equipment

1. Six foot or so length of dimensional lumber. 2. Low friction pivot system. 3. Plastic rods capable of producing opposite charges. 4. Wool cloth.

C HAPTER 40: ELECTROSTATIC I NDUCTION 17 18 C HAPTER 41

ELECTRIC FIELDS

19 Demo 17-07 Van de Graaff Generator

This demonstration shows the operation of a Van de Graaff generator and includes a discussion of how the generator works.† The operating generator is shown in Figure 1 with paper streamers on the dome. A major section of this demonstration is an animated sequence explaining the operation of a Van de Graaff generator.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-27, Van de Graaff Generator. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ec-1, Electrostatic Generator.

20 C HAPTER 41: ELECTRIC F IELDS Van de Graaff Generator / Script Demo 17-07

This Van de Graaff generator can be used to develop very high voltages. This animation shows how the generator works.

A rubber belt passes over a pair of rollers, with the bottom roller driven by a motor. The upper roller is contained inside a smooth hollow metal sphere. A high voltage DC source is connected to a metal comb positioned near the bot- tom roller; it transfers charge to the moving belt by corona discharge.

When the belt enters the hollow sphere, it passes by another metal comb which is electrically connected to the sphere. The charge is pulled off the belt by the comb and moves to the outside of the sphere. Since the electric field inside the charged sphere is nearly zero, the comb can continue to remove charge even after there is a large amount of charge on the sphere. The voltage on the outside of the sphere continues to increase until it reaches the break- down voltage of the surrounding air.

Equipment

1. Van de Graaff electrostatic generator. The demonstration is primarily a theoretical animation explaining how a Van de Graaff electrostatic generator works. 2. AC power.

C HAPTER 41: ELECTRIC F IELDS 21 Demo 17-08 Van de Graaff with Streamers

In this demonstration a group of paper streamers are positioned around the Van de Graaff dome.† When the dome is charged, the streamers extend out- ward, following the electric field lines from the dome, as shown in Figure 1.

Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ec-3, Lines of Force.

22 C HAPTER 41: ELECTRIC F IELDS Van de Graaff with Streamers / Script Demo 17-08

We’ll show the shape of the electric field around this Van de Graaff machine using these paper streamers.

When the machine is turned on, the sphere and the streamers become charged. Electrostatic forces push the streamers away until they follow the electric field lines of the sphere.

A person standing on an insulated platform with their hand on the machine shows a similar effect.

Equipment

1. Van de Graaff electrostatic generator with the sphere equipped with an array of low mass paper streamers. 2. Van de Graaff electrostatic generator without streamers. 3. Elevated stand with the maximum amount of insulation from the floor as possible. 4. AC power.

C HAPTER 41: ELECTRIC F IELDS 23 Demo 17-09 Van de Graaff and Wand

This demonstration illustrates the difference between the electric field created by a small sphere and that of a sharp point held near the dome of a Van de Graaff.† The electric field generated near a charged surface increases as the radius of the surface becomes smaller. For the case of a small sphere, the field builds up to a point where the breakdown occurs in the form of a large spark, as seen in Figure 1. On the other hand, the field in the region of the sharp end of the wand quickly becomes very large as the dome begins to charge. The air therefore breaks down continuously at a very low potential difference between the dome and the wand, and the charge on the dome never rises to a value at which a large spark can occur.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-32, Discharge of Electricity from a Point. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ec-3, Lines of Force.

24 C HAPTER 41: ELECTRIC F IELDS Van de Graaff and Wand / Script Demo 17-09

We’ll use this Van de Graaff high voltage generator and a grounded metal wand to illustrate how electrical discharge differs between sharp and blunt objects.

When the Van de Graaff generator is turned on, paper streamers fan out and follow the shape of the electric field around the sphere.

The wand is grounded, when its blunt end is brought near the sphere. There is little effect on the streamers until a large spark passes to the wand. The streamers collapse momentarily, but quickly rise back up until the next spark.

When the pointed end of the wand is brought near, the streamers collapse and stay down. The sharp point drains the charge off the sphere without visible sparks.

Equipment

1. Van de Graaff electrostatic generator with its sphere equipped with an array of low mass paper streamers. 2. Electrically grounded metal wand with a small sphere on one end and a sharp point on the other end. 3. Long grounding wire. 4. AC power.

C HAPTER 41: ELECTRIC F IELDS 25 Demo 17-10 Electric Field

This demonstration illustrates the electric fields of a number of different charge configurations, including a coaxial small disc and large ring, parallel plates, and concentric circles.† The field can be seen by immersing small bits of an electrically polarizable material in a liquid between the two conductors under study. Polarization of the charge in that material causes the small particles to align, rendering the electric field visible, as seen in Figure 1 and on the video.

Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Eb-1, Electric Fields between Electrodes.

26 C HAPTER 41: ELECTRIC F IELDS Electric Field / Script Demo 17-10

We’ll use this apparatus to show the electric field configuration around various charged objects.

Particles suspended in the fluid inside this chamber will line up in the pres- ence of an external electric field. The alignment shows the lines of the electric field.

If we put this small metal cylinder on top of the chamber and charge it using this pistol, this is the shape of the electric field from the metal cylinder.

Here is the electric field between a pair of parallel metal plates, one of which is positive and the other negative.

Here is the field between two oppositely charged circular conductors. Notice the absence of an electric field inside the inner conductor.

Equipment

1. Electric field demonstrator. 2. Overhead projector and screen.

C HAPTER 41: ELECTRIC F IELDS 27 Demo 17-11 Lightning Rod

The purpose of a lightning rod is to avoid lightning by continuously discharg- ing the clouds above the lightning rod before the charge can build up and cause lightning.† A lightning rod, being a sharp point, creates a large local elec- tric field in the same way as the wand of Demonstration 17-09, thus discharg- ing the clouds at a relatively low potential. The video shows lightning created by a large Wimshurst machine discharging to a house. As the grounded light- ning rod is raised from the center of the house, as seen in Figure 1, the charge is collected at a low potential, and the lightning discharge ceases.

Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Eb-7, Lightning Rod.

28 C HAPTER 41: ELECTRIC F IELDS Lightning Rod / Script Demo 17-11

We’ll use this model house and a high voltage electrostatic generator to demonstrate how a lightning rod works.

One side of the generator is connected to a metal ball inside the chimney. This sharp rod is also electrically connected to the same side of the generator. The other end of the generator is hooked to this copper sphere, which simulates a charged cloud in the atmosphere.

When the generator is cranked with the lightning rod down, large bolts strike the chimney.

When the lightning rod is raised, the bolts stop.

When the lightning rod is lowered, the large bolts resume.

Equipment

1. Toepler-Holtz electrostatic generator. 2. Model house equipped with a lightning rod through the ridge of its roof, which can be raised and lowered remotely, supporting a chimney with the necessary metal contact point and the associated wiring to connect the chimney and the lightning rod to one of the generators. 3. Model cloud (a brass sphere) with chain and hook so the cloud can be hung from the other terminal of the generator at an appropriate distance above the chimney. 4. Two large Leyden jars connected to the Toepler-Holtz will maximize the effect by having the system build up a much higher voltage before producing the lightning bolt.

C HAPTER 41: ELECTRIC F IELDS 29 Demo 17-12 Pinwheel

A pinwheel is the electrostatic version of a water sprinkler, as shown in Figure 1, but it works in a very different way.† This demonstration is often incorrectly explained using the rocket principle, suggesting that the motion of the pin- wheel is a reaction to the motion of the electrons leaving the tips of the pin- wheel, much the same as the water sprinkler.

Electrons from the generator leave the pinwheel at the points, just as in Demonstrations 17-09 and 17-11 above. This charge then collects on the adja- cent air molecules, creating a large cloud of gas charged the same as the tips of the pinwheels. The electrostatic repulsion between the charged tips of the pinwheel and the charged clouds of gas then propels the pinwheel.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-38, Electric Reaction Wheel. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Eb-10, Electrostatic Pinwheel.

30 C HAPTER 41: ELECTRIC F IELDS Pinwheel / Script Demo 17-12

We’ll place this pinwheel on top of a Van de Graff generator and turn it on.

The high voltage from the generator causes corona discharge from the points of the pinwheel. The discharge ionizes nearby air then repels it strongly from the point, making the wheel spin.

Equipment

1. Van de Graaff electrostatic generator. 2. Pinwheel. 3. Needle point support stand for the pinwheel.

C HAPTER 41: ELECTRIC F IELDS 31 Demo 17-13 Point and Candle

This demonstration, sometimes called “electric wind,” uses an electrical dis- charge from a point to blow a candle flame as shown in Figure 1.† This effect is due to Coulomb repulsion or attraction between the charge of the point and the positive ions in the flame.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-37, “Electric Wind.” Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Eb-3, Electric Wind.

32 C HAPTER 41: ELECTRIC F IELDS Point and Candle / Script Demo 17-13

This sharp point fastened to a high-voltage electrostatic generator will be used to demonstrate coronal discharge.

When we place the point on the terminal, the resulting coronal discharge blows the flame strongly to the side.

Equipment

1. Toepler-Holtz electrostatic generator, with one terminal equipped with a very sharp point projecting out horizontally. 2. Candle on a long non-conductive handle. 3. Source of flame.

C HAPTER 41: ELECTRIC F IELDS 33 Demo 17-14 Faraday Cage

This demonstration illustrates Gauss’ law: the electric field inside a conducting surface must be zero.† An electroscope is inductively charged by holding a charged rod a few inches above the plate on the top of the electroscope, as seen in Figure 1. When the screen is placed over the electroscope, the electro- scope is shielded from the charged rod, and feels no electric field when the charged rod is moved close to the electroscope plate.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-30, Absence of Electric Field within a Closed Conductor. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ea-20, Shielded Electroscope.

34 C HAPTER 41: ELECTRIC F IELDS Faraday Cage / Script Demo 17-14

We will use this electroscope to show that electric fields are affected by a con- ductive metal screen.

If a positively charged rod is brought near the electroscope, the electric field of the rod attracts negative charges to the top of this support arm.

That leaves a net positive charge on the bottom half, which deflects the pointer.

We’ll place this grounded metal cage over the top of the electroscope and repeat the demonstration.

What will happen when we bring the charged rod near the electroscope?

There is no deflection at all. The electric field of the charged rod cannot pene- trate the metal screen.

Equipment

1. Electroscope. 2. Plastic rod. 3. Wool cloth. 4. Hail screen enclosure that has the appropriate sizing and geometry to cover the electroscope.

C HAPTER 41: ELECTRIC F IELDS 35 Demo 17-15 Faraday Ice Pail

This demonstration shows that the charge on a conductor resides on the out- side of the conductor using the classical Faraday ice pail experiment.† A regular old mop bucket, isolated from ground, is charged to a high potential using a Wimshurst machine. Charge is then “scraped” off the outside of the pail using small conducting spheres and deposited on an electrometer, causing the elec- trometer to deflect and thus indicating the presence of charge on the outside of the pail, as seen in Figure 1. When a similar procedure is used to scrape charge off the inside of the bucket, it is found that no charge is obtained, thus demonstrating that the charge resides on the outside of the conductor.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstrations E-13, Electrostatic Induction— Faraday’s Ice-pail Experiment, and E-28, Location of Charge on Insulated Hollow Conductors. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ea-7, Charges on Conductors.

36 C HAPTER 41: ELECTRIC F IELDS Faraday Ice Pail / Script Demo 17-15

This Wimshurst machine is an electrostatic generator which produces approxi- mately 100,000 volts. We’ll use it to put a charge on this metal pail.

Now that we’ve put some charge on the pail, we’ll use this metal sphere on a plastic rod to find out how the charge is distributed.

Where will we find the charge—on the inside surface of the pail, the outside surface, or both?

Touching the sphere to the inside surface and then to the electroscope shows that there is no charge on the inside surface of the pail.

Touching the sphere to the outside surface and then to the electroscope shows that the charge is all on the outside surface.

Equipment

1. Wimshurst electrostatic generator. 2. Non-conductive stand to isolate the bucket. 3. Galvanized bucket. 4. Chain with a loop on one end and a metal sphere attached to the other end to transfer charge from the Wimshurst to the inside of the bucket. 5. Non-conducting rod with a hook on the end to manipulate the chain without discharging the bucket. 6. Second non-conducting rod with a metal sphere to serve as a probe. 7. Electroscope.

C HAPTER 41: ELECTRIC F IELDS 37 Demo 17-16 Smoke Precipitation

If an electrical discharge is created within a confined volume of smoke, the smoke particles will develop a net charge. If a plate of the opposite charge exists nearby, the charged smoke particles will be drawn to that plate, precipi- tating the smoke on the plate.† A device using this idea, shown in Figure 1, is called an electrostatic smoke precipitator.

Electrostatic smoke precipitators, sometimes called electrostatic scrubbers, are commonly used in cleaning the air from industrial smoke stacks.

Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Eb-12, Electrostatic Precipitator.

38 C HAPTER 41: ELECTRIC F IELDS Smoke Precipitation / Script Demo 17-16

Electrostatic precipitators are often used to remove pollutants from industrial smoke before it reaches the atmosphere.

To demonstrate the effect, we’ll use this acrylic tube with a metal conductor at each end connected to an electrostatic generator.

Smoke is put into the chamber with each of the conductors hooked up to one end of the generator.

The high voltage that is generated sweeps the smoke particles out of the tube.

Equipment

1. Smoke precipitation tube. 2. Electrical lead with alligator clips on both ends. 3. Source of smoke (see Demonstration 13-7). 4. Wimshurst electrostatic generator.

C HAPTER 41: ELECTRIC F IELDS 39 Demo 17-17 Electron Discharge Tube with Wheel

An electrical discharge inside a low-pressure tube can be used to roll a small paddlewheel along a track, illustrated in Figure 1. The device, formed by an axle with four small mica paddles, rotates in the direction of the electron motion.† When the direction of the discharge is reversed, the rotation of the paddlewheel reverses.

Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ep-9, Paddlewheel.

40 C HAPTER 41: ELECTRIC F IELDS Electron Discharge Tube with Wheel / Script Demo 17-17

Like all particles, electrons in motion have momentum. We’ll use this discharge tube to demonstrate that fact.

The tube is evacuated, with an electrode at either end and a glass wheel with mica paddles which is free to roll along the wheel.

If we turn up this high voltage power source and send a beam of electrons through the tube, electrons striking the paddles of the wheel roll it along the tube.

When we reverse the direction of the voltage, the wheel reverses direction.

Equipment

1. Crooke’s tube with a fluorescent paddle wheel and horizontal roller track; and its support stand. 2. Appropriate electrical leads. 3. Induction coil with reversing switch. 4. Battery eliminator.

C HAPTER 41: ELECTRIC F IELDS 41 42 C HAPTER 42

RESISTANCE AND DC CIRCUITS

43 Demo 17-18 Resistance Wires

The resistance of a wire is directly proportional to its length and its resistivity, and inversely proportional to its area.† The effect of length, area, and resistivity are shown in this demonstration, which is seen in Figure 1. The resistance R is deduced by measuring the current I in each wire when the same voltage V is individually applied to each wire, using Ohm’s law:

V R = I

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-175, Dependence of Resistance on Length and Area of Conductor.

44 C HAPTER 42: RESISTANCE AND DC CIRCUITS Resistance Wires / Script Demo 17-18

These five wires have varying resistivities, diameters, and lengths.

We’ll apply a constant voltage to each and measure the current that flows through them.

This center wire, labeled C, will be the basis for all comparisons.

Each of the other wires differ from C in only one respect. Here is the current through C at a constant 6 volts.

Wire A is identical to C, in diameter and length, but is made of a different material.

Here is the current flowing through A at 6 volts.

Wire B is made of the same material as C and is the same length, but has twice the diameter. Here is the current through B at 6 volts.

Wires D and E are the same material as C and are the same diameter, but D is half the length of C and E is one-fourth the length of C. Here is the current through D.

Here is the current through E.

Equipment

1. Resistance board—a vertical board supporting five wires of differing resistivities, diameters, and lengths. 2. Battery. 3. Ammeter. 4. Appropriate electrical leads.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 45 Demo 17-19 Ohm’s Law

Ohm’s law specifies the relationship between the voltage V , the current I , and resistance R for a resistive circuit element:

V = IR

As the voltage across a resistor is raised, the current increases linearly as the voltage, as shown in Figure 1.† The equipment used is shown in Figure 2.

Figure 1 Figure 2

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Eo-1, Ohm’s Law.

46 C HAPTER 42: RESISTANCE AND DC CIRCUITS Ohm’s Law / Script Demo 17-19

We’ll use this resistor and this battery to demonstrate the relation between volt- age and current in a conductor. By hooking to the battery here,… here, or here, we can get 2, 4, or 6 volts from the battery.

We’ll apply each of those voltages to this resistor and measure the current that flows in each case with this ammeter.

Here is the current at 2 volts.

Here is the current at 4 volts.

Here is the current at 6 volts.

Equipment

1. Rheostat. 2. Battery 3. Voltmeter. 4. Ammeter. 5. Appropriate electrical leads.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 47 Demo 17-20 Heated Wire

This demonstration shows that the resistance of iron increases as its tempera- ture increases.† As a coil of iron wire, in series with a light bulb, is heated, its resistance increases. The voltage across the coil therefore increases, so the voltage across the light bulb decreases, and the bulb becomes dimmer, as shown in Figure 1. Iron has a positive temperature coefficient of resistance, as do most conductors.

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstrations E-163, Effect of Temperature on Resistance, and E-164.

48 C HAPTER 42: RESISTANCE AND DC CIRCUITS Heated Wire / Script Demo 17-20

This iron wire in series with this small lamp will be used to demonstrate the effect of heating on resistance.

A current is sent through the wire and the bulb, which lights brightly.

When a gas flame is lit beneath the wire, current through the wire drops and the bulb dims.

Equipment

1. Small lamp wired in series with a “flatted” loop (arranged without shorting on an insulated stand) of iron wire, held just above a gas heating element that heats the majority of the coil at once. 2. Battery eliminator. 3. Ammeter. 4. Appropriate electrical leads. 5. Length of rubber tubing. 6. Supply of natural gas. 7. Source of flame. 8. AC power.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 49 Demo 17-21 Cooled Wire

This demonstration shows that the resistance of copper decreases as its tem- perature decreases.† As a coil of copper wire, in series with a light bulb, is cooled in a liquid nitrogen bath, its resistance decreases. The voltage across the coil therefore decreases, so the voltage across the light bulb increases, and the bulb becomes brighter, as shown in Figure 1. Copper has a positive tem- perature coefficient of resistance, as do most conductors.

Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Eg-4, Temperature Dependence of Resistance.

50 C HAPTER 42: RESISTANCE AND DC CIRCUITS Cooled Wire / Script Demo 17-21

This coil of copper wire in series with this lamp will be used to show the effect of cooling on electrical resistance.

We’ll run a small current through the coil and the lamp, which lights dimly.

When the coil is cooled by dipping it into liquid nitrogen, the brightness of the bulb increases dramatically.

Equipment

1. Coil of thin copper wire in series with a 6-volt lamp with both mounted on opposite ends of a non-conductor. 2. Battery eliminator. 3. Dewar of liquid nitrogen. 4. AC power.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 51 Demo 17-22 Electron Motion Model

This demonstration uses models of electron motion to illustrate two types of electron current.† If electrons are accelerated across a potential difference through a vacuum, they will continue to accelerate as they move, similar to the case of small spheres rolling unimpeded down an inclined plane. On the other hand, in a resistive medium, such as a wire, the conduction electrons interact with the atomic electrons around the atoms of the wire, limiting the speed with which the electron current moves. This can be simulated by spheres rolling down an inclined plane in which a large number of nails have been randomly inserted, as shown in Figure 1.

Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Eg-1, Model of Resistance.

52 C HAPTER 42: RESISTANCE AND DC CIRCUITS Electron Motion Model / Script Demo 17-22

Electrons travel differently when they are free in a vacuum than they do inside a material such as metal.

We’ll use these ball bearings to simulate electrons and show the differences between the two situations.

A board with side rails is used to model the motions of the electrons in a vac- uum when there is an electric field present.

This board has a number of nails in the surface of the wood, corresponding to the atoms in a piece of metal. When the bearings are added and the board is tilted as before, collisions with the nails slow the bearings, just as collisions with atoms slow the progress of electrons through a metal.

Equipment

1. Vacuum/atomic array analog—made by dividing a square piece of plywood in half and by adding rectangular walls on the same side of the plywood, driving a sizable number of small finish nails into one section. 2. Pivot. 3. Large number of steel ball bearings.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 53 Demo 17-23 Series/Parallel Resistors

This demonstration illustrates the difference between the currents and the volt- ages in series and parallel resistor circuits using the circuits shown in Figure 1.† When two resistors are in series (Figure 2), the same current must flow through both, so the voltage of the circuit is divided between them, and less total current will flow. On the other hand, when the two resistors are in paral- lel (Figure 3), each has the entire voltage across it, so separate currents flow in each, and the total current is two times the current through a single resistor.

Figure 1 Figure 2

Figure 3

† Sutton, Demonstration Experiments in Physics, Demonstration E-177, Resistors in Parallel and in Series.

54 C HAPTER 42: RESISTANCE AND DC CIRCUITS Series/Parallel Resistors / Script Demo 17-23

We’ll hook these two identical wire resistors together in three different configu- rations and apply the same voltage to each.

This ammeter will measure the current that flows through each of the configu- rations.

Here is a current that flows through a single wire with 6 volts applied.

If we hook the two wires in parallel, how much current will flow?

Twice as much current flows as with a single wire.

If we hook the wires in series, how much current will flow?

Now only half as much current flows as through a single wire.

Equipment

1. Two identical horizontal lengths of wire supported on a vertical board. 2. Battery. 3. Ammeter. 4. Appropriate electrical leads.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 55 Demo 17-24 Series/Parallel Light Bulbs

This demonstration illustrates the difference between series and parallel circuits using light bulbs.† Light bulbs in parallel across a 110 VAC circuit glow with their normal intensity. On the other hand, light bulbs in series across a 110 VAC circuit must share the voltage, so both become dimmer than normal, as seen in the video and in Figure 1.

Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Eh-1, Series and Parallel Light Bulbs.

56 C HAPTER 42: RESISTANCE AND DC CIRCUITS Series/Parallel Light Bulbs / Script Demo 17-24

Here are three light bulbs. We’ll hook the first bulb up to house current, then hook the other two bulbs in parallel to the first, one at a time.

The brightness of the bulbs stays constant as each is hooked in.

Now we’ll again hook up the first bulb, then hook the other two bulbs in series with it, one by one. How will the brightness of the bulbs change?

The bulbs dim as each new bulb is added.

Equipment

1. Three light bulbs and lamp sockets, along with banana plug sockets, mounted on a board with a power cord wired in parallel. 2. Three light bulbs and lamp sockets, along with banana plug sockets, mounted on a board with a power cord wired in series. 3. Appropriate electrical leads. 4. AC power.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 57 Demo 17-25 Wheatstone Bridge

A Wheatstone bridge is a device that can be used to measure resistances very accurately.† If three of the resistances are very well known, the resistance of a fourth (unknown) resistance can be determined very accurately. The circuit is shown in Figure 1, and the actual setup, which uses light bulbs as the resis- tances, is shown in Figure 2. A proportionality exists between pairs of resistors when the variable resistance is adjusted so that no current flows in the light bulb which is wired across the diamond. The resistance of the unknown is then equal to the value of the variable resistance.

Figure 1 Figure 2

† Sutton, Demonstration Experiments in Physics, Demonstrations E-155, Wheatstone-bridge Network, and E-156, Wheatstone Bridge—Slide-wire Form. Freier and Anderson, A Demonstration Handbook for Physics, Demonstrations Eg-6, Wheatstone Bridge, and Eh-2, Light Bulb Wheatstone Bridge.

58 C HAPTER 42: RESISTANCE AND DC CIRCUITS Wheatstone Bridge / Script Demo 17-25

A wheatstone bridge is a device used to make sensitive measurements of resis- tance.

Four resistors are arranged in a diamond configuration, with a small light bulb connected across the top and bottom corners.

These two resistors are equal in value. This resistor is variable, and this resistor is the “unknown” resistor we are trying to measure.

We’ll put 110 volts AC across the right and left corners of the bridge, then slide the variable resistor back and forth. When the slide is at an extreme position to the right or left, there is a voltage across the top and bottom corners as indi- cated by the light.

When the resistor is near the center the bulb goes out.

Now there is no voltage across the top and bottom points, and the value of the unknown resistor can easily be calculated in terms of the two known resis- tances and the value of the variable resistance.

Equipment

1. Wheatstone bridge made up of a slide wire rheostat, light bulbs, and lamp sockets, with a vertical support system. 2. AC power.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 59 Demo 17-26 Galvanometer as Voltmeter and Ammeter

A galvanometer, which is sensitive to small electrical currents, can be used as either a voltmeter or an ammeter by wiring it in the appropriate manner.† The setup used for this demonstration is shown in Figure 1.

When a small resistance is wired in parallel with the galvanometer, it functions as an ammeter, as in Figure 2. The ammeter is used to measure the current in several circuits.

When a large resistance is wired in series with the galvanometer, it functions as a voltmeter, as in Figure 3. The voltmeter is used to measure the terminal volt- age of several batteries.

Figure 1 Figure 2

Figure 3

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstrations Ej-6, Converting a Galvanometer to a Voltmeter, and Ej-7, Converting a Galvanometer to an Ammeter.

60 C HAPTER 42: RESISTANCE AND DC CIRCUITS Galvanometer as Voltmeter and Ammeter / Script Demo 17-26

This galvanometer will be used to show the electrical connections that change a galvanometer into an ammeter and a voltmeter.

Running a 25-milli-amp current through the galvanometer drives it to full scale.

If we want to measure larger currents, we must connect a shunt across the ammeter, like this. The reading now goes nearly to zero. Because the meter now requires 1 amp to go to full-scale deflection. If we increase the current we again get a reading.

To measure voltage, we connect a specific resistance in series with the gal- vanometer, like this.

Now we have a 10-volt full-scale voltmeter, which we can use to measure the voltage of this 1.5-volt battery.

A 6-volt battery produces a larger deflection.

Equipment

1. Lecture table galvanometer. 2. Variable resistor. 3. Batteries (two 1 1/2 volts, one 6 volt). 4. Appropriate electrical leads.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 61 Demo 17-27 Conservation of Current

At a node, or junction of three or more wires in a complex circuit, the alge- braic sum of the currents entering (or leaving) the junction must be equal to zero; that is, the current entering the junction must be equal to the current leaving the junction.† This rule, known as Kirchhoff’s junction rule, is demon- strated on the video using the apparatus shown in Figure 1.

Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstrations Eo-4, Continuity of Current, and Eo-7, Superposition of Currents.

62 C HAPTER 42: RESISTANCE AND DC CIRCUITS Conservation of Current / Script Demo 17-27

The amount of current entering a circuit junction or node must equal the amount of current leaving the node.

We’ll use this setup of batteries, resistors, and ammeters to show that is true.

The ammeters are set up to read the current flowing into and out of this circuit node.

Here are the currents at the node at the first resistor setting.

The total current flowing into the node is equal to the total current flowing out.

Here are the currents flowing into and out of the node after the values of the resistors are changed.

Equipment

1. Three slide wire rheostats. 2. Three ammeters. 3. Two 6-volt batteries. 4. Appropriate electrical leads.

C HAPTER 42: RESISTANCE AND DC CIRCUITS 63