The Video Encyclopedia of Physics Demonstrationsjedlik.phy.bme.hu/f29/dvd/17_brochure.pdf · Encyclopedia of Physics ... As charge builds up on the shells, the process of induction

TheVideoEncyclopedia

ofPhysicsDemonstrations™

Explanatory Material By: Dr. Richard E. BergUniversity of Maryland

Scripts By: Brett CarrollUniversity of Washington

Equipment List By: John A. DavisUniversity 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 ofthis series; to Gerhard Salinger for his support and encouragement during theproduction of this series; and to Joan Abend, without whom all this would nothave 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 informationstorage 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.

D I S C S E V E N T E E N

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

5

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I N D U C T I O N

Two electrically isolated metal spheres are placed in contact, and a chargedrod is held adjacent to one of the spheres as shown in Figure 1. When thespheres are separated, keeping the charged rod in its position as shown, theywill 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.

Demo 17-01 Electrostatic Induction

6 C H A P T E R 4 0 : E L E C T R O S T A T I C I N D U C T I O N

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, InductionCharging.

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 theelectroscope to deflect.

Now we’ll start with both spheres neutral. The spheres are placed in contactwith 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. Whenone 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 spheresare equal and opposite.

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

Electrostatic Induction / Script Demo 17-01

C H A P T E R 4 0 : E L E C T R O S T A T I C I N D U C T I O N 7

Equipment

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

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

Demo 17-02 Metal Rod Attraction


Figure 1

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. Whenthe plastic rod is held near the neutral metal rod, the rod is attracted andswings toward the plastic.

We’ll repeat that with a rubber rod, which develops a negative charge whenrubbed 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 orrepelled by the two rods.

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

Metal Rod Attraction / Script Demo 17-02


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.

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

Demo 17-03 Electrophorus


Figure 1

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

Here is a device called an electrophorus, which generates electric charge in aninteresting way. It consists of an acrylic sheet and a round metal plate attachedto an insulating arm. If we rub the acrylic with a wool cloth, the acrylicacquires 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 liftedoff the acrylic and brought up to a second metal plate that is grounded, aspark jumps between the plates. If we repeat the procedure without rechargingthe 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 metalplate is lowered onto the positively charged acrylic, the negative charges in themetal are pulled to the bottom of the plate. That leaves a net positive chargeon the top surface of the plate. When the top surface is connected to ground,electrons conducted from the ground neutralize the upper surface. The platenow has a net negative charge. The positive charge is still on the acrylic, sothe cycle can be repeated.

Electrophorus / Script Demo 17-03


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.

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 oneare used in several of the following demonstrations.

Demo 17-04 Induction Generator


Figure 1

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

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 asa voltage doubler. An acrylic disc with small pieces of metal foil near its outeredge 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 withconductive brushes at either end contacts two of the foil pieces at the positionsshown.

Assume the two metal shells start out with a small imbalance of charge, so thatone is more positive than the other. That charge imbalance induces charges onthe two foil pieces which are in contact with the brushes. When the discrotates, these foil pieces break contact with the brushes, each coming awaywith a small charge that is opposite that of the metal shell on that side. Eachfoil piece that follows comes away with the same charge. When these chargedfoil pieces reach the center of the opposite shell, they contact a conductivebrush 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 chargingprocess.

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

Induction Generator / Script Demo 17-04


Equipment

Wimshurst machine.

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

Demo 17-05 Kelvin Water Dropper


Figure 1

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

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

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

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

This animation shows how the generator works.

Assume a small charge imbalance initially exists between the cans. The canwhich is more positive is connected to this ring and the positive charge on thering 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 chargedcan. The charges on the two cans increase until the voltage is high enough toflash the neon lamp.

Kelvin Water Dropper / Script Demo 17-05


Equipment

1. Kelvin water dropper electrostatic generator built by soldering a brass rod up and away fromthe upper rim of two tin cans at a 45° angle, whose end is soldered to a brass ring. One hasa 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 beingbrought near the other rod, leaving an appropriate spark gap.

6. Supply of water.

This experiment demonstrates one effect of polarization of water molecules ina wooden beam.† A charged rod is held near a wooden beam that is mountedon 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 fieldis 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.

Demo 17-06 Wooden Needle


Figure 1

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

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.

Wooden Needle / Script Demo 17-06


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.

18

C H A P T E R 4 1

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19

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

Demo 17-07 Van de Graaff Generator

20 C H A P T E R 4 1 : E L E C T R I C F I E L D S

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, ElectrostaticGenerator.

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

A rubber belt passes over a pair of rollers, with the bottom roller driven by amotor. The upper roller is contained inside a smooth hollow metal sphere. Ahigh 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 combwhich is electrically connected to the sphere. The charge is pulled off the beltby the comb and moves to the outside of the sphere. Since the electric fieldinside the charged sphere is nearly zero, the comb can continue to removecharge even after there is a large amount of charge on the sphere. The voltageon the outside of the sphere continues to increase until it reaches the break-down voltage of the surrounding air.

Van de Graaff Generator / Script Demo 17-07

C H A P T E R 4 1 : E L E C T R I C F I E L D S 21

Equipment

1. Van de Graaff electrostatic generator. The demonstration is primarily a theoretical animationexplaining how a Van de Graaff electrostatic generator works.

2. AC power.

In this demonstration a group of paper streamers are positioned around theVan 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.

Demo 17-08 Van de Graaff with Streamers


Figure 1

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

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

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

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

Van de Graaff with Streamers / Script Demo 17-08


Equipment

1. Van de Graaff electrostatic generator with the sphere equipped with an array of low masspaper 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.

This demonstration illustrates the difference between the electric field createdby a small sphere and that of a sharp point held near the dome of a Van deGraaff.† The electric field generated near a charged surface increases as theradius of the surface becomes smaller. For the case of a small sphere, the fieldbuilds 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 endof the wand quickly becomes very large as the dome begins to charge. The airtherefore breaks down continuously at a very low potential difference betweenthe dome and the wand, and the charge on the dome never rises to a value atwhich a large spark can occur.

Demo 17-09 Van de Graaff and Wand


Figure 1

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

We’ll use this Van de Graaff high voltage generator and a grounded metalwand to illustrate how electrical discharge differs between sharp and bluntobjects.

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

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

When the pointed end of the wand is brought near, the streamers collapse andstay down. The sharp point drains the charge off the sphere without visiblesparks.

Van de Graaff and Wand / Script Demo 17-09


Equipment

1. Van de Graaff electrostatic generator with its sphere equipped with an array of low masspaper streamers.

2. Electrically grounded metal wand with a small sphere on one end and a sharp point on theother end.

3. Long grounding wire.4. AC power.

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

Demo 17-10 Electric Field


Figure 1

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

We’ll use this apparatus to show the electric field configuration around variouscharged 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 electricfield.

If we put this small metal cylinder on top of the chamber and charge it usingthis 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 whichis positive and the other negative.

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

Electric Field / Script Demo 17-10


Equipment

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

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 andcause 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 createdby 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 chargeis collected at a low potential, and the lightning discharge ceases.

Demo 17-11 Lightning Rod


Figure 1

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

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

One side of the generator is connected to a metal ball inside the chimney. Thissharp rod is also electrically connected to the same side of the generator. Theother end of the generator is hooked to this copper sphere, which simulates acharged cloud in the atmosphere.

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

When the lightning rod is raised, the bolts stop.

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

Lightning Rod / Script Demo 17-11


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 theassociated 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 otherterminal 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 thesystem build up a much higher voltage before producing the lightning bolt.

A pinwheel is the electrostatic version of a water sprinkler, as shown in Figure1, but it works in a very different way.† This demonstration is often incorrectlyexplained 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 inDemonstrations 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 tipsof the pinwheels. The electrostatic repulsion between the charged tips of thepinwheel and the charged clouds of gas then propels the pinwheel.

Demo 17-12 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.

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 pointsof the pinwheel. The discharge ionizes nearby air then repels it strongly fromthe point, making the wheel spin.

Pinwheel / Script Demo 17-12


Equipment

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

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 effectis due to Coulomb repulsion or attraction between the charge of the point andthe positive ions in the flame.

Demo 17-13 Point and Candle


Figure 1

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

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

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

Point and Candle / Script Demo 17-13


Equipment

1. Toepler-Holtz electrostatic generator, with one terminal equipped with a very sharp pointprojecting out horizontally.

2. Candle on a long non-conductive handle.3. Source of flame.

This demonstration illustrates Gauss’ law: the electric field inside a conductingsurface must be zero.† An electroscope is inductively charged by holding acharged rod a few inches above the plate on the top of the electroscope, asseen 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 thecharged rod is moved close to the electroscope plate.

Demo 17-14 Faraday Cage


Figure 1

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

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 ofthe rod attracts negative charges to the top of this support arm.

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

We’ll place this grounded metal cage over the top of the electroscope andrepeat 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.

Faraday Cage / Script Demo 17-14


Equipment

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

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 regularold mop bucket, isolated from ground, is charged to a high potential using aWimshurst machine. Charge is then “scraped” off the outside of the pail usingsmall conducting spheres and deposited on an electrometer, causing the elec-trometer to deflect and thus indicating the presence of charge on the outsideof the pail, as seen in Figure 1. When a similar procedure is used to scrapecharge off the inside of the bucket, it is found that no charge is obtained, thusdemonstrating that the charge resides on the outside of the conductor.

Demo 17-15 Faraday Ice Pail


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 onConductors.

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 aplastic rod to find out how the charge is distributed.

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

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

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

Faraday Ice Pail / Script Demo 17-15


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.

If an electrical discharge is created within a confined volume of smoke, thesmoke particles will develop a net charge. If a plate of the opposite chargeexists 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, iscalled an electrostatic smoke precipitator.

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

Demo 17-16 Smoke Precipitation


Figure 1

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

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

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

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

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

Smoke Precipitation / Script Demo 17-16


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.

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

Demo 17-17 Electron Discharge Tube with Wheel


Figure 1

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

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

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

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

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

Electron Discharge Tube with Wheel / Script Demo 17-17


Equipment

1. Crooke’s tube with a fluorescent paddle wheel and horizontal roller track; and its supportstand.

2. Appropriate electrical leads.3. Induction coil with reversing switch.4. Battery eliminator.

42

C H A P T E R 4 2

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D C C I R C U I T S

43

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 resistivityare shown in this demonstration, which is seen in Figure 1. The resistance R isdeduced by measuring the current I in each wire when the same voltage V isindividually applied to each wire, using Ohm’s law:

R =

V

I

Demo 17-18 Resistance Wires

44 C H A P T E R 4 2 : R E S I S T A N C E A N D D C C I R C U I T S

Figure 1

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

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

We’ll apply a constant voltage to each and measure the current that flowsthrough 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 currentthrough C at a constant 6 volts.

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

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 twicethe 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 ishalf the length of C and E is one-fourth the length of C. Here is the currentthrough D.

Here is the current through E.

Resistance Wires / Script Demo 17-18

C H A P T E R 4 2 : R E S I S T A N C E A N D D C C I R C U I T S 45

Equipment

1. Resistance board—a vertical board supporting five wires of differing resistivities, diameters,and lengths.

2. Battery.3. Ammeter.4. Appropriate electrical leads.

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

V = I R

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

Demo 17-19 Ohm’s Law


Figure 1 Figure 2

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

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 thatflows in each case with this ammeter.

Here is the current at 2 volts.



Ohm’s Law / Script Demo 17-19


Equipment

1. Rheostat.2. Battery3. Voltmeter.4. Ammeter.5. Appropriate electrical leads.

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, itsresistance increases. The voltage across the coil therefore increases, so thevoltage across the light bulb decreases, and the bulb becomes dimmer, asshown in Figure 1. Iron has a positive temperature coefficient of resistance, asdo most conductors.

Demo 17-20 Heated Wire


Figure 1

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

This iron wire in series with this small lamp will be used to demonstrate theeffect 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 andthe bulb dims.

Heated Wire / Script Demo 17-20


Equipment

1. Small lamp wired in series with a “flatted” loop (arranged without shorting on an insulatedstand) of iron wire, held just above a gas heating element that heats the majority of the coilat 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.

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, iscooled in a liquid nitrogen bath, its resistance decreases. The voltage acrossthe coil therefore decreases, so the voltage across the light bulb increases, andthe bulb becomes brighter, as shown in Figure 1. Copper has a positive tem-perature coefficient of resistance, as do most conductors.

Demo 17-21 Cooled Wire


Figure 1

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

This coil of copper wire in series with this lamp will be used to show theeffect 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 thebulb increases dramatically.

Cooled Wire / Script Demo 17-21


Equipment

1. Coil of thin copper wire in series with a 6-volt lamp with both mounted on opposite ends ofa non-conductor.

2. Battery eliminator.3. Dewar of liquid nitrogen.4. AC power.

This demonstration uses models of electron motion to illustrate two types ofelectron current.† If electrons are accelerated across a potential differencethrough a vacuum, they will continue to accelerate as they move, similar to thecase of small spheres rolling unimpeded down an inclined plane. On the otherhand, in a resistive medium, such as a wire, the conduction electrons interactwith the atomic electrons around the atoms of the wire, limiting the speed withwhich the electron current moves. This can be simulated by spheres rollingdown an inclined plane in which a large number of nails have been randomlyinserted, as shown in Figure 1.

Demo 17-22 Electron Motion Model


Figure 1

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

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

We’ll use these ball bearings to simulate electrons and show the differencesbetween 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 tothe atoms in a piece of metal. When the bearings are added and the board istilted as before, collisions with the nails slow the bearings, just as collisionswith atoms slow the progress of electrons through a metal.

Electron Motion Model / Script Demo 17-22


Equipment

1. Vacuum/atomic array analog—made by dividing a square piece of plywood in half and byadding rectangular walls on the same side of the plywood, driving a sizable number of smallfinish nails into one section.

2. Pivot.3. Large number of steel ball bearings.

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 flowthrough both, so the voltage of the circuit is divided between them, and lesstotal 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 ineach, and the total current is two times the current through a single resistor.

Demo 17-23 Series/Parallel Resistors


Figure 1 Figure 2

Figure 3

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

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.

Series/Parallel Resistors / Script Demo 17-23


Equipment

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

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

Demo 17-24 Series/Parallel Light Bulbs


Figure 1

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

Here are three light bulbs. We’ll hook the first bulb up to house current, thenhook 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 inseries with it, one by one. How will the brightness of the bulbs change?

The bulbs dim as each new bulb is added.

Series/Parallel Light Bulbs / Script Demo 17-24


Equipment

1. Three light bulbs and lamp sockets, along with banana plug sockets, mounted on a boardwith a power cord wired in parallel.

2. Three light bulbs and lamp sockets, along with banana plug sockets, mounted on a boardwith a power cord wired in series.

3. Appropriate electrical leads.4. AC power.

A Wheatstone bridge is a device that can be used to measure resistances veryaccurately.† If three of the resistances are very well known, the resistance of afourth (unknown) resistance can be determined very accurately. The circuit isshown 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 resistorswhen the variable resistance is adjusted so that no current flows in the lightbulb which is wired across the diamond. The resistance of the unknown isthen equal to the value of the variable resistance.

Demo 17-25 Wheatstone Bridge


Figure 1 Figure 2

† Sutton, Demonstration Experiments in Physics, Demonstrations E-155, Wheatstone-bridgeNetwork, 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.

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 bulbconnected across the top and bottom corners.

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

We’ll put 110 volts AC across the right and left corners of the bridge, then slidethe variable resistor back and forth. When the slide is at an extreme position tothe 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 theunknown resistor can easily be calculated in terms of the two known resis-tances and the value of the variable resistance.

Wheatstone Bridge / Script Demo 17-25


Equipment

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

2. AC power.

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

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

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

Demo 17-26 Galvanometer as Voltmeter and Ammeter


Figure 1

Figure 3

Figure 2

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

This galvanometer will be used to show the electrical connections that changea 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 theammeter, like this. The reading now goes nearly to zero. Because the meternow requires 1 amp to go to full-scale deflection. If we increase the current weagain 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 thevoltage of this 1.5-volt battery.

A 6-volt battery produces a larger deflection.

Galvanometer as Voltmeter and Ammeter / Script Demo 17-26


Equipment

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

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 tozero; that is, the current entering the junction must be equal to the currentleaving the junction.† This rule, known as Kirchhoff’s junction rule, is demon-strated on the video using the apparatus shown in Figure 1.

Demo 17-27 Conservation of Current


Figure 1

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

The amount of current entering a circuit junction or node must equal theamount 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 circuitnode.

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 flowingout.

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

Conservation of Current / Script Demo 17-27


Equipment

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

Documents

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