the video encyclopedia of physics demonstrations

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 I X T E E N

Chapter 36 Kinetic Theory

Demo 16-01 Pressure vs. Volume.......................................................6

Demo 16-02 Pressure vs. Temperature ..............................................8

Demo 16-03 Temperature Increase Simulation................................10

Demo 16-04 Pressure vs. Volume Simulation ..................................12

Demo 16-05 Equipartition of Energy Simulation .............................14

Demo 16-06 Mercury Kinetic Theory ...............................................16

Demo 16-07 Brownian Motion .........................................................18

Demo 16-08 Brownian Motion Simulation.......................................20

Demo 16-09 Diffusion .......................................................................22

Demo 16-10 Diffusion Simulation ....................................................24

Demo 16-11 Bromine Diffusion........................................................26

Demo 16-12 Gaussian Curve ............................................................28

Demo 16-13 Free Expansion Simulation ..........................................30

Chapter 37 Crystals and Low Temperatures

Demo 16-14 Superconductors...........................................................34

Demo 16-15 Crystal Models ..............................................................36

Demo 16-16 Faults in Crystal ............................................................38

Chapter 38 Thermoelectricity

Demo 16-17 Thermistor ....................................................................42

Demo 16-18 Thermoelectric Magnet ................................................44

Demo 16-19 Thermoelectric Heat Pump..........................................46

Demo 16-20 Thermocouple ..............................................................48

Chapter 39 Electric Charges

Demo 16-21 Electrostatic Rods .........................................................50

Demo 16-22 Electrostatic Rod and Cloth .........................................52

Demo 16-23 Electrostatic Ping-Pong Deflection ..............................54

Demo 16-24 Electrostatic Ping-Pong Balls .......................................56

Demo 16-25 Conductors and Insulators...........................................58

Demo 16-26 Piezoelectric Sparker....................................................60

5

C H A P T E R 3 6

K I N E T I C T H E O R Y

This demonstration uses a moveable plunger in a calibrated tube and a pres-sure gauge to illustrate Boyle’s law:

PV = Constant

Several values of pressure and volume are taken using the apparatus in Figure 1.

Demo 16-01 Pressure vs. Volume

6 C H A P T E R 3 6 : K I N E T I C T H E O R Y

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration M-319, Compressibility of Air—Boyle’s Law. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Hg-1, Gas Lawwith Hypodermic Syringe.

This syringe is connected to a pressure gauge. We’ll use this apparatus todemonstrate the relationship between pressure and volume for a gas such asair.

The total volume of the air in the syringe and gauge can be read on the scaleat the side. We’ll start with a 20-cubic centimeter volume of air at normalatmospheric pressure, about 14.7 pounds per square inch.

When the volume is decreased to 10 cubic centimeters, the pressure increasesto about 29 pounds per square inch.

If the volume is instead increased to 40 cubic centimeters, the pressure dropsto about 7 pounds per square inch.

Pressure vs. Volume / Script Demo 16-01

C H A P T E R 3 6 : K I N E T I C T H E O R Y 7

Equipment

1. Boyle’s law apparatus.2. Overhead projector and screen.

A hollow sphere filled with air is connected to a pressure gauge. As the tem-perature of the air in the large sphere is varied, the pressure changes inverselyas the temperature:

Using the apparatus in Figure 1, data points are taken on the video at the tem-perature of boiling water, ice water, and room temperature. The curve of P asa function of T is extrapolated to zero pressure to determine the theoreticalvalue of absolute zero in centigrade degrees.

P

T= Constant

Demo 16-02 Pressure vs. Temperature


Figure 1

We’ll use this pressure gauge connected to a hollow copper sphere to demon-strate how the pressure of a gas varies with temperature.

With the air in the sphere at room temperature, 21 degrees celsius, the pres-sure is about 14.2 pounds per square inch.

When we dip the sphere in ice water to reduce its temperature to zero degreescelsius, the pressure decreases to 13 pounds per square inch.

Dipping the sphere in boiling water at 100 degrees celsius increases the pres-sure to 17.5 pounds per square inch.

If we mark these three points on a graph of pressure vs. temperature, theyappear to lie in a line. Extrapolating that line backwards indicates that thepressure of the gas would reach zero at approximately minus 270 degreescentigrade.

Pressure vs. Temperature / Script Demo 16-02


Equipment

1. Hollow copper sphere connected via a length of plastic tubing to a projection pressuregauge.

2. Overhead projector.3. Projection screen.4. Large beaker of ice water.5. Large beaker of boiling water.6. Ring stand and ring clamp.7. Meker burner.8. Length of rubber hose.9. Supply of natural gas.10. Source of flame.

Using small metal spheres to represent the atoms of a gas, the molecularmotion of a gas can be simulated.† As the kinetic energy of the “molecules” isincreased, the “temperature” is increased, as simulated using the apparatusshown in Figure 1.

Demo 16-03 Temperature Increase Simulation


Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Hh-5, MechanicalModel of Kinetic Motion.

We’ll use this molecular motion demonstrator to simulate the increasing speedof gas molecules with increasing temperature.

The outer frame shakes back and forth, causing spheres to move and collide ina manner similar to that of the molecules in a gas.

If we increase the shaking speed, the spheres move with a higher averagespeed. The average speed of gas molecules increases in much the same way asthe temperature of the gas is increased.

Temperature Increase Simulation / Script Demo 16-03


Equipment

1. Molecular motion demonstration (commercially available) and its collection of associatedspheres.

2. Leveling platform for the overhead projector.3. Overhead projector and screen.4. Source of power.5. Two electrical leads.

This demonstration shows a model that uses small metal balls to simulate gasmolecules.† As the volume of the container decreases and the average speed ofthe “molecules” increases, the rate at which the “molecules” strike the sides ofthe container increases, so the “pressure” in the container increases, as shownin the video. The apparatus, shown in Figure 1, is available commercially froma number of scientific supply houses.

Demo 16-04 Pressure vs. Volume Simulation


Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Hh-4, Two-Dimensional Kinetic Motion.

We’ll use this molecular motion demonstrator to simulate the increase in pres-sure of a sample of gas when its volume is decreased.

The motion of the spheres in the frame corresponds to the motion of the gasmolecules. The pressure on the side of the frame caused by spheres strikingthe frame simulates the pressure of a gas on the walls of its container.

When we decrease the volume inside the frame, the spheres strike the framemore frequently and at higher speeds, simulating the increase of pressure in agas when the volume is reduced.

Pressure vs. Volume Simulation / Script Demo 16-04


Equipment

Same as Demonstration 16-03 with the addition of a bar with magnets bonded to both ends sothe total length just fits inside the vibrating framework on the demonstrator.

This demonstration uses two sizes of small metal balls to simulate gases of var-ious molecular weight.† Because all balls develop the same average kineticenergy, the smaller balls move at a higher average speed, as can be seen usingthe apparatus of Figure 1.

Demo 16-05 Equipartition of Energy Simulation


Figure 1


We’ll use these spheres moving in a frame to simulate the equipartition ofenergy among gas molecules of different mass.

These spheres all weigh the same, and travel at a low average speed. Whenwe add a set of lighter spheres to the frame, they travel at a higher averagespeed. The average kinetic energy in the motion of the lighter spheres isapproximately equal to the average energy in the heavier spheres.

Equipartition of Energy Simulation / Script Demo 16-05


Equipment

Same as Demonstration 16-03 but with medium plastic spheres and the addition of small plasticspheres.

A sealed test tube contains mercury under vacuum and small bits of glass thatfloat on the mercury. As the mercury is heated, it begins to boil, and energeticmercury vapor causes the bits of glass to jump wildly about in the upper partof the tube, illustrating that the heated mercury atoms develop a great deal ofkinetic energy.† The increased agitation of the mercury, easily seen in the video,is shown in Figure 1.

Demo 16-06 Mercury Kinetic Theory


Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Hh-1, Model forKinetic Theory of Gases.

We’ll use this glass tube containing glass chips floating in a small amount ofmercury to demonstrate kinetic theory.

When the mercury is heated over a gas flame, the mercury boils rapidly. Theenergetic mercury vapor pushes some of the glass chips up into the air, wherethey dance vigorously.

Mercury Kinetic Theory / Script Demo 16-06


Equipment

1. Enclosed glass tube containing colored glass chips and a small amount of mercury.2. Ring stand and test tube clamp.3. Meker burner.4. Length of rubber tubing.5. Supply of natural gas.6. Source of flame.7. Shadow project:

a. light sourceb. lensc. projection screen

Brownian motion is the random atomic and molecular motion always presentin any gas or liquid. It can be rendered visible by inserting smoke into a smallcell and watching the motion of the smoke particles resulting from collisionswith the air molecules.† A microscope can be used to see the motion of thesmoke particles, which are shown in Figure 1.

It should be noted that any change of the motion of the smoke particle cannotbe obtained by a collision with one atom of gas. It requires a statistical net ofabout 104 collisions, with air molecules of the average molecular speed, onone side of the smoke particle relative to the other side to modify the motionof your average smoke particle.

Demo 16-07 Brownian Motion


Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Hh-3, BrownianMotion.

Brownian motion provides direct evidence of the existence of atoms and mole-cules using nothing more complicated than an ordinary microscope.

We’ll fill this small chamber with smoke particles from a match, then watch theparticles under a magnification of 100 times. Notice the random jiggling motionof the smoke particles, caused by air molecules colliding with the much largerparticles of smoke.

Brownian Motion / Script Demo 16-07


Equipment

1. Microscope.2. Light source.3. Smoke cell fitted with a squeeze bulb on one side and a cork on the other side.4. Matches.

This simulation uses a molecular motion model with small metal balls to simu-late gas molecules. If a larger disc is placed in the simulator along with theballs, the motion of the balls when they strike the larger object will jostle thelarger object about, simulating molecular motion as in Figure 1.

The observer should not be deceived by this rather simple model into believ-ing that Brownian motion for the case of smoke particles is this simple. In real-ity, because the air molecules are so much smaller relative to the smokeparticles than this model illustrates, it requires a net number of about 104 airmolecules striking one side of the smoke particle relative to the other side tocreate any change in the motion of the smoke particle.

Demo 16-08 Brownian Motion Simulation


Figure 1

We’ll use these small spheres moving in a frame to simulate Brownian motion.

The spheres represent molecules of a gas in random motion. When we add alarger disc to the frame to simulate a large smoke particle in the gas, the disc isjostled side to side just as a smoke particle is jostled by the motion of thesmaller molecules in a surrounding gas.

Brownian Motion Simulation / Script Demo 16-08


Equipment

Same as Demonstration 16-03 adding a dozen or so small spheres and a larger plastic disc.

Small molecules can move in the space between porous materials by a processknown as diffusion. The process of diffusion is illustrated on this video formethane gas and for helium using a porous clay jar.† It is also shown on thevideo that air diffuses very slowly through the clay jar material. The gases dif-fuse through the jar into the glass tube, forming bubbles in the beaker at thebottom of the tube, as seen in Figure 1.

Demo 16-09 Diffusion


Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstrations Hi-1, Diffusionof Hydrogen, and Hi-2, Diffusion of CO2.

This porous clay jar will be used to demonstrate diffusion of gas moleculesthrough a porous barrier.

The clay jar is originally filled with air. A long glass tube extends down fromthe jar into a beaker of water. When another beaker is placed over the clay potand filled with methane gas at atmospheric pressure, the pressure inside theclay pot increases and pushes gas bubbles out of the end of the tube.

When the surrounding beaker is filled with helium, gas bubbles out very vigor-ously.

If air is now blown back into the beaker, the pressure inside the pot drops dra-matically and draws water back up the tube.

This animation shows how the smaller molecules of natural gas and heliumpenetrate the porous walls of the jar more easily than the larger air moleculescan escape. That increases the number of gas molecules inside the jar, andthus the pressure.

Diffusion / Script Demo 16-09


Equipment

1. Porous clay jar. The jar is fitted with an airtight cap with a long glass tube projecting throughit.

2. Support system to hold the jar upside down inside a glass beaker so the glass tube is placedinto another beaker with water in it.

3. Three long lengths of rubber tubing.4. Source of natural gas.5. Source of helium.6. Source of compressed air.

Diffusion through a porous material is simulated using the molecular motionsimulator and a baffle with a small hole.† Small molecules will readily passthrough the hole, but larger molecules will not, as shown in Figure 1, whichwas made after a long shaking time.

Demo 16-10 Diffusion Simulation


Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Hh-4, Two-Dimensional Kinetic Theory.

These small spheres moving in a frame will be used to simulate diffusion of agas through a porous barrier.

We’ll put in this barrier which has a small gap in the center.

Now we’ll put all the large and small spheres on the same side of the barrier.

When we shake the frame to jostle the spheres, the small spheres easily escapethrough the gap in the barrier, while the larger spheres cannot move through.The small spheres are analogous to small gas molecules, which can diffusemore easily through a porous barrier than larger gas molecules can.

Diffusion Simulation / Script Demo 16-10


Equipment

Same as Demonstration 16-03 with the addition of a bar that divides the vibrating framework ofthe demonstrator in half, but has an opening near the bottom edge of the bar midway along itslength; with a collection of large and small spheres.

Any molecule interacts with the surrounding air molecules as it diffusesthrough the air, limiting the speed with which it diffuses.† Conversely, the mol-ecules will diffuse through a partially evacuated tube at a much faster speed.By dipping the tubes into liquid nitrogen, the bromine gas is initially collectedat the bottom end of two tubes, one containing air at atmospheric pressureand one partially evacuated.‡ When the tubes are allowed to warm, thebromine rapidly fills the evacuated tube (right), but diffuses slowly through theair-filled tube (left), as illustrated in Figure 1.

Demo 16-11 Bromine Diffusion


Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Hi-4, Diffusion ofBromine.

‡ Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Hj-9, BromineCryophorus.

When a liquid evaporates and becomes a gas, the gas diffuses outward into itssurroundings. We’ll use these two bromine tubes to show how that diffusioncan be affected by the presence of other gasses.

This tube contains only bromine, while this tube contains a mixture ofbromine and air.

The bromine is liquified in the bottom of both tubes by dipping them in liquidnitrogen.

When the tubes are allowed to warm and the bromine evaporates, there is aclear difference in the way the bromine diffuses into a vacuum and the way itdiffuses into air.

Bromine Diffusion / Script Demo 16-11


Equipment

1. Two glass tubes, one containing bromine only, the other containing bromine and air (usecaution).

2. Ring stand.3. Two right-angle clamps.4. Two three-fingered clamps.5. Dry ice/alcohol bath, or a dewar of liquid nitrogen.6. Gloves.

A commercially available device is used to illustrate the Gaussian curve. As alarge number of small metal balls falls through the opening at the top of thedevice, each ball encounters a maze of spikes that direct the ball randomly toeither side of the spike. The result is that the number of balls in the bins at thebottom of the device on the average follow the “Gaussian curve,” shown inFigure 1.

Demo 16-12 Gaussian Curve


Figure 1

This device has many equally spaced steel pins set inside a chamber. Whenthe chamber is tipped, hundreds of small steel balls roll down from the top.The balls collide with the pins and are scattered to both sides as they rolldown landing in small chambers at the bottom.

The resulting distribution of balls approximates a curve, known as a Gaussiancurve.

Gaussian Curve / Script Demo 16-12


Equipment

1. An array of steel pins located above a series of vertical columns, both situated just below afunneling chamber that “feeds” small steel spheres down through the pins and into thecolumns; all of which is enclosed with clear plastic and held on an incline.

2. Overhead projector and screen.

The molecular model is used to illustrate the concept of free expansion.† In afree expansion the average energy of the molecules remains the same. In thevideo, as illustrated in Figure 1, the “molecules” initially moving about in one-half of the volume of the simulator are allowed to move into the second halfby removing the barrier. In this process the average molecular speed remainsunchanged.

Demo 16-13 Free Expansion Simulation


Figure 1


We’ll use this molecular motion simulator to show the free expansion of a gasinto a vacuum.

At first the balls are moving in only half the frame. When we remove the cen-ter barrier, the balls expand freely to the other half of the frame.

Free Expansion Simulation / Script Demo 16-13


Equipment

Same as Demonstration 16-03 with the addition of a solid barrier bar, dividing the vibratingframework, which can be easily removed.

C H A P T E R 3 7

C R Y S T A L S A N D

L O W T E M P E R A T U R E S

33

This demonstration shows the levitation of a tiny rare earth magnet over ahigh-temperature superconductor. The disc of superconducting material iscooled by immersing it in a liquid nitrogen bath, as shown in Figure 1. A num-ber of recent articles, along with numerous references given therein, discussboth the theoretical and practical aspects of modern, high-temperature super-conducting materials.†

Demo 16-14 Superconductors

34 C H A P T E R 3 7 : C R Y S T A L S A N D L O W T E M P E R A T U R E

Figure 1

† P. J. Ouseph, Levitation of a Magnet over a Superconductor, The Phys. Teach. 28, 205-209(1990). Robert J. Birgeneau, The Richtmeyer Memorial Lecture (January 1989): Novel magneticphenomena and high-temperature superconductivity in lamellar copper oxides, Am. J. Phys. 58,28-40 (1990).

This disc is made of a superconducting material that conducts current with noresistance at low temperatures.

When the disc is cooled in liquid nitrogen, a small powerful magnet will levi-tate above the disc.

Superconductors / Script Demo 16-14

C H A P T E R 3 7 : C R Y S T A L S A N D L O W T E M P E R A T U R E 35

Equipment

1. Super conducting kit.2. Supply of liquid nitrogen

This demonstration shows several crystal models: sodium chloride, calciumcarbonate, and carbon, in the form of graphite and of diamond.

Demo 16-15 Crystal Models


Figure 1

Here are models of different crystals, with small spheres of different sizes andcolors representing different types of atoms in the structures.

This is a model of the structure of sodium chloride, with the sodium atomsrepresented by the white spheres, and chlorine by the red.

This is a model of a calcium carbonate, or calcite crystal, with the calciumatoms represented by the grey spheres, carbon by the black, and oxygen bythe red spheres.

Pure carbon can assume more than one crystal structure. This is graphite, aform in which the atoms are strongly connected in a common plane, with onlyweak connections to the plane below.

Another crystal structure of carbon is diamond, in which each carbon atom isconnected symmetrically to four others in a tetragonal shape.

Crystal Models / Script Demo 16-15


Equipment

Crystal models made from various sizes of wooden spheres and metal rods:a. sodium chlorideb. calcium carbonatec. graphited. diamond

A monolayer of metal spheres forms a simulation of a crystal, with each sphererepresenting one of the lattice sites in the crystal. When the crystal is formedby shaking the spheres and allowing them to settle, fault lines appear alongwhich the crystal has been imperfectly joined, as is readily seen in the videoand shown in Figure 1.

Demo 16-16 Faults in Crystal


Figure 1

Most natural crystals have fault planes running through them. At these faultplanes the crystals are easy to cleave.

This model shows how crystal faults develop. Ball bearings represent the indi-vidual atoms in a crystal. They can stack together quite cleanly for the mostpart, but occasionally a fault line will develop in the array.

Faults in Crystal / Script Demo 16-16


Equipment

1. Calcite crystal.2. Model fabricated by separating two sheets of glass totally around all edges with appropriate

sized shim stock and then loading with a high number of small metal spheres. Then bond alltogether so the spheres are free to move, but not escape.

3. Shadow project with an overhead projector and projection screen.

C H A P T E R 3 8

T H E R M O E L E C T R I C I T Y

41

A thermistor is a passive component, whose resistance changes with tempera-ture. In combination with the appropriate electronics it can therefore be usedto measure temperature.† In this demonstration the resistance of a thermistor ismeasured at different temperatures, as illustrated in Figure 1. The thermistorhas a negative coefficient of resistivity, that is, its resistance decreases as thetemperature increases.

Demo 16-17 Thermistor

42 C H A P T E R 3 8 : T H E R M O E L E C T R I C I T Y

Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-169, Electric Thermometers.

The electrical resistance of most materials increases when they are heated.Here is a device known as a thermistor that shows the opposite effect.

At room temperature the thermistor has a resistance of about 12,000 ohms, asshown on this ohm meter.

When the thermistor is heated by squeezing it between two fingers its resis-tance decreases dramatically.

Dipping the thermistor in ice water to cool it increases its resistance.

Thermistor / Script Demo 16-17

C H A P T E R 3 8 : T H E R M O E L E C T R I C I T Y 43

Equipment

1. Thermistor.2. Two clip leads.3. Multimeter with ohm meter capacity.4. Cup of ice water.

A thermoelectric junction uses the temperature difference across the boundarybetween two appropriate different conductors to .generate a large electric cur-rent.† In this demonstration the thermoelectric current generated by such ajunction is used to power a large magnet, which in turn is used to hold upheavy weights, as seen in Figure 1.

Demo 16-18 Thermoelectric Magnet


Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstrations E-179, Thermoelectric Effect—Seebeck Effect, and E-182, Thermoelectric Magnet. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Et-3,Thermoelectric Magnet.

This pair of different metals has been bonded together to form a large thermo-couple.

When one side of the thermocouple is heated and the other cooled, an electri-cal current results. The current will be used to run this electromagnet and holdup a large weight.

The current from the thermocouple easily holds the weight on the magnet. Ifthe heat and ice water are removed, the magnet soon drops the weight.

Thermoelectric Magnet / Script Demo 16-18


Equipment

1. Large thermocouple.2. Support system to suspend the thermocouple and hold the other components.3. Beaker of ice water.4. Bunsen burner.5. Length of rubber hose.6. Supply of natural gas.7. Source of flame.8. Thermocouple face plate with hook.9. Weight hanger.10. Several heavy slotted weights.

The inverse of the process used in a thermoelectric generator can be used totransfer heat across a thermoelectric junction.† In this demonstration, shown inFigure 1, a current is passed through the junction, cooling one side of thejunction and heating the other side; heat is being transferred from one side tothe other. When the current is reversed, the heating and cooling reverse.

Demo 16-19 Thermoelectric Heat Pump


Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstrations E-180, Peltier Effect, and E-181,Seebeck and Peltier Effects in One Thermocouple. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Et-2,Thermoelectric Cooler.

This thermoelectric heat pump consists of many pairs of two different types ofmaterial wired in series.

Aluminum blocks on either side of the pump act as reservoirs for heat whichwill be pumped from one block to the other. The temperature of both sides isinitially the same.

When we run an electrical current through the heat pump, the block on theleft becomes colder while the block on the right heats up.

When we reverse the direction of the current, the block on the right becomescolder while the block on the left heats up.

Thermoelectric Heat Pump / Script Demo 16-19


Equipment

1. Thermoelectric heat pump assembly.2. Battery eliminator.3. Two electrical leads.4. Two thermometers.5. Two aluminum blocks fitted to accommodate the thermometers.

A thermocouple consists of a junction (or a collection of junctions in seriesand/or parallel) between two different metals.† The two metals are chosensuch that a voltage is generated across the junction which is a function of thetemperature of the junction. Such a thermocouple, shown in Figure 1, can becalibrated and used to measure temperature, as demonstrated in the video.

Demo 16-20 Thermocouple


Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-179, Thermoelectric Effect—Seebeck Effect. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Et-1,Thermocouple.

This pair of different metals has been twisted together to form a simple thermo-couple.

If we hook the thermocouple to an ammeter and place the tip near a flame,current flows through the ammeter.

The closer the thermocouple gets to the flame, the more current flows.

Thermocouple / Script Demo 16-20


Equipment

1. Large thermocouple.2. Lecture table galvanometer.3. Meker burner.4. Length of rubber tubing.5. Supply of natural gas.6. Source of flame.

C H A P T E R 3 9

E L E C T R I C C H A R G E S

51

When an acrylic rod is rubbed with wool the rod develops a positive charge,and when a rubber rod is rubbed with wool it develops a negative charge.†

Rods charged by these procedures are used in this demonstration to illustratethat charges of the same sign repel each other and charges of the oppositesign attract each other, using one hand-held rod and one mounted on a rota-tor, as seen in Figure 1.

Demo 16-21 Electrostatic Rods

52 C H A P T E R 3 9 : E L E C T R I C C H A R G E S

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-1, ElectrostaticCharges.

When one material comes in contact with another material, electric charge canbe transferred from one to the other.

We’ll use these rods and a wool cloth, to demonstrate the effect. The cloth isfirst rubbed along an acrylic rod, which is set on a rotation stand. If a secondacrylic rod is rubbed with a cloth and brought near the first, the two rods repelone another because they both have the same type of charge.

Let’s repeat this demonstration using two rubber rods.

If we use one acrylic and one rubber rod, the two rods attract.

Electrostatic Rods / Script Demo 16-21

C H A P T E R 3 9 : E L E C T R I C C H A R G E S 53

Equipment

1. Two sets of plastic rods of differing composition (giving opposite charges).2. Wool cloth.3. Bearing pivot system on a stand with minimal friction.

When an acrylic rod is charged positive by rubbing it with wool the electronsmove to the wool with which it was rubbed.† Similarly, when a rubber rod ischarged negative by rubbing it with wool, the electrons come from the woolwith which it was rubbed. Thus the wool develops a negative charge whenrubbed on acrylic, and a positive charge when rubbed on rubber. This demon-stration illustrates the existence of these residual charges by demonstrating theforce between the rods and the wool with which they were rubbed, using theapparatus illustrated in Figure 1.

Demo 16-22 Electrostatic Rod and Cloth


Figure 1

† Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ea-1, ElectrostaticCharges.

Both of these acrylic rods have been rubbed with a wool cloth. They repel oneanother because each has a positive charge.

If a rubber rod is negatively charged by rubbing it with wool and is thenbrought near a positively charged acrylic rod, the two rods attract each otherbecause of their opposite charges.

But what type of charge develops on the cloth used to charge the rods? Whenthis acrylic rod is positively charged and placed on the stand, it is attracted bythe cloth.

This rubber rod and its cloth show the same effect. When any twomaterials are rubbed together they develop opposite types of charge.

Electrostatic Rod and Cloth / Script Demo 16-22


Equipment

Same as Demonstration 16-21.

If two isolated Ping-Pong balls, coated with conductive material, are chargedby the charged rods of the preceding two demonstrations, the Coulomb forcebetween them can be demonstrated.† Using the apparatus shown in Figure 1,the video demonstrates that the force between two balls with the same chargeis repulsive, but the force between two balls with the opposite charge is attrac-tive.

Demo 16-23 Electrostatic Ping-Pong Deflection


Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-7, Force between Charges—Coulomb’s Law. Freier and Anderson, A Demonstration Handbook for Physics, Demonstrations Ea-5,Electrostatic Repulsion, and Ea-6, Repulsion and Attraction.

These pairs of Ping-Pong balls coated with conductive paint will be used toshow electrostatic attraction and repulsion.

This pair of balls hang on wires from a metal support, so they are electricallyconnected. If we touch a negatively charged rod to the metal, both ballsacquire negative charge and separate because of the repulsive forces betweenthem.

This pair of balls is electrically insulated from each other—we’ll touch onewith a positively charged rod and the other with a negatively charged rod. Theballs acquire opposite charges and attract each other, until they touch and neu-tralize their charges.

Electrostatic Ping-Pong Deflection / Script Demo 16-23


Equipment

1. Two pairs of Ping-Pong balls that have been coated with electrically conductive paint andsuspended with very low mass electrically conductive rods from pivot systems where one iselectrically conductive and the other is non-conductive.

2. Two rods to produce opposite charges.3. Wool cloth.

If a Ping-Pong ball, coated with conducting paint, is placed between the platesof a large parallel plate capacitor, as shown in Figure 1, the ball moves backand forth rapidly between the plates. When the ball touches one plate itreceives the charge of that plate and is immediately attracted to the other plate.When it touches the second plate its initial charge is replaced by the oppositecharge and it is immediately attracted to the first plate, whence the process isrepeated, as presented on the video.

Demo 16-24 Electrostatic Ping-Pong Balls


Figure 1

This Wimshurst machine is used to produce very high voltages. One side ofthe generator becomes positive, the other negative, until the voltage betweenthe electrodes is great enough to ionize the air between them.

We’ll use the Wimshurst to put opposite charges on a pair of metal plates.

A small pith ball tethered by a thread shows the attractive force between theopposing charges—it picks up a small amount of charge from the bottom plateand is lifted toward the top plate by an electrostatic force. If we drop a con-ductive ball through this tube and into the space between the plates, the ballbounces energetically between the two plates, transferring charge betweenthem.

When the Wimshurst machine stops, the plates are rapidly neutralized by thebouncing ball. When the charge is mostly gone, the ball stops.

Electrostatic Ping-Pong Balls / Script Demo 16-24


Equipment

1. Wimshurst machine.2. Pair of horizontal electrically isolated aluminum discs, with retaining rods (non-conducting)

spaced around the upper rim of the lower disc at intervals less than the diameter of a Ping-Pong ball.

3. Small weight attached to a pith ball by a short tether so its total length is approximately halfthe distance of separation for the metal discs.

4. Two clip leads.5. Non-conductive injection tube whose inside diameter is somewhat greater than the diameter

of the Ping-Pong ball.6. Several Ping-Pong balls with an electrically conductive coating.

A conducting rod and an insulating rod are mounted on an electrometer, asshown in Figure 1. When the conducting rod is contacted by a separatecharged rod, the charge immediately flows to the electrometer, as shown in thevideo. When the insulating rod is contacted by a separate charged rod, nocharge flows, as can be observed by the lack of response of the electrometer.

Demo 16-25 Conductors and Insulators


Figure 1

This electroscope can detect the presence of electric charge. When a chargedrod touches the top of the electroscope, the pointer deflects. We’ll use theelectroscope to compare the way charges move in different materials.

A rod with two arms is plugged into the top of the electroscope. The right armis an aluminum rod, while the left arm is acrylic. When we touch a chargedrod to the aluminum arm, the electroscope deflects, showing that charge hasmoved through the aluminum arm. Aluminum is an electrical conductor.

If we touch a charged rod to the acrylic arm, there is no deflection. Acrylic isan insulator and does not conduct electric charge.

Conductors and Insulators / Script Demo 16-25


Equipment

1. Electroscope 2. Plastic rod.3. Wool cloth.4. Electrical conductive/non-conductive “T,” made by joining an aluminum rod and an equal

length acrylic rod together at the “T’s” leg which is fitted to plug right into the electroscope.

The piezoelectric effect uses stress in certain crystals to generate an electricpotential. One type of piezoelctric device is the piezoelectric sparker, in whichthe stress in the crystal generates a spark, as shown in Figure 1 and on thevideo. Such sparkers are used in the laboratory to start gas burners, and arecommonly used in automatic starters for gas home appliances.

Demo 16-26 Piezoelectric Sparker


Figure 1

† Sutton, Demonstration Experiments in Physics, Demonstration E-186, Piezoelectric Effect inRochelle Salt Crystals. Freier and Anderson, A Demonstration Handbook for Physics, Demonstration Ea-9,Piezoelectric Pistol.

This device contains a small piezoelectric crystal which can be squeezed bypressing down on this lever arm.

When the arm is pressed, the high voltage generated in the crystal causes aspark to jump between these two electrodes.

When the crystal is released, another spark jumps.

This animation shows the voltage being generated by the crystal.

Piezoelectric Sparker / Script Demo 16-26


Equipment

1. Piezoelectric demonstrator.2. Electroscope.3. Electrical lead.

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