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The Video Encyclopedia of Physics Demonstrations ™

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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
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