The
Experimental and Historical Foundations of Electricity(paperback, 268 pages; ISBN 978-0986492631)
The
Experimental and Historical Foundations of Electricity
(paperback, 268 pages; ISBN 978-0986492631)
Andre Koch Torres Assis
In the early 1990’s I discovered the work of Norberto Cardoso Ferreira, of the Institute of Physics at the University of S˜ao Paulo, USP, Brazil. One of his research interests was to experimentally demonstrate the most important aspects of electricity utilizing very simple and easily available materials. I had the opportunity to visit him at USP in 1993. During this visit he gave me a small set of experimental materials made of thin cardboard, plastic straws, tissue paper, paper fasteners, etc. He showed me how to perform the main experiments and also showed me his book Plus et Moins: Les Charges ´Electriques. I became fascinated with what I learned, realizing how it was possible to experimentally envision very profound physical phenomena dealing with easily found materials. I kept this material as a treasure for 10 years, but neither used nor developed it during this period. I am extremely thankful to Norberto Ferreira for what I learned from him. Recently I discovered other works by Ferreira, as always extremely rich and creative. I also learned during discussions with his students, like Rui Manoel de Bastos Vieira and Emerson Izidoro dos Santos.
In 2005 I met Alberto Gaspar and discovered his book Experiˆencias de Ciˆencias para o Ensino Fundamental.3 I also learned a great deal from his book and other of his works.
Between 2004 and 2007 I taught classes to high school science teachers in the Teia do Saber project of the Secretary of Education of the State of S˜ao Paulo, in Brazil. It was a great privilege to be invited to participate in this project. The support I received from the Secretary of Education and from the Coordinating Group of Educational Projects of the University of Campinas, GGPE—UNICAMP, as well as the rich contacts with high school science teachers who took our classes, were extremely productive and stimulating for me. I also profited greatly from many exchanges of ideas with professors at the University of Campinas who participated in this project. As part of my activities, I decided to teach the high school science teachers what I had learned with Norberto Ferreira. As a result, I returned to the experiments with the further motive of writing this book, in order to share all this fascinating material with a wider audience.
Presentation and Acknowledgments 7 1 Introduction 11 2 Electrification by Friction 15 2.1 The Beginning of the Study of Electricity . . . . . . . . . . . . . 15 2.2 The Amber Effect . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.3 Exploring the Attraction Exerted by Rubbed Bodies . . . . . . . 20 2.4 Which Bodies are Attracted by the Rubbed Plastic? . . . . . . . 21 2.5 Is it Possible to Attract Liquids? . . . . . . . . . . . . . . . . . . 23 2.6 Gilbert and Some of His Electrical Experiments . . . . . . . . . . 25 2.7 What Rubbed Substances Attract Light Bodies? . . . . . . . . . 28 2.8 Gilbert’s Nomenclature: Electric and Non-Electric Bodies . . . . 29 3 The Versorium 33 3.1 Fracastoro’s Perpendiculo and Gilbert’s Versorium . . . . . . . . 33 3.2 Making a Versorium . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.2.1 Versorium of the First Kind . . . . . . . . . . . . . . . . . 36 3.2.2 Versorium of the Second Kind . . . . . . . . . . . . . . . . 37 3.2.3 Versorium of the Third Kind . . . . . . . . . . . . . . . . 40 3.3 Experiments with the Versorium . . . . . . . . . . . . . . . . . . 41 3.4 Is it Possible to Map the Electric Force? . . . . . . . . . . . . . . 43 3.5 Is There Action and Reaction in Electrostatics? . . . . . . . . . . 46 3.6 Fabri and Boyle Discover Mutual Electrical Action . . . . . . . . 50 3.7 Newton and Electricity . . . . . . . . . . . . . . . . . . . . . . . . 55 4 Electrical Attraction and Repulsion 59 4.1 Is There Electrical Repulsion? . . . . . . . . . . . . . . . . . . . . 59 4.2 Guericke’s Experiment with a Floating Down Feather . . . . . . 62 4.3 Du Fay Recognizes Electrical Repulsion as a Real Phenomenon . 68 4.4 The Electric Pendulum . . . . . . . . . . . . . . . . . . . . . . . . 71 4.5 Discharge by Grounding . . . . . . . . . . . . . . . . . . . . . . . 76 4.6 Gray’s Electric Pendulum . . . . . . . . . . . . . . . . . . . . . . 78 4.7 The Du Fay Versorium . . . . . . . . . . . . . . . . . . . . . . . . 79 4.8 The ACR Mechanism . . . . . . . . . . . . . . . . . . . . . . . . 82 4.9 Gray’s Pendulous Thread . . . . . . . . . . . . . . . . . . . . . . 85 4.10 Mapping the Electric Force . . . . . . . . . . . . . . . . . . . . . 87 4.11 Hauksbee and the Mapping of Electric Forces . . . . . . . . . . . 91 5 Positive and Negative Charges 95 5.1 Is There Only One Kind of Charge? . . . . . . . . . . . . . . . . 95 5.2 Du Fay Discovers Two Kinds of Electricity . . . . . . . . . . . . 106 5.3 Which Kind of Charge does a Body Acquire by Friction? . . . . 111 5.4 The Triboelectric Series . . . . . . . . . . . . . . . . . . . . . . . 119 5.5 Are Attractions and Repulsions Equally Frequent? . . . . . . . . 124 5.6 Variation of the Electric Force as a Function of Distance . . . . . 125 5.7 Variation of the Electric Force with the Quantity of Charge . . . 127 6 Conductors and Insulators 133 6.1 The Electroscope . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 6.2 Experiments with the Electroscope . . . . . . . . . . . . . . . . . 136 6.3 Which Bodies Discharge an Electroscope by Contact? . . . . . . 142 6.3.1 Definitions of Conductors and Insulators . . . . . . . . . . 143 6.3.2 Bodies which Behave as Conductors and Insulators in the Usual Experiments of Electrostatics . . . . . . . . . . . . 146 6.4 Which Bodies Charge an Electroscope by Contact? . . . . . . . . 148 6.5 Fundamental Components of a Versorium, an Electric Pendulum, and an Electroscope . . . . . . . . . . . . . . . . . . . . . . . . . 150 6.6 Influence of the Electric Potential Difference upon the Conducting or Insulating Behaviour of a Body . . . . . . . . . . . . . . . 151 6.6.1 Substances which Behave as Conductors and Insulators for Small Potential Differences . . . . . . . . . . . . . . . 155 6.7 Other Aspects which have an Influence upon the Conducting and Insulating Properties of a Substance . . . . . . . . . . . . . . . . 156 6.7.1 The Time Necessary in order to Discharge an Electrified Electroscope . . . . . . . . . . . . . . . . . . . . . . . . . 157 6.7.2 The Length of a Substance which Comes into Contact with an Electrified Electroscope . . . . . . . . . . . . . . . 157 6.7.3 The Cross-Sectional Area of a Substance which Comes into Contact with an Electrified Electroscope . . . . . . . 158 6.8 Electrifying a Conductor by Friction . . . . . . . . . . . . . . . . 158 6.9 Conservation of Electric Charge . . . . . . . . . . . . . . . . . . . 159 6.10 Gray and the Conservation of Electric Charges . . . . . . . . . . 164 6.11 A Short History of the Electroscope and the Electrometer . . . . 165 7 Differences between Conductors and Insulators 173 7.1 Mobility of Charges on Conductors and Insulators . . . . . . . . 173 7.2 Charge Collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 7.3 The Electric Polarization of Conductors . . . . . . . . . . . . . . 177 7.3.1 Aepinus and Electric Polarization . . . . . . . . . . . . . . 181 7.4 Attractions and Repulsions Exerted by a Polarized Body . . . . . 182 7.5 Utilizing Polarization to Charge an Electroscope . . . . . . . . . 187 7.5.1 First Procedure of Electrification by Induction . . . . . . 187 7.5.2 Second Procedure of Electrification by Induction . . . . . 189 7.5.3 Third Procedure of Electrification by Induction . . . . . . 191 7.6 The Electric Polarization of Insulators . . . . . . . . . . . . . . . 192 7.7 Does an Electrified Body Attract a Conductor or an Insulator More? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 7.7.1 Discussion of Gray’s Electric Pendulum . . . . . . . . . . 197 7.8 Forces of Non-Electrostatic Origin . . . . . . . . . . . . . . . . . 198 7.9 Microscopic Models of Conductors and Insulators . . . . . . . . . 199 7.10 Can Two Bodies Electrified with Charges of the Same Sign Attract One Another? . . . . . . . . . . . . . . . . . . . . . . . . . . 202 7.11 The Conductivity of Water . . . . . . . . . . . . . . . . . . . . . 206 7.12 Is it Possible to Electrify Water? . . . . . . . . . . . . . . . . . . 207 7.12.1 Kelvin’s Electrostatic Generator . . . . . . . . . . . . . . 208 7.13 The Conductivity of Air . . . . . . . . . . . . . . . . . . . . . . . 211 7.14 How to Discharge an Electrified Insulator? . . . . . . . . . . . . . 212 7.15 A Small Piece of Paper is Attract with a Greater Force when Above an Insulator or a Conductor? . . . . . . . . . . . . . . . . 215 8 Final Considerations 219 8.1 Changing Names and Meanings: From Electric and Non-Electric Bodies to Insulators and Conductors . . . . . . . . . . . . . . . . 219 8.2 Simple and Primitive Facts about Electricity . . . . . . . . . . . 220 8.3 Description of the Amber Effect . . . . . . . . . . . . . . . . . . . 223 Appendices 231 A Definitions 231 B Stephen Gray and the Discovery of Electrical Conduction 233 B.1 Gray’s Electrical Generator . . . . . . . . . . . . . . . . . . . . . 234 B.2 The Discovery of Electrification by Communication . . . . . . . . 236 B.3 Exploring the Discovery and Awakening the Hidden Electricity of Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 B.4 Gray Discovers Conductors and Insulators . . . . . . . . . . . . . 241 B.5 Discovery that What makes a Body Behave as a Conductor or as an Insulator Depends upon Its Intrinsic Properties . . . . . . . . 246 B.6 Discovery that Electrification by Communication Happens at a Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 B.7 The Experiment with the Suspended Boy . . . . . . . . . . . . . 251 B.8 Discovery that Free Charges are Distributed over the Surface of Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 B.9 Discovery of the Power of Points . . . . . . . . . . . . . . . . . . 256 B.10 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Bibliography 259
About
the Author
Andre Koch Torres Assis was born in Brazil (1962) and educated at the University of Campinas – UNICAMP, BS (1983), PhD (1987). He spent the academic year of 1988 in England with a post-doctoral position at the Culham Laboratory (United Kingdom Atomic Energy Authority). He spent one year in 1991-92 as a Visiting Scholar at the Center for Electromagnetics Research of Northeastern University (Boston, USA). From August 2001 to November 2002, and from February to May 2009, he worked at the Institute for the History of Natural Sciences, Hamburg University (Hamburg, Germany) with research fellowships awarded by the Alexander von Humboldt Foundation of Germany. He is the author of Weber’s Electrodynamics (1994), Relational Mechanics (1999), Inductance and Force Calculations in Electrical Circuits (with M. A. Bueno, 2001), The Electric Force of a Current (with J. A. Hernandes, 2007), and Archimedes, the Center of Gravity, and the First Law of Mechanics (2008). He has been professor of physics at UNICAMP since 1989, working on the foundations of electromagnetism, gravitation, and cosmology.