The Britannica Guide to Electricity and Magnetism · This book examines electricity and magnetism,...
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Transcript of The Britannica Guide to Electricity and Magnetism · This book examines electricity and magnetism,...
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ContentsIntroduction x
chapter 1: An Introduction to Electricity and Magnetism 1
Historical Background 2Early observations and Applications 3Emergence of the Modern Sciences of Electricity and Magnetism 4
Pioneering Efforts 5Invention of the Leyden Jar 6Formulation of the Quantitative Laws of Electrostatics and Magnetostatics 8
Foundations of Electrochemistry and Electrodynamics 10
Experimental and Theoretical Studies of Electromagnetic Phenomena 13Discovery of the Electron and Its Ramifications 20Special Theory of Relativity 22
Development of Electromagnetic Technology 24
chapter 2: Electricity 29Electrostatics 29
Electric Charge 30Static Electricity 31Capacitance 47
Dielectric constants of Some Materials (At Room Temperature) 53Direct Electric Current 55 Electric Resistivities (At Room Temperature) 60
Conductors, Insulators, and Semiconductors 60
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Electromotive Force 64Direct-Current Circuits 68Resistors in Series and Parallel 70Kirchhoff ’s Laws of Electric Circuits 72
Alternating Electric Currents 74Transient Response 75Alternating-Current Circuits 76Reactance 83
Electrical units 85Ampere 85Coulomb 85Electron Volt 86Faraday 86Henry 86ohm 87Siemens 87Volt 88Watt 88Weber 88
chapter 3: Electric Properties of Matter 89
Piezoelectricity 89Electro-optic Phenomena 92Thermoelectricity 93Thermionic Emission 95Secondary Electron Emission 96Photoelectric Conductivity 97Electroluminescence 98Bioelectric Effects 100Pyroelectricity 102Superconductivity 103
Discovery 103Thermal Properties of Superconductors 105Magnetic and Electromagnetic Properties of Superconductors 111
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Higher-Temperature Superconductivity 117
chapter 4: Magnetism 122Fundamentals 122 Typical Magnetic fields 126Magnetic Field of Steady Currents 126Magnetic Forces 131
Lorentz Force 132Repulsion or Attraction Between Two Magnetic Dipoles 136
Magnetization Effects in Matter 140Magnetohydrodynamics 145Magnetic Properties of Matter 146
Induced and Permanent Atomic Magnetic Dipoles 149Diamagnetism 151Paramagnetism 153Ferromagnetism 154
curie Temperatures for Some ferromagnetic Substances 157
Antiferromagnetism 160 Néel Temperature of Antiferromagnetic Substances 161
Ferrimagnetism 161Magnets 162
Magnetization Process 163Powder Magnets 168High Anisotropy and Alnico Alloys 169Rare-Earth 169Barium Ferrites 170Permeable Materials 170
Magnetic units 172Gauss 172oersted 173Tesla 173
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chapter 5: The Fundamentals of Electromagnetism 174
Coulomb’s Law 176Principle of Charge Conservation 178Electric Fields and Forces 179Magnetic Fields and Forces 185Interaction of a Magnetic Field with a Charge 187Effects of Varying Magnetic Fields 190
Faraday’s Law of Induction 191Self-Inductance and Mutual Inductance 194
Effects of Varying Electric Fields 197Electromagnets 202
Solenoids 205Relays 207Design of Large Electromagnets 209Principal Applications 210
Conclusion 213
chapter 6: Biographies 215André-Marie Ampère 215François Arago 219Jean-Baptiste Biot 221Charles-Augustin de Coulomb 222Michael Faraday 223Luigi Galvani 227William Gilbert 231Joseph Henry 232Fleeming Jenkin 233Gustav Robert Kirchhoff 234James Clerk Maxwell 235Louis-Eugène-Félix Néel 239Georg Simon ohm 240Hans Christian Ørsted 241
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Jean-Charles-Athanase Peltier 242Peter Peregrinus of Maricourt 242John Henry Poynting 243William Sturgeon 244Alessandro Volta 245Wilhelm Eduard Weber 246Sir Charles Wheatstone 246
Appendix: Other concepts 248Ammeter 248Barkhausen Effect 249Displacement Current 249Electric Displacement 250Electron optics 252Ferrite 252Ferroelectricity 254Galvanometer 255Gauss’s Law 255Gunn Effect 256Magnetic Mirror 257Magnetic Monopole 257Magnetic Pole 258Magnon 259Pinch Effect 260Polaron 260Seebeck Effect 261Skin Effect 261Space Charge 261Thomson Effect 262
Glossary 263Bibliography 265Index 267
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7 Introduction 7
When they think of electricity, most people proba-bly think of many of the appliances and electronic
equipment in their homes, ranging from light bulbs to refrigerators to computers. They also may think of the outlets, fuse boxes, and wires that deliver the electric-ity, or even the static that builds up on clothes that just come out of the dryer. Perhaps they think of something more grandiose, like a bolt of lightning tearing across a stormy sky.
What animates these appliances, flows through the wires, sticks the clothes together, and lights up the sky has its root in a basic property of matter called electric charge. Electricity and magnetism are actually two faces of elec-tric charge. Electricity is sparks and shocks; magnetism seems something else entirely. The simple bar magnet works only on certain metals. on larger scales there is the attraction of the compass to the north pole of Earth’s magnetic field.
This book examines electricity and magnetism, two physical phenomena that underlie much of the technol-ogy that makes the modern world work. The biographies of leading figures in electromagnetic science also are dis-cussed in detail.
For all their differences, both these phenomena have one thing in common: moving electric charges. Charge is fundamental. Matter is either charged or uncharged. This property of charge also comes in a bit called an elemen-tary charge, or -1 e. This property is quantized. That is, charge is almost always observed in integer multiples of e; for example, -4 e or -5 e is possible, but -4.5 e, -4.6 e, or any amount of charge between -4 and -5 are not. (There is an exception in the quarks, the subatomic particles that make up the proton and the neutron. Quarks have charge of 2/3 or -1/3 e. However, free quarks are never observed
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outside of particle accelerators and so do not affect this book’s discussion of electricity.)
The study and the use of electricity involves the obser-vation and manipulation of these charges. The carrier of the charge, -1 e, is the electron. The electron is one of the lightest subatomic particles and one of the key compo-nents of atoms, along with neutrons and protons.
Like masses, charges have a force between them that is an inverse square law. The force decreases as the distance is squared; if two charges are moved twice as far apart as they were previously, the force between them is one-quarter of what it was before. In the case of masses, the force is that of gravity. For charges, the force is called the Coulomb force. objects can have either positive or nega-tive charge. This means that unlike gravity, the charge can be either attractive or repulsive. A positive charge attracts a negative charge, and vice versa. A positive charge repels a positive charge, and a negative charge repels a negative charge. If it was possible for negative mass to exist, gravity would act like the Coulomb force and the universe would be very different.
The modern study of electricity began with the English doctor William Gilbert, who was physician to Queen Elizabeth I and King James I. In 1600, Gilbert coined the word “electricity” for the attraction that brought together two objects that had been charged by being rubbed together. He attributed this electricity to a fluid, which, because of his medical training, he called a “humour,” after the fluids that were thought to influence personality. This may seem like a long way away from subatomic particles with fundamental irreducible properties. However, the basics of Gilbert’s description contained the seed for the further study and explanation of electricity.
once charges had been discovered, the next logical step was to somehow store them. This was done in 1745 when
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the Dutch mathematician Pieter van Musschenbroek invented the Leyden jar. This device was a glass jar con-taining water stopped up by a cork with a wire going through it. The jar was charged by connecting devices that generated static electricity to the wire. If the wire was touched, the jar discharged and gave the person touching the wire an electric shock. As was the case with many an 18th-century electrical apparatus, this was used to perform scientific tricks that were sometimes edify-ing and but sometimes ridiculous, such as being made into the shape of a cane and being placed into a rack to shock the unwary. As historian Jessica Riskin put it, “The Leyden jar ultimately became the whoopie cushion of the 1740s.” Despite its ribald origins, the Leyden jar was the ancestor of the capacitor, a device usually consisting of two parallel plates that is used in many pieces of elec-trical equipment to store charge.
The other obvious thing to do with charge was to move it from one place to another, to make it flow. Such a flow is called a current. The current in most homes is known as alternating current, or AC, in which current flows in a periodic pattern of 60 cycles or Hertz per second. AC succeeded direct current (DC) in the late 19th century. The change to the more efficient AC from DC was one of the greatest technological feuds in American history, with the Wizard of Menlo Park, Thomas Alva Edison, on the side of direct current, and the eccentric inven-tor, Nikola Tesla on the side of alternating current. Both Edison and Tesla resorted to lurid demonstrations to bol-ster their case. Tesla lit lamps with alternating current that flowed through his body. one demonstration of Edison’s showed the dangers of AC when he electrocuted an ele-phant in 1904, an event that was filmed for posterity.
once charge can be stored and can flow, one can har-ness it for human needs. There are various ways this can
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be accomplished. Light comes from electricity. In old-fashioned filament bulbs, the resistance of the filament material to the flow of electricity heats the wire, causing it to emit light. In compact fluorescent bulbs, the interior, which contains argon and mercury vapor, is ionized and emits ultraviolet radiation, which is absorbed and rera-diated by phosphors on the inside of the tube. Electrical energy can also be converted into mechanical energy to move an object. For example, in the home, an electric motor can be used to spin the blades of a blender. This requires electromagnetism.
The concept of magnetism was understood long before William Gilbert conducted his first experiments with electricity. “one feels the stone’s own power,” said the Roman poet Lucretius about the magnet. Hundreds of years before the Renaissance, before modern science, a 13th-century Frenchman, Peter Peregrinus of Maricourt, mapped a magnetic field. The magnet was spherical, and its field had two poles. By this time, the compass had come into use in navigation, and it was William Gilbert himself who realized the movement of the compass, its attraction to the North, meant that Earth was a giant magnet. Just as Peregrinus’s small magnet had north and south poles, Earth’s polar regions also had magnetic poles.
unlike electricity, there is no “magnetic charge,” no elementary particle that in itself carries some quantity of magnetism. Scientists have tried very hard to find such particles, called monopoles, without success. There are merely the north and south poles.
In the humble bar magnet, the moving charges are those of the electrons orbiting around the nuclei of the atoms in the material. These atoms have what is called a dipole moment, which is the product of the current of the moving electrons and the area of their orbits. These dipole moments give rise to the magnetic field. In the
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case of Earth, the mechanism is complicated (the “mag-netic field” is produced by many different currents) but is mainly due to fluid iron moving because of the convection caused by temperature differences between colder and hotter areas in Earth’s core. This movement creates an electric current and thus a magnetic field.
The connection between magnetism and electricity surprised 19th-century scientists. The discovery hap-pened with one of those happy accidents that occur so often in science. While setting up some electrical equip-ment for a lecture in April 1820, the Danish physicist Hans Christian oersted discovered that when a current passed through a wire, a nearby compass needle moved. English physicist Michael Faraday took the next logical step and produced an electric current by changing a magnetic field. The discovery of this type of charge, called induction, led to the first electromagnetic generators. These generators originally were used to spark the powerful light of arc lamps and subsequently the gentler radiance of incandes-cent lamps.
Since magnetic fields could cause currents and cur-rents could cause magnetic fields, it was only a matter of time before they could be mathematically described in a single theory. In 1864 the Scottish physicist James Clerk Maxwell did just that using four simple equations. To state them in simple English, rather than in the more abstruse terms of vector calculus that they are usually stated in, they are: 1) the electric field diverges outward from elec-tric charges, 2) there are no magnetic monopoles, 3) an electric field arises from a changing magnetic field, and 4) a magnetic field arises from a changing electric field. Not only did these equations explain how the compass needle moved in oersted’s demonstration, they also gave a speed for how fast this effect traveled. That speed was 300,000 kilometres per second—the speed of light, which
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had been known since the 17th century. Light was an elec-tromagnetic wave.
Although it may seem as if electricity and magnetism should march toward their unification in electromagne-tism, there are many interesting byways that make this happen, such as superconductivity. The same resistance of an incandescent lamp’s filament to the flow of electric cur-rent operates in every wire and electrical device, causing some loss of efficiency. A material in which the resis-tance is zero is called superconducting. Many materials can be superconductors, but they usually have to be very cold, at temperatures close to absolute zero. Scientists have embarked on the quest for a material that would be superconducting at room temperature, but they have not gotten there yet.
Some of the leading personalities in the history of elec-tromagnetic science were not single-minded savants but figures who ranged over the whole of what was then physi-cal science. For example, Coulomb studied the strength of materials and frictional forces. In fact, he used the same apparatus, a torsion balance, for many of his experiments. Maxwell, the great unifier of electricity and magnetism, made great advances in the study of thermodynamics. He also demonstrated a process of colour photography.
The study of electricity and magnetism is but one part of the broad field of physics, but it contains much of what makes science rich and interesting. From the simple sparks and magnets have emerged one of the key forces of the universe.
