Electrostatic Field
When we touch a metal doorknob after walking on a carpet on a dry day, we sometimes feel a shock on the fingertips as a small crackle. If it is dark, we can see a spark when inserting a key into a keyhole. This is the same phenomenon as thunder. This phen
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Electrostatic Field
1.1 Electric Charge in Vacuum When we touch a metal doorknob after walking on a carpet on a dry day, we sometimes feel a shock on the fingertips as a small crackle. If it is dark, we can see a spark when inserting a key into a keyhole. This is the same phenomenon as thunder. This phenomenon is brought about by electric charge in substances. The usual frictional electricity we experience also comes from electric charges. Electric charge build-up in a substance that causes various kinds of electric phenomena, including the above examples. Matter is a substance that obeys universal gravity laws, and its magnitude is quantitatively described in terms of mass. In the case of electric phenomena, an amount of electric charge quantitatively describes the phenomena and the same term, “electric charge”, is also used to mean the amount of electric charge. Unlike mass, there are two kinds of electric charge, positive and negative. The components of the electric charge are the proton with positive charge and electron with negative charge. The electric charge of a proton is called the elementary electric charge and its magnitude is e D 1:602 189 2 1019 C; where the unit [C] is coulomb. The electric charge of an electron is e. The elementary electric charge is the minimum amount of electric charge, and any electric charge is its integral multiple. Since e is sufficiently small, electric charge can be regarded as a continuous quantity in many cases. This is similar to the fact that an amount of water can be regarded as a continuous quantity in usual cases. On an atomic scale the nucleus of an atom is composed of protons and neutrons, which are electrically neutral, and electrons stay in orbits around the nucleus. There are innumerable positive and negative electric charges in substances. Since the size of each atom is very small, electrons and protons can be regarded as being
T. Matsushita, Electricity and Magnetism: New Formulation by Introduction of Superconductivity, Undergraduate Lecture Notes in Physics, DOI 10.1007/978-4-431-54526-2__1, © Springer Japan 2014
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1 Electrostatic Field
in the center of each atom on a macroscopic scale. As a result, the positive and negative charges cancel each other to yield an electrically neutral state. Ionic crystals composed of equal amounts of positive and negative ions can also be regarded as electrically neutral on the macroscopic scale, since the distance between these ions is sufficiently small. Sometimes the electric charge is not balanced. In such a case the electric charge that remains after cancellation causes various electric phenomena. There are two kinds of electric charge that cause electric phenomena: one is true electric charge, which can be transferred outside a substance and the other is polarization charge, which is locally bound around a nucleus and cannot be transferred outside. The former charge appears on the surface of a conductor and will be covered in Chap. 2, and the latter appears on the surface of a dielectric and will be covered i
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