Page 281 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 23

Electricity

ELECTRIC CHARGE

Electric charge, like mass, is one of the basic properties of certain of the elementary particles of which all matter
is composed. There are two kinds of charge, positivecharge and negativecharge. The positive charge in ordinary
matter is carried by protons, the negative charge by electrons. Charges of the same sign repel each other, charges
of opposite sign attract each other.
The unit of charge is the coulomb (C). The charge of the proton is +1.6 × 10 −19 C, and the charge of the
electron is −1.6 × 10 −19 C. All charges in nature occur in multiples of ±e =±1.6 × 10 −19 C.
According to the principle of conservation of charge, the net electric charge in an isolated system always
remains constant. (Net charge means the total positive charge minus the total negative charge.) When matter is
created from energy, equal amounts of positive and negative charge always come into being, and when matter is
converted to energy, equal amounts of positive and negative charge disappear.

ATOMS AND IONS
An atom of any element consists of a small, positively charged nucleus with a number of electrons some distance
away. The nucleus is composed of protons (charge +e, mass = 1.673 × 10 −27 kg) and neutrons (uncharged,
mass = 1.675 × 10 −27 kg). The number of protons in the nucleus is normally equal to the number of electrons
around it, so the atom as a whole is electrically neutral. The forces between atoms that hold them together as
solids and liquids are electric in origin. The mass of the electron is 9.1 × 10 −31 kg.
Under certain circumstances an atom may lose one or more electrons and become a positive ion,oritmay
gain one or more electrons and become a negative ion. Many solids consist of positive and negative ions rather
+
than of atoms or molecules. An example is ordinary table salt, which is made up of positive sodium ions (Na )
−
and negative chlorine ions (Cl ). Solutions of such solids in water also contain ions. Sparks, flames, and X-rays
have the ability to ionize gases. Ions of opposite sign in a gas come together soon after being formed, and the
excess electrons on the negative ions pass to the positive ones to form neutral molecules. A gas can be maintained
in an ionized state by passing an electric current through it (as in a neon sign) or by bombarding it with X-rays
or ultraviolet light (as in the upper atmosphere of the earth, where the radiation comes from the sun).

COULOMB’S LAW
The force one charge exerts on another is given by Coulomb’s law:

q 1 q 2
Electric force = F = k
r 2
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