TABLE OF CONTENTS Any observed charge q, positive or negative, is in magnitude an integral multiple of
where ? e is the charge on a single electron. Note that C (coulomb) is the derived unit for electric charge, since 1 C = 1 A s. One coulomb is a large amount of charge; hence the microcoulomb ( ?C) is frequently used.
An electric current in a wire corresponds to the transport of a certain amount of electric charge through a fixed cross section in a certain time interval. Electric current is measured in amperes (A).
If q 1 and q 2 are magnitudes of two charges, separated by a distance r, then the force between them is
This force is repulsive when q 1 and q 2 are of the same sign and attractive when they are of opposite signs. By experiment, it is found that
For mathematical reasons, it is convenient to replace b by another constant, ? 0, called the permissivity of empty space and defined by
In terms of ? 0, Coulomb's law is written
Forces between point charges act independently. Suppose that charges q 1, q 2, q 3, are fixed in an inertial frame. Then the force F 1 on a charge due to q 1 is computed by Coulomb's law as if q 2, q 3,
