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Chapter One: Electrostatic

The observed fact is that all charged particles can be divided into
two classes such that all members of one class repel each other, while
attracting members of the other class. If two small electrically charged
bodies A and B, some distance apart, attract one another, and if A attracts
some third electrified body C, then we always find that B repels C.
Contrast this with gravitation: there is only one kind of gravitational mass,
and every mass attracts every other mass.

1.3 SI Unit of Charge

The SI unit of charge is the coulomb (C). The coulomb is defined in
terms of the unit of electric current the ampere, which, in turn, is defined
in terms of the forces current-carrying conductors exert on one another.

1.4 Kinds of Electric Charge

Electric charge can be transferred from one object

to another by physical contact. Suppose that a

rubber rod is rubbed with fur and a glass rod is

rubbed with silk. If the rubber rod is placed

against a pith ball (a small ball of wadded paper)

and removed, the rubber rod is found to repel the Fig. (1.5 ): Electric force
pith ball. Similarly, if the glass rod is placed against between similar and
a second pith ball and removed, the glass rod is opposite charges.

found to repel the second pith ball. However, it is found that the rubber

rod attracts the second pith ball and the glass rod attracts the first pith

ball. Experiments like this show that there are two types of electric

charge. Like charges repel one another and unlike charges attract one
another. Figure (1.5 and 1.6) illustrate the above explanation.

University of Babylon/ College of Science Department of Physics

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