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Capacitance of a conductor. If a charge q is given to an isolated conductor, its voltage is increased by an amount V. For a given conductor, the ratio Q/V is independent of Q and depends only on the size and shape of the conductor. The ration Q/V is called the capacitance of the conductor, and is denoted by C.

. : C = Q/V. Hence the capacitance of the conductor is defined as the ratio of the charge given to the increase in the potential of the conductor.

    The capacitance of a conductor is also defined as the amount of charge that should be given to it to increase its potential by unity.

    The unit of capacitance is farad. A conductor has a capacitance of one farad, if a charge of 1 coulomb given to it raises its potential by 1 volt.

1 μ F = 10-6 F; 1pF = 10-12 F.

Principle of a Capacitor. Suppose an insulated metallic plate A is given a positive charge Q and its potential is V. Its capacitance C = Q/V. Let another insulated metal plate B be brought near A. Negative charge is induced on that side of B which is nearer to A. An equal positive charge is induced on the other side of B. The negative charge on B decreases the potential of A. The positive charge on B increases of A. But the negative charge on B is nearer to A than the positive charge on B. so the net effect is that the potential of A decreases. Thus the capacitance of A is increased.

The Positive charge on B is neutralized by connecting the back side of B to earth. Then the potential of A decreases still further. Thus the capacitance of A is considerably increased.

A capacitor in general consists of two conductors one positively charged and the other earthed. The conductors are called plates. The capacitance depends on the geometry of the conductors and the permittivity of the medium separating them. A capacitor is a device for storing charge.

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