To understand the principle of a capacitor, let us consider an insulated metal plate A. let some positive charge be given to this plate, till its potential becomes maximum. No further charge can be given to this plate as it would leak out.
Now consider another reinsulated metal plate B held near the plate A. by induction a negative charge is produced on the nearer face of B and an equal positive change develops on the farther face of B, the induced negative charge tends to decrease the potential of And the induced negative charge is closer to A, it is more effective. The overall potential of A reduces and hence some more charge can be given to A to raise its potential to maximum. Thus capacity of the conductor A has increased, by bringing another uncharged conductor B in its vicinity.
Now connect the plate B to earth the induced positive charge on B being free flows to earth, the induced negative charge on B. However stays on as it is bound to positive charge on A. due to the induced negative charge on B, potential of A is greatly reduced. Thus a large amount of charge can be given t A to raise it to the maximum potential.
Hence we conclude that the capacitance of an insulated conductor is increased considerably by bringing near it an uncharged earthed conductor.
This is the principle of the capacitor.
Such an arrangement of two conductors separated by a dielectric medium is said to form a capacitor or condenser.
If the conductors are plane, the condenser is called a parallel plate capacitor. When the conductors are spherical the capacitor is called a spherical capacitor and when the conductors are cylindrical it is called cylindrical capacitor.
Note: Capacitors or condensers are used in many electrical appliances, like oscillator circuits in radio tuning, in ignition system of engines in vehicles, in filter circuits in electric fan and in electric motor etc.
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