**CAPACITORS AND
CAPACITANCE**

A capacitor is a system of two conductors
separated by an insulator. The conductors have charges; say Q_{1} and Q_{2},
and potentials V_{1} and V_{2}.

Usually, in practice, the two conductors have
charges Q and – Q, with potential difference V = V_{1} – V_{2} between them.

The conductors may be so charged by connecting
them to the two terminals of a battery. Q is called **the charge of the capacitor**, though this, in fact, is the charge on
one of the conductors – the total charge of the capacitor is zero.

The electric field in the region between the
conductors is proportional to the charge Q. That is, if the charge on the
capacitor is, say doubled, the electric field will also be doubled at every
point.

Potential difference V is the work done per unit
positive charge in taking a small test charge from the conductor 2 to 1 against
the field.

Consequently, V is also proportional to Q, and the
ratio Q/V is a constant C.

The constant C is called the **capacitance of the capacitor**. C is independent
of Q or V, as stated above.

The capacitance C depends
only on the geometrical configuration (shape, size, separation) of the system
of two conductors.

The SI unit of capacitance is
1 farad (=1 coulomb volt^{-1}) or 1 F = 1 C V^{–1}. A capacitor
with fixed capacitance is symbolically shown as ---||---.

A capacitor with large
capacitance can hold large amount of charge Q at a relatively small V.

High potential difference
implies strong electric field around the conductors.

The maximum electric field
that a dielectric medium can withstand without break-down (of its insulating
property) is called its **dielectric strength**;
for air it is about 3 × 10e6 Vm^{–1}.