A capacitor is a system of two conductors separated by an insulator. The two conductors have charges Q and – Q, with potential difference V = V1 – V2 between them.
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. The potential difference V between two plates V is also proportional to Q, and the ratio Q/V is a constant: Q ∝ V or Q = CV
C = Q/V ------- (1)
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 --||--. The charge of the capacitor leaks away due to the reduction in insulating power of the intervening medium.
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 × 106 Vm–1. For a separation between conductors of the order of 1 cm or so, this field corresponds to a potential difference of 3 × 104 V between the conductors. Thus, for a capacitor to store a large amount of charge without leaking, its capacitance should be high enough so that the potential difference and hence the electric field do not exceed the break-down limits. Put differently, there is a limit to the amount of charge that can be stored on a given capacitor without significant leaking. In practice, a farad is a very big unit; the most common units are its sub-multiples 1 μF = 10–6 F, 1 nF = 10–9 F, 1 pF = 10–12 F, etc.
