Electrostatics

Abstract

Capacitance is the property by which the elements in an electric circuit store an electric charge. A capacitor is a circuit element or piece of apparatus designed to have a capacitance of definite or controllable value, and consists essentially of two electric conductors separated from each other by an insulating material called the dielectric. The simplest form of capacitor comprises two flat metal plates mounted on insulating supports, facing each other and parallel, and with a small gap intervening. Imagine the space surrounding the plates to be completely evacuated. If the plates are connected to a battery, a current of electricity flows for a brief instant, thus causing the passage of a charge of Q coulombs of electricity round the circuit. The passage of this charge may be observed by a suitable galvanometer (a ballistic galvanometer) connected in the circuit, the galvanometer giving a ‘kick’ or ‘throw’ when the battery is connected. The magnitude of this throw is proportional to the quantity Q coulombs passing through the galvanometer to charge the capacitor. The passage of the charge Q causes the potential difference between the two plates to become equal to the potential difference of the battery, one plate thus becoming positively charged and the other negatively charged, as shown in figure 1.1.