Why is capacitance important?
Capacitance is the measure of a device known as a capacitor to hold a voltage. or potential difference in charge, in equilibrium. In its simplest form, a capacitor consists of a set of two conductive parallel plates separated by an arbitrarily small distance, dx. However, the capacitor is really useless until it is placed in a circuit with a battery or power source that provides a given voltage.
In a DC (direct current) circuit, current will flow from a battery to one of the plates. As electrons accumulated on a plate, their electric fields will repel electrons on the second plate and simultaneously attract positive charges and force them to accumulate on the opposite plate. Because the plates do not touch, an electric equilibrium cannot be achieved, and an electric field between the plates is the result.
As the electrons from the second plate are repelled, they are forced to travel back through the circuit, until they reach the encounter the same electrons that repelled them in the first place. At this point, the electrons will change directions again until an equilibrium is reached. This is analogous to the motion of a mass hanging from a spring bouncing up and down, until finally the mass reaches an equilibrium point. When an equilibrium is reached, the voltage across the capacitor will theoretically match the voltage from the original source.
When the capacitor is removed from the power source, the circuit is broken, and the capacitor maintains the potential difference across the plates until it is introduced into another circuit. This property of capacitors is extremely useful in electrical devices because it allows a burst of charge to be delivered at exactly the right moment.
One old school example of a capacitor is seen in flash photography. When a picture was taken, the charge from a capacitor was quickly released causing a filament to burn brightly and illuminate people's faces while the film captured the image!