Capacitance

Imagine a large metal ball connected to the positive of a power supply, the other terminal of which is earthed. What will happen when the switch is closed.

The power supply will try to make a current flow by forcing charge onto the ball. But the more charge there is on the ball the harder it will be to get more charge on. With this power supply there will be a maximum amount of charge we can get on the ball.

This maximum will depend on what voltage we use.
(We will get twice as much charge on if we double the voltage)

It will also depend on a property of the ball called its capacitance.
(This mainly depends on its surface area. A bigger ball will carry a lot more charge)

The capacitance of the ball is defined by this equation;  C = Q / V

Capacitance is measured in Farads (F) after Michael Faraday.

In the film Cool Hand Luke the character played by Paul Newman makes a bet that he can eat 50 boiled eggs in 1 hour. His stomach is the capacitor. The eggs are the charge. The man pushing them in is the power supply. The number of eggs eaten every minute decreases exponentially just like the current in the charging circuit. Does he do it? Well, you'll have to see the film to find out.

the capacitors we find in electrical circuits are not metal balls but pairs of metal plates, usually rolled up like a Swiss roll, with an insulator between them

This diagram shows the symbol for a capacitor. Notice that a resistor is usually included in the circuit. When the switch is closed the plate connected to the positive terminal becomes positively charged and the other plate negatively charged.

Example

A 220 µF capacitor is connected to a 6V d.c. supply. How much charge will it store when fully charged?

Q = C V    = 220 x 10^-6 x 6      = 1.32 x 10^-3 C or 1.32 mC