What is a capacitor

A capacitor is a simple device for storing electrical energy, but although it is simple it is an essential component in almost all complex circuits. It consists of two metal plates which are separated by an insulator.

A simple capacitor

Making a capacitor

To make capacitors that are small and easy to handle they are usually made from aluminium foil with thin insulation, which can be rolled up into a small tube. The insulation is needed because otherwise the charge would just flow from one plate to the other.


If we connect a cell or any other electricity supply, the electrons are sucked off one plate and pushed onto the other.

Capacitor in a DC circuit

What does a capacitor do?

Capacitors store energy in the form of an electrical charge. The general equation for the energy from electricity is that:

energy = charge x potential difference.

But in a capacitor the PD rises as more charge is stored and falls as the capacitor discharges (as you can see on the sketch graph). The energy is still calculated by that equation but because the PD (voltage) varies from zero to a maximum as the capacitor charges up, we must take account of the changing voltage by measuring the area under the graph.

Capacitor graph

The bigger the potential difference (voltage) of the supply, the bigger the charge (number of electrons moved). The ability of a capacitor to store charge is measured in Farads. If the capacitor had a value of one Farad then a voltage of one volt would be able it to store a charge of one coulomb. this gives the equation:


In fact a one farad capacitor would be huge. Most capacitors are measured in microfarads (10-6) or picofarads (10-9).