Capacitance and energy stored in a capacitor can be calculated or determined from a graph of charge against potential. Charge and discharge voltage and current graphs for capacitors.

Part of

A capacitor is a gap in a circuit with space for charge on the 'plates' shown as the horizontal lines.

When a capacitor is charged, electrons on the lower plate repel electrons from the upper plate, which then move to the positive terminal of the supply.

The voltage applied to charge the capacitor (circuit 1 below) is measured with a voltmeterand charge accumulated is measured by removing the charged capacitor from circuit 1 and connecting it to a coulomb meter (circuit 2).

By varying the charging voltage and measuring the associated charge a graph can be drawn.

The gradient of this graph is equal to the capacitance of the capacitor.

And the area under the graph is the energy stored by the capacitor.

This can be combined with the equation for capacitance above to give two alternative equations for energy stored.

and

- Question
A capacitor is charged up with a cell.

(a) What charge is stored?

(b) What energy is stored?

a)

b)

- The S.I. unit of charge is the Coulomb,
- The S.I. unit of capacitance is the Farad,
- A capacitor of capacitance 1 Farad will store 1 Coulomb of charge with the potenial difference across it is 1 volt