Capacitors store small amounts of energy that can be accessed quickly. The energy stored by a capacitor is equal to the work done by the battery. This is given by the area under a p.d.-charge graph.


W =frac{1}{2}frac{Q^{2}}{C}


Capacitors are found in electronic devices that don’t need to store much charge but which need to discharge quickly. For example:

  1. Flash photography: short pulse of high current gives a brief, bright flash
  2. Back-up power supplies: can be used to store charge. Release stored energy very quickly so generate high output power. E.g. for emergency lighting.
  3. Smoothing out p.d.: convert a.c. power to d.c. power by charging during peaks and discharging during troughs, having used diode to restrict current to one direction. Smooths out output voltage to give direct voltage of a constant value. maintaining a constant output. Keep the ripple small (small decrease in output p.d. as capacitor discharges) by making the time period much larger than the period of the alternating voltage.