Capacitors are common electronic devices consisting of two conducting metal plates separated by a very thin layer of insulating material, called the dielectric. Unlike chemical cells, capacitors really do store electrical potential energy because, in use, the two plates carry equal and opposite separated charges.
Historically capacitors have been (and continue to be) used in AC circuits, especially radio circuits, because their impedance is frequency dependent. They are used in smoothing circuits to iron out fluctuations in p.d., for example in surge suppressors. In recent decades large-value capacitors (> 1 F) have become increasingly used as backups for power supply.
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