If you look at a catalogue of electronic components you'll find an enormous variety of sizes and types of capacitor. However, for most purposes we can divide capacitors into two basic types:- dielectric and electrolytic.

A capacitor acts as a charge store. It contains a pair of metal plates separated by a thin sheet of insulating material. Left to themselves the plates are electrically neutral - the number of positive protons in each exactly equals the number of negative electrons. However, if we connect wires to the plates and apply and external voltage we can drag electrons off one plate and push them on to the other.

This takes energy, i.e. we have to do work charging the capacitor. The result is a capacitor with one plate positively charged and the other negatively charged. The energy used to move charge is stored by this imbalance. If we connect two plates together with a resistor, the electrons 'rush back home' releasing their energy again. The voltage between the plates of a charged capacitor is proportional to the amount of charge moved. The charge/voltage ratio for any specific capacitor is called it's capacitance.

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