Capacitors are electrical components that store charge so that there is a potential difference across them. They consist of two conductive plates separated by a non-conductive material.

The capacitance of a capacitor is the ratio of the charge on the capacitor to the potential difference between its plates. A capacitor of 1 farad will store 1 coulomb of charge when the potential difference across it is 1 volt. If the capacitance of a capacitor is increased, it will store more coulombs of charge for every volt of potential difference across its plates, i.e. a capacitor of 2 farads will store 2 coulombs of charge when the potential difference across it is 1 volt.

The following relationship links capacitance, charge and potential difference and can be used to solve problems involving these quantities.

$$ C=\dfrac{Q}{V} $$

A capacitor stores electrical charge, which means it stores electrical energy which can be released at a later time to use.

The total energy stored in a charged capacitor is equal to the area under a charge-potential difference graph.

The following relationships involve energy, charge, capacitance, and potential difference and can be used to solve problems involving these quantities.

$$ E=\dfrac{1}{2}QV=\dfrac{1}{2}CV^2=\dfrac{1}{2}\dfrac{Q^2}{C} $$

When connected to a power source, a capacitor begins charging so that one plate accumulates a negative charge, while the other accumulates a positive charge, creating a potential difference between the plates. Once fully charged, the capacitor can be connected to a circuit and discharged so that the stored electrical energy is released and the potential difference between the plates is reset to zero again. Both of these processes have effects on the voltage across a capacitor and current in the circuit.

The following is the experimental procedure to investigate the variation of current in and the voltage across discharging and charging capacitors.

A conclusion from these experiments is that, in the charging cycle of an RC circuit, as the capacitor charges, the current in the circuit decreases and the potential difference across the capacitor increases.