Alternating and direct current
An electric current flows either as a direct current or as an alternating current.
On a voltage-time graph this would appear as a straight horizontal line at a constant voltage.
Car batteries, dry cells and solar cells all provide a direct current (dc) that only flows in one direction.
On a voltage-time graph, this would appear as a curve alternating between positive and negative voltages - the positive and negative values indicate the direction of current flow.
Power stations usually provide an alternating current (ac). In the UK, the mains electrical supply is generated at a frequency of 50 hertz (Hz) and is delivered to houses at about 230 volts (V).