Alternating current

Most of our everyday uses of electricity are when electrons are moving in conductors – this is current flow as opposed to static electricity.

Direct current (d.c.) is a one-way flow of electrons from negative to positive terminals of a power supply.

Alternating current (a.c.) is an oscillation of electrons or 'back and forward' movement.

One full cycle of alternating current is shown in the diagram below:

Graph of current against time for AC showing sine wave rising from origin to peak, falling to reach same but negative peak before rising up to x-axis. Time for this to occur is labelled "one cycle".

In the first half of the cycle the current flow is in one direction. In the second half the current flow reverses.

Oscilloscopes are used to display a.c. waveforms. The y-axis plots the voltage across a component. The x-axis shows how this changes over time.

A circuit diagram of an oscilloscope, resistor and a.c. supply connected in parallel.

An oscilloscope trace allows us to determine two important quantities of an a.c. signal: frequency and peak voltage.


Why is alternating current used so extensively around the world?

Transformers can increase and decrease voltages in a.c. supply networks. Using transformers reduces current flow in transmission lines and saves energy when supplying electricity over large distances.