Alternating Current (AC)

Alternating current (AC) is an electric current that reverses its direction at regular intervals. The electrons in the wire oscillate back and forth rather than flowing steadily in one direction.

How AC works

A generator (alternator) produces AC by rotating a coil in a magnetic field. As the coil turns, the direction of the induced current reverses every half-turn. The result is a current that follows a sinusoidal wave pattern.

The frequency of mains AC in Malaysia and the UK is 50 Hz. This means the current reverses direction 100 times per second (50 complete cycles).

AC vs DC

Property	AC	DC
Direction	Reverses periodically	Flows in one direction only
Source	Generator, mains supply	Battery, solar cell
Voltage	Can be stepped up or down easily with a transformer	Cannot be changed with a transformer
Transmission	Used for long-distance power transmission	Used in electronic circuits

Why AC is used for mains electricity

AC can be stepped up to high voltage using a transformer. High voltage means lower current for the same power (P = IV), which reduces energy losses in the cables (P = I squared R). At the consumer end, transformers step the voltage back down to a safe level (230 V in Malaysia).

AC on an oscilloscope

An oscilloscope displays AC as a wave. The height of the wave shows the peak voltage. The horizontal spacing between identical points on the wave gives the period. Frequency = 1 / period.

To find the peak voltage: count the number of divisions from the centre line to the peak and multiply by the Y-gain setting (volts per division).

To find the period: count the number of divisions for one complete wave and multiply by the time-base setting (seconds per division).