- What is alternating current?
- Shape of a sine wave
- Period and Frequency
- Peak voltage
- Peak-to-peak voltage
- RMS voltage
- Average voltage

The current flowing through a circuit powered by a battery flows in one direction - from the positive to the negative. This is called direct current (DC).

Alternating currents flow in one direction and then reverse and flow in the other. In the UK and other European countries the direction of current flow supplied to a building changes fifty times per second. This rate is called the frequency, measured in Hertz (Hz for short). The frequency of the mains supply in the UK is therefore 50 Hz.

The animation shows current from an AC source (the circle with a sine wave in it) flowing through a resistor.

The current flowing in an AC circuit does not abruptly change direction but instead takes takes the form of a sine wave.

The voltage from the supply gradually increases from zero, reaches a maximum and gradually returns back to zero. It then changes polarity, gradually increasing from zero, reaches a maximum and gradually returns back to zero. This repeats until the power is switched off.

On the diagram the red parts are called the positive half-cycle and the blue parts the negative half-cycle.

This process happens so quickly that you are not aware that it is happening.

The time it takes for one complete cycle to complete is called the periodic time (T) measured seconds.

Frequency is a measure of how many times the cycle repeats in one second. Frequency is the reciprocal of time and can be calculated using:

Therefore:

The peak voltage is taken between the horizonal axis and the highest peak on the positive or negative half-cycles.

The peak-to-peak voltage is taken between the peaks.

RMS stands for Root Mean Square and is the way we usually express AC voltages derived from the mains supply. For example the supply to your house will be around 230 V RMS, we usually just miss the RMS bit off in practice.

Another measure of an AC voltage is the average voltage. It is a measure we use less frequently.