Mains supply and batteries

ac and dc

Cells, batteries and the supply are all sources of electrical energy. We can use an oscilloscope to examine electrical signals to see how they change over time. This allows us to see differences between the electricity provided by a and by the mains.

An oscilloscope gives the following display for the electricity from a battery.

Oscilloscope trace of direct current: a straight horizontal line at the top of the screen — Figure caption,
Oscilloscope trace showing battery voltage: a straight horizontal line at the top of the screen.

The signal is a horizontal line showing constant voltage. This means that it does not change over time.

Cells and batteries supply direct current (\(dc\)). This means that in a circuit with an energy supply from a cell or battery, the current is always in the same direction in the circuit.

The oscilloscope gives the following display for the electricity from the mains.

Oscilloscope trace of alternating current: a sine wave shape across the screen starting at 0 V. — Figure caption,
Oscilloscope trace of the voltage from an alternating supply.

The signal is constantly changing value between positive and negative.

The voltage from the mains supply is alternating. This produces an alternating (\(ac\)) in circuits. This means that the current in a circuit using a mains supply continually changes direction backwards and forwards in a circuit.

The current changes direction 100 times per second, making the frequency of the mains supply 50 hertz (\(50Hz\)).

The of the mains supply in Britain is 230 (\(230V\)).