Physics

2.12 - AC & DC Current

7 min

Alternating current

UK mains supply

Direct current and oscilloscopes

note 1 of 3

Alternating and direct current

There are two types of electric current: alternating current (a.c.) and direct current (d.c.).

Direct current

In direct current, the charges move in one direction only, so the current always flows the same way around the circuit.
It's produced by a direct potential difference - a p.d. that stays positive, or stays negative, rather than switching between the two.
Cells and batteries supply direct current.

Alternating current

In alternating current, the current is constantly changing direction, flowing back and forth around the circuit.
It's produced by an alternating potential difference, in which the positive and negative ends keep swapping over.
The mains electricity supplied to homes is an alternating current.

note 2 of 3

UK mains supply

The UK mains supply is an alternating current supply at about 230 V.
Its frequency is 50 Hz (50 cycles per second).

note 3 of 3

Showing a.c. and d.c. on an oscilloscope trace

A cathode ray oscilloscope (CRO) displays how potential difference changes with time, so the trace it draws is a graph of voltage against time.

Reading the traces

Direct voltage - Gives a horizontal straight line, because the p.d. doesn't change with time. The height of the line above the centre line shows the size of the d.c. p.d.
Alternating voltage - Gives a wave that rises and falls in a regular, repeating pattern, going positive for part of the time and negative for the rest.
Graphs of current against time have the same shape as the corresponding voltage-against-time traces.

Two graphs of voltage against time, headed direct voltage and alternating voltage: the first has a flat orange line held above the axis, the second an orange wave swinging equally above and below it.

key terms

TermDefinition

Direct currentA current where the charges move in one direction only, flowing the same way around the circuit.

Alternating currentA current that is constantly changing direction, flowing back and forth around the circuit.

Direct potential differenceA potential difference that stays positive or stays negative, rather than switching between the two.

Alternating potential differenceA potential difference in which the positive and negative ends keep swapping over.

OscilloscopeAn instrument that displays how a potential difference changes with time, as a graph of potential difference against time. It is also called a cathode ray oscilloscope.

end of notes

Key points to remember

Alternating and direct current

There are two types of electric current: alternating current (a.c.) and direct current (d.c.).

Direct current

In direct current, the charges move in one direction only, so the current always flows the same way around the circuit.

It's produced by a direct potential difference - a p.d. that stays positive, or stays negative, rather than switching between the two.

Cells and batteries supply direct current.

Alternating current

In alternating current, the current is constantly changing direction, flowing back and forth around the circuit.

It's produced by an alternating potential difference, in which the positive and negative ends keep swapping over.

The mains electricity supplied to homes is an alternating current.

Showing a.c. and d.c. on an oscilloscope trace

A cathode ray oscilloscope (CRO) displays how potential difference changes with time, so the trace it draws is a graph of voltage against time.

Reading the traces

Direct voltage - Gives a horizontal straight line, because the p.d. doesn't change with time. The height of the line above the centre line shows the size of the d.c. p.d.

Alternating voltage - Gives a wave that rises and falls in a regular, repeating pattern, going positive for part of the time and negative for the rest.

Graphs of current against time have the same shape as the corresponding voltage-against-time traces.