Mains Electricity » Alternating and Direct Current

Two kinds of electricity supply

A torch and a toaster both need a current, but they do not get the same kind. The torch runs on a battery. The toaster runs on the mains. The current from each supply behaves in a different way, and you need to know that difference for the exam.

There are only two kinds to learn. Their short names are written in small letters with full stops: d.c. and a.c.

Key terms:

• Direct current (d.c.): a current that flows in one direction only.
• Alternating current (a.c.): a current that keeps reversing its direction.
• Mains electricity: the electricity supplied to the sockets and lights in a building.
• Trace: the line on a graph or screen that shows how the voltage of a supply changes with time.

Direct current

A cell or battery supplies direct current. One end of a cell is always the positive terminal and the other end is always the negative terminal. They never swap over. So the cell always pushes the charge the same way round the circuit, and the current flows in one direction only.

Anything that runs on cells or a battery works on d.c.: a television remote control, a wall clock, a wireless doorbell, a mobile phone. The voltage of a cell is also steady. A new 1.5 V cell gives 1.5 V this second, the next second and the second after that.

If you turn a cell round in its holder, the current flows the other way round the circuit. It is still d.c., because it still flows in one direction only. It does not keep changing.

Alternating current

Mains electricity is alternating current. The mains supply does not have a fixed positive side and a fixed negative side. Its voltage keeps changing direction, so it pushes the charge one way, then the other way, then back again. The current reverses many times every second, for as long as the appliance is switched on.

In an a.c. circuit the charge does not travel all the way round. It moves backwards and forwards in the wires. Energy is still transferred to the appliance, which is why a mains lamp lights and a mains heater gets hot.

In the UK the mains voltage is about 230 V. You do not have to memorise that number for this spec point, but you will see it in many questions. What you must know is that mains is a.c. and cells and batteries give d.c.

Voltage-time traces

You cannot see a current, so how can you tell the two kinds apart? One way is to plot a graph of the voltage of the supply against time. The line on the graph is called a trace. Some instruments draw this trace on a screen for you.

The graph has time along the bottom and voltage up the side. The important line is the zero line, a horizontal line where the voltage is 0 V. It runs across the middle of the graph. Above the zero line the voltage is in one direction (positive). Below it the voltage is in the opposite direction (negative).

Follow the a.c. trace with your finger. The voltage grows to its largest value in one direction, shrinks to zero, grows to the same largest value in the opposite direction, then shrinks to zero again. The wave is the same size above the line as below it.

What happens to the d.c. trace if you reverse the connections to the battery? The voltage is now in the opposite direction, so the flat line appears the same distance below the zero line. It is still flat and still stays on one side, so it is still d.c.

Comparing a.c. and d.c.

	Direct current (d.c.)	Alternating current (a.c.)
Direction of the current	One direction only	Keeps reversing
Supplied by	A cell or battery	The mains
Shape of the trace	Flat horizontal line	Smooth wave
Where the trace sits	On one side of the zero line	Equally above and below the zero line
Does the trace cross zero?	No	Yes, again and again

The quickest test for any trace is the last row. If the trace crosses the zero line, the supply is a.c. If it stays on one side, the supply is d.c.

The power supply symbols

You met the two power supply symbols in Electrical Units and Circuit Symbols. Now you can see why they look the way they do.

• d.c. power supply: two small circles marked + and −. The signs are there because the positive and negative sides are fixed.
• a.c. power supply: two small circles with a short wavy line (~) between them. The wavy line is a tiny picture of the a.c. trace. There are no + and − signs, because neither side stays positive.

In an exam, read the supply symbol in a circuit diagram carefully. It tells you at once whether the current in the circuit flows one way or keeps reversing.

Exam-style question

A student plots voltage-time traces for two different electricity supplies. Both graphs have the zero line across the middle.

Trace X is a horizontal straight line, three squares above the zero line.

Trace Y is a smooth wave that rises three squares above the zero line and falls three squares below it, again and again.

(a) State which trace shows an alternating current supply. [1 mark]

(b) Describe the difference between the current from supply X and the current from supply Y. [2 marks]

(c) One of the supplies is a battery. State which one, and give a reason for your answer. [2 marks]

(d) The connections to the battery are reversed. Describe the trace that the student now sees. [2 marks]