3.1.2 Alternating and Direct Current

3.1.2 Alternating and Direct Current: Compare alternating and direct current, interpret oscilloscope traces, and use frequency and period to describe mains electricity.

A direct current always flows in the same direction around a circuit.

Direction

Charge flows one way only, from one terminal of the supply to the other.

Sources

Cells, batteries and solar cells supply direct current.

Trace

On an oscilloscope a steady direct supply appears as a flat horizontal line.

Uses

Portable electronic devices such as torches, phones and remote controls run on direct current.

Safety note

Mains alternating current is dangerous, so appliances are fitted with plugs, fuses and earthing to protect users.

An alternating current repeatedly changes direction and size.

Direction

The charge carriers move forwards and backwards, reversing direction regularly.

Sources

Generators and the mains socket supply alternating current.

Mains in the UK

The supply is about 230 V with a frequency of 50 Hz, meaning 50 complete cycles each second.

Trace

An oscilloscope shows a repeating wave that rises above and falls below the zero line.

Why used

Alternating supplies are easy to transform to different voltages, so power stations can send electricity over long distances efficiently.

Feature	Direct current	Alternating current
Direction	Constant	Reverses regularly
Typical source	Battery	Mains supply
Oscilloscope trace	Flat line	Repeating wave

Frequency can be found from the wave shown on the screen.

Count the number of complete waves shown across the screen of the oscilloscope.

Use the time-base setting to work out the total time represented across the screen.

Calculate the frequency using f = number of waves ÷ time, giving the answer in hertz.

Find the period, the time for one cycle, using T = 1 ÷ f.

Read the peak voltage as the greatest height of the trace above the zero line, using the voltage-gain setting.

Question: An oscilloscope shows exactly 2 complete cycles of an a.c. signal in a time of 40 ms. Calculate the frequency and the period.

Convert the time: 40 ms = 0.040 s.
Frequency f = number of waves ÷ time = 2 ÷ 0.040 = 50 Hz.
Period T = 1 ÷ f = 1 ÷ 50 = 0.020 s (20 ms).

Answer: The frequency is 50 Hz and the period is 0.020 s.

3.1.2 – Key Points for the Exam

Direct current: Current that flows in one direction only.
Alternating current: Current that changes direction regularly.
Frequency: The number of complete cycles each second, measured in hertz.
Period: The time taken for one complete cycle, equal to 1 ÷ frequency.

Common Mistakes

Saying that alternating current flows in only one direction.
Mixing up frequency (cycles per second) with the voltage of the supply.
Forgetting to convert milliseconds to seconds when finding frequency from the time base.
Calling a battery an a.c. source because it has two terminals.