Alternating and Direct Current — Electrical Power & Mains Electricity | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 10% of your exam marks.

stable

Medium

Stable10%

P = IV and energy calculations, plus mains electricity safety, appear in most series.

Direct current (d.c.)

A direct current (d.c.) is a current that keeps flowing in just one direction around a circuit, starting at the source's positive end and arriving back at its negative end
The two terminals of a d.c. source keep their roles unchanged: one stays at the higher potential (the positive end) and the other stays at the lower potential (the negative end) for the whole time the source is connected
Sources of d.c. include cells, batteries, solar panels and d.c. packs used in school labs

Alternating current (a.c.)

An alternating current (a.c.) is a current whose direction continually reverses, flowing one way, then the opposite way, then back again, many times every second
The supply's two terminals swap polarity in step with the current; neither terminal is permanently positive
The frequency of an a.c. supply is the number of complete cycles per second, measured in hertz (Hz)
In the United Kingdom, the mains supply is a.c. at a frequency of 50 Hz and a voltage of about 230 V
Sources of a.c. include power-station generators and the sockets in any home

Exam tip

Distinguish d.c. from a.c.

What comes up: state or explain the difference between direct current and alternating current.

Write (two marks): (1) d.c. flows in one direction only; (2) a.c. continuously reverses direction.

Watch out: saying a.c. "changes" without specifying that it reverses direction may not score the second mark — the credited idea is that the direction changes, not just that the current fluctuates.

Comparison of d.c. and a.c. at a glance

Feature	Direct current (d.c.)	Alternating current (a.c.)
Direction of flow	One fixed direction	Continually reversing
Source terminals	One fixed positive, one fixed negative	Two identical terminals that swap polarity
Current-vs-time graph	Flat horizontal line above the zero axis	Sine wave above and below the zero axis
Typical sources