Direct & Alternating Current (Cambridge International IGCSE Physics)

Spec point — 4.2.6 · Know the difference between direct current (d.c.) and alternating current (a.c.)

Direct & alternating current

• There are two types of current:

  • direct current (d.c.)
  • alternating current (a.c.)

Direct current

• A direct current (d.c.) is defined as:

A steady current, constantly flowing in the same direction in a circuit, from positive to negative

• The potential difference across a cell in a d.c. circuit travels in one direction only

  • The current travels from the positive terminal to the negative terminal

• A d.c. power supply has a fixed positive terminal and a fixed negative terminal

• Electric cells, or batteries, produce direct current (d.c.)

Alternating current

• An alternating current (a.c.) is defined as:

A current that continuously changes its direction, going back and forth around a circuit

• An alternating current power supply has two identical terminals that change from positive to negative and back again

  • The alternating current always travels from the positive terminal to the negative terminal
  • Therefore, the current changes direction as the polarity of the terminals changes

• The frequency of an alternating current is the number of times the current changes direction back and forth each second

  • In the UK, mains electricity is an alternating current with a frequency of 50 Hz and a potential difference of around 230 V

Graphs of direct current and alternating current

Two graphs showing the variation of current with time for alternating current and direct current

Comparing direct and alternating current

• The following table summarises the differences between d.c. and a.c.

Exam Hint If asked to explain the difference between alternating and direct current, sketch the graphs shown above: a well-sketched (and labelled) graph can earn you full marks.

AC & DC current video

Spec point — 4.2.6 · nan