A.C. & D.C. (Edexcel IGCSE Physics (Modular))

A.C. & D.C.

• Mains electricity can be supplied by alternating current (a.c.) or direct current (d.c.) from a cell or battery

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.)

A cell or battery provides a d.c. supply to a circuit

Circuits powered by cells or batteries use a d.c. supply

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 vs alternating current on an oscilloscope screen

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

Comparing alternating current & direct current

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

Direct current vs. alternating current table