AC & DC (Edexcel IGCSE Physics)

• 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

Direct Current (d.c.)	Alternating Current (a.c.)
Continuous and in one direction	Constantly changing direction
Produced by cells and batteries	Produced by electrical generators i.e. mains electricity
Has a positive and negative terminal	Has two identical terminals