2.2.4 AC & DC

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

A current that is steady, 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
• A d.c. power supply has a fixed positive terminal and a fixed negative terminal
• Electric cells, or batteries, produce direct current (d.c.)

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

• A series circuit consists of a string of components connected in a loop between the ends of the power supply
• In a series circuit:
  • The current is the same at all points
  • The potential difference of the power supply is shared between the individual components

• Two disadvantages of series circuits are:
  • If one of the components breaks, all of the others will stop working
  • The components cannot be controlled (switched on and off) separately

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

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

• On an oscilloscope, direct current and alternating current are represented in the following way:

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

• An a.c. power supply has two identical terminals
• The potential difference between the terminals varies as a sine curve, or sinusoidally
• 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

Comparing Direct & Alternating Current

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