Alternating Current & Direct Current (WJEC GCSE Science (Double Award)): Revision Note

Alternating Current & Direct Current

Direct Current

• 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 is in one direction only

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

• Cells, and batteries, produce direct current (d.c.)

An Example of a D.C Circuit

Circuits powered by cells or batteries use direct current

Alternating Current

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

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

• The potential difference of an a.c. circuit changes polarity

• In an a.c. supply, the positive and negative terminals alternate, which causes the current to continually switch direction

• Power packs and the mains supply, produce alternating current (a.c.)

An Example of an A.C. Circuit

Circuits powered by power packs or the mains supply use alternating current

Comparing Direct and Alternating Current

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

Oscilloscope Traces of Direct and Alternating Current

Direct current is shown as a flat line, because the current does not change, alternating current is shown as a sine curve because the current oscillates back and forth

• An a.c. power supply has two identical terminals

• The potential difference between the terminals varies as a sine curve, or sinusoidally

• This is because 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 voltage of around 230 V

Table of Comparison for Direct and Alternating Current