AC & DC (OCR Gateway GCSE Physics: Combined Science)

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 travels in one direction only
  • This means the current is only positive or only negative

• 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

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 a.c. power supply has two identical terminals that switches between positive and negative
  • The current is therefore defined as positive or negative, depending on which direction it is flowing at that time

• 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
• 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

Comparing AC & DC

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

Direct Current vs. Alternating Current Table