AC & DC
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
Examiner Tip
If you are asked to explain the difference between alternating and direct current, sketching and labelling the graphs shown above can earn you full marks.
