Explain the difference between direct and alternating voltage

The idea

A direct voltage keeps the same polarity, so the potential difference stays on one side of zero and current flows one way around the circuit; a battery gives this kind of supply. An alternating voltage repeatedly reverses polarity, so the potential difference swings between positive and negative values and the current changes direction each cycle; the UK mains at 50 Hz is an example. The key contrast is direction: d.c. is unidirectional and steady in sign, while a.c. changes direction periodically. A good answer links the voltage behaviour to the resulting current direction and can name a source of each type. When choosing an option, check that it captures the reversal of an alternating supply and the one-way nature of a direct supply, and reject options that confuse voltage with frequency or claim a.c. never changes value.

What earns the marks

• States that a direct voltage keeps a constant polarity so current flows in one direction.
• States that an alternating voltage reverses polarity periodically so current changes direction.
• Links the type of voltage to an everyday source, such as a battery for direct and the mains for alternating.
• Rejects options that misdescribe alternating voltage as steady or direct voltage as reversing.

Common mistakes and how to put them right

• Claims that alternating voltage stays constant.

  Put it right: It reverses direction each cycle.

• Describes direct voltage as changing direction.

  Put it right: Direct voltage keeps one polarity.

• Confuses the size of the voltage with its type.

  Put it right: The difference is about direction, not magnitude.

Exam tips

• Focus on the word 'direction' when comparing the two, since that is the central difference.
• Match each type to a familiar source to confirm your choice.
• Rule out any option that says an alternating supply is steady or a direct supply reverses.