Explain the difference between direct and alternating voltage

The idea

Direct voltage keeps one polarity, so the potential difference across a component always pushes charge the same way round the circuit; a cell or battery gives this steady d.c. supply. Alternating voltage repeatedly reverses its polarity, so the potential difference changes direction at a fixed frequency, and the mains supply in the UK is an example at about 50 Hz. To explain the difference, describe both the direction behaviour and the change over time: d.c. stays constant in direction (and typically in size), while a.c. swings positive and negative about zero. A useful method is to sketch both voltage–time graphs: a flat horizontal line for d.c. and a wave crossing the axis for a.c.

What earns the marks

• States that direct voltage maintains a single, unchanging polarity so charge is pushed one way round the circuit.
• States that alternating voltage reverses its polarity periodically, so the potential difference changes direction at a regular frequency.
• Identifies a cell or battery as a source of direct voltage and the mains supply as a source of alternating voltage.
• Describes the voltage–time graph as a steady horizontal line for d.c. and a wave that crosses zero for a.c.

Common mistakes and how to put them right

• Confuses voltage with current and describes only charge flow.

  Put it right: Keep the answer focused on the potential difference and its direction.

• Claims that alternating voltage changes size but never direction.

  Put it right by stating that a.c. reverses polarity periodically.

• Assumes all batteries give alternating voltage.

  Put it right by recalling that cells and batteries supply direct voltage.

Exam tips

• Read every option fully before choosing, because distractors often swap the words direct and alternating.
• Link each option to a real source, such as a battery for d.c. and the mains for a.c., to test whether it makes sense.
• Check whether an option mentions direction, size or both, since the key difference is the reversal of direction.