Direct and alternating potential difference: Students should be able to explain the difference between direct and alternating potential difference.

The idea

Potential difference (p.d.) drives current and is measured in volts (V). In a direct p.d., the polarity stays fixed, so current flows one way around the circuit; a cell or battery gives direct p.d., and its voltage is steady, as shown by a horizontal line on a voltage–time graph. In an alternating p.d., the polarity swaps repeatedly, so current reverses direction; the mains supply in the UK alternates at about 50 Hz, completing 50 cycles each second, and its voltage–time graph is a wave crossing zero. The peak value of the UK mains is about 325 V, while the quoted value of 230 V is an average effect, not the peak. To explain the difference, state the direction of current, the shape of the voltage–time graph and a named source for each type.

What earns the marks

• Direct p.d. has constant polarity, so current flows in one direction only; a cell or battery is a common source.
• Alternating p.d. repeatedly reverses polarity, so current changes direction; the mains supply is a common source.
• A voltage–time graph for direct p.d. is a horizontal line, whereas for alternating p.d. it is a wave that crosses the zero line.
• UK mains alternating p.d. has a frequency of about 50 Hz, meaning 50 cycles per second.
• The quoted 230 V for UK mains is not the peak value; the peak is about 325 V.
• Explanations should link the p.d. pattern to the resulting current direction rather than merely naming the two types.

Common mistakes and how to put them right

• Saying alternating current flows both ways at once.

  Put it right by stating that the direction reverses repeatedly over time, so at any instant it flows one way.

• Treating 230 V as the peak mains voltage.

  Put it right by noting that 230 V is an average value and the peak is about 325 V.

• Describing a battery as alternating because it can be recharged.

  Put it right by stating that batteries supply direct p.d. and only the charging supply may be alternating.

Exam tips

• Sketch both voltage–time graphs and label the axes with potential difference in V and time in s to make the contrast visible.
• Use the phrase 'reverses direction' for alternating and 'one direction' for direct, then name a source for each.
• If asked to compare, give one similarity (both measured in volts) and then two differences to secure the explanation.