A student connects two resistors in series in a circuit and then reconnects them in parallel using the same power supply. Explain why the total resistance of the parallel circuit is less than the total resistance of the series circuit.

AQA GCSE Physics (8463) — 4.2.1 Current, potential difference and resistance · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

In a series circuit the total resistance is the sum of the individual resistances, so R_total,series = R1 + R2.
In a parallel circuit the total resistance is given by 1/R_total,parallel = 1/R1 + 1/R2, which is always less than the smallest individual resistance. Because the current can split between the two branches, there are two paths for charge to travel. This makes it easier for charge to flow, so the overall opposition to current flow (resistance) is reduced.

Examiner tips

  • Use the formula for series and parallel resistance to justify the comparison.
  • Explain that current splits in parallel, giving more paths and lower resistance.
  • Mention the smallest resistance rule to show why R_parallel < R_series.

Common mistakes

  • Confusing the series and parallel formulas, e.g. using R_total,parallel = R1 + R2.
  • Failing to mention that current splits in parallel, or that the resistance is less than the smallest resistor.
  • Using vague language like "more current" without linking it to reduced resistance.

Mark scheme (4 marks)

  1. In series, the total resistance is the sum of the individual resistances
  2. In parallel, the total resistance is less than the smallest individual resistance
  3. In parallel, charge (current) has more than one path / branch to travel through
  4. Because current can split between branches, it is easier for charge to flow, so overall opposition to current flow (resistance) is reduced

Key terms in this question

resistance · series · parallel

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