A student notices that when she connects a longer wire between two points in a circuit, the lamp in the circuit becomes dimmer. Explain why increasing the length of the wire causes the current in the circuit to decrease.

Eduqas GCSE Physics — 7.1 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)

1. The resistance of a wire is directly proportional to its length.
2. A longer wire contains more atoms/ions for electrons to collide with, increasing resistance.
3. Higher resistance opposes the flow of charge, so the current is reduced.
4. With the same potential difference, V = IR gives I = V/R; a larger R gives a smaller I, making the lamp dimmer.

Examiner tips

  • Use the proportionality R∝L to justify the first point. Show the link V = IR to explain the decrease in current. Keep the answer concise and use correct terminology (resistance, current, potential difference).

Common mistakes

  • Confusing resistance with voltage drop. Failing to mention the V = IR relationship. Using vague phrases like "more resistance" without explaining why.

Mark scheme (4 marks)

  1. Resistance of the wire increases as its length increases
  2. A longer wire has more collisions between electrons and the atoms/ions of the wire
  3. Greater resistance means a greater opposition to the flow of charge/current
  4. For the same potential difference, a higher resistance results in a lower current (correct link using V = IR or equivalent reasoning)

Key terms in this question

current

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