A student sets up a circuit with a battery and a single lamp. The student then adds a second identical lamp in series with the first lamp. Describe what happens to the brightness of the original lamp when the second lamp is added in series, and explain why this change occurs.

Eduqas GCSE Physics — 1.1 Electric circuits · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The original lamp becomes dimmer.
Adding a second identical lamp in series increases the total resistance of the circuit.
Because the resistance rises, the current through the circuit decreases.
With less current flowing through the original lamp, less electrical energy is converted to light per second, so it is dimmer.

Examiner tips

  • Use the exact phrase "becomes dimmer". Show the chain: series → ↑R → ↓I → ↓brightness. Mention "identical lamp" to justify equal resistance addition.
  • Avoid saying "the lamp is dimmer" without linking to current change.

Common mistakes

  • Saying the lamp becomes brighter. Forgetting to explain that resistance increases. Using vague terms like "less power" without linking to current.

Mark scheme (4 marks)

  1. The original lamp becomes dimmer (less bright)
  2. Adding a lamp in series increases the total resistance of the circuit
  3. Because resistance increases, the current in the circuit decreases
  4. Less current through the original lamp means less energy is transferred per second, so it is dimmer

Key terms in this question

series

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