A student connects a battery, a lamp, and a switch in a series circuit. The student then adds a second identical lamp in series with the first lamp. Explain what happens to the brightness of the original lamp when the second lamp is added, and why.

OCR GCSE Physics A: Gateway Science (J249) — P3.2 Simple circuits · Explain · 4 marks · View as Markdown

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

A student is investigating how adding components to a series circuit affects the behaviour of the circuit. The battery used has a fixed voltage.

Model answer (4 marks)

The original lamp becomes dimmer when the second lamp is added.
Adding a second lamp in series increases the total resistance of the circuit.
Because the battery voltage is fixed, the increased resistance causes the current in the circuit to decrease.
A smaller current through the lamp means less power is transferred, so the lamp is dimmer.

Examiner tips

  • Use the word ‘dimmer’ to show the change in brightness. Explain the chain: added lamp → ↑R → ↓I (V fixed) → ↓P → dimmer. Show the relationship P=IV or P=I²R to link current to brightness.

Common mistakes

  • Saying the lamp becomes brighter or no change. Forgetting that the battery voltage is fixed. Using the wrong formula for power (e.g. P=V²/R instead of P=I²R).

Mark scheme (4 marks)

  1. The original lamp becomes dimmer (after the second lamp is added)
  2. Adding a second lamp in series increases the total resistance of the circuit
  3. Because the voltage of the battery is fixed, the increased resistance causes the current in the circuit to decrease
  4. A smaller current through the lamp means less power is transferred, so the lamp is dimmer

Key terms in this question

series circuit

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