A torch contains a battery connected to a bulb in a simple circuit. When a second identical bulb is added in series with the first bulb, both bulbs glow more dimly than the single bulb did. Explain why both bulbs glow more dimly when connected in series.

WJEC GCSE Physics (Wales) — 1.1 Electric circuits · Explain · 4 marks · View as Markdown

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

A torch contains a battery connected to a single bulb. A student then adds a second identical bulb in series with the first bulb in the circuit.

Model answer (4 marks)

Adding a second identical bulb in series increases the total resistance of the circuit. Because the resistance increases, the current supplied by the battery decreases. The battery voltage is now shared between the two bulbs, so each bulb receives only half the original voltage. With less current and lower voltage across each bulb, less electrical energy is converted to light per second, so both bulbs glow dimmer.

Examiner tips

  • Mention the increase in total resistance first, then the drop in current, then the voltage division, finally the reduced power per bulb.
  • Use the correct terms: resistance, current, voltage, power, energy per second.
  • Show the logical sequence: R↑ → I↓ → V per bulb↓ → P↓ → dimmer.
  • Keep the answer concise – 4 marks, one point per sentence.

Common mistakes

  • Failing to state that the resistance increases when a second bulb is added.
  • Confusing current with voltage – saying the voltage increases instead of decreases.
  • Not linking the reduced current/voltage to the lower power (energy per second).

Mark scheme (4 marks)

  1. Adding a second bulb in series increases the total resistance of the circuit
  2. Because resistance increases, the current in the circuit decreases
  3. The voltage of the battery is shared (divided) between the two bulbs
  4. Less current through each bulb (and lower voltage across each bulb) means less energy is transferred per second, so the bulbs are dimmer

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