A parallel circuit contains two branches. One branch contains a filament lamp and the other branch contains a fixed resistor. Explain what happens to the total resistance of the circuit and the current through the fixed resistor when the filament lamp is disconnected from the circuit.

Edexcel A-Level Physics (9PH0) — 10.1 Stars, life cycles and cosmology · Explain · 5 marks · View as Markdown

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

A parallel circuit contains two branches connected to a 6 V battery. One branch contains a filament lamp with a resistance of 12 Ω and the other branch contains a fixed resistor with a resistance of 6 Ω.

Model answer (5 marks)

When the lamp is disconnected the circuit no longer has two parallel branches – only the fixed resistor remains, so the circuit becomes effectively a single resistor in series with the battery. The total resistance therefore increases from the parallel value (3 Ω) to the single‑resistor value (6 Ω). The voltage across the fixed resistor is still the full 6 V of the battery, so its current is I=V/R=6 V/6 Ω=1 A, unchanged from before the lamp was removed.

Examiner tips

  • State that the circuit changes from parallel to a single resistor, giving the new total resistance.
  • Explain that the voltage across the fixed resistor remains 6 V, so its current is unchanged.
  • Use the formula I=V/R to show the unchanged current.
  • Mention the numerical values to demonstrate the change in total resistance.

Common mistakes

  • Confusing the total resistance of the parallel circuit with that of the single resistor.
  • Assuming the current through the fixed resistor changes because the lamp is removed.
  • Failing to recognise that the voltage across the fixed resistor stays at 6 V.

Mark scheme (5 marks)

  1. When the lamp is disconnected, the circuit changes from parallel to series (or: only the fixed resistor branch remains).
  2. The total resistance of the circuit increases.
  3. Because in a parallel circuit the total resistance is less than the smallest individual resistance, so removing a branch always increases total resistance.
  4. The potential difference across the fixed resistor remains 6 V (equal to the supply voltage).
  5. Therefore the current through the fixed resistor does not change (stays the same), because the potential difference across it and its resistance are both unchanged.

Key terms in this question

parallel · total resistance · current · fixed resistor

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