A torch contains three identical cells connected in series, each with an electromotive force (EMF) of 1.5 V. The torch has two identical bulbs connected in parallel with each other, and this parallel combination is connected in series with a switch and the cells. Explain what happens to the brightness of the remaining bulb and the current drawn from the cells when one bulb breaks and stops working.

WJEC A-Level Physics (Wales) — 2.2 DC circuits · Explain · 5 marks · View as Markdown

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

A torch contains three identical cells connected in series. The torch circuit has two identical bulbs connected in parallel with each other, and this parallel combination is connected in series with a switch and the cells.

Model answer (5 marks)

The three cells give a total EMF of 4.5 V (3×1.5 V). This voltage is applied across the parallel pair of bulbs. In a parallel circuit each branch has the same potential difference, so the working bulb still has 4.5 V across it. Because the voltage across the bulb is unchanged, its brightness remains the same. When one bulb fails the parallel branch is removed, so the total resistance of the circuit increases. With the EMF unchanged, the total current drawn from the cells therefore decreases.

Examiner tips

  • State the total EMF and explain it is applied across the parallel bulbs.
  • Explain that each parallel branch has the same voltage, so the remaining bulb keeps 4.5 V.
  • Show that unchanged voltage gives unchanged brightness.
  • Note that removing a branch raises total resistance.

Mark scheme (5 marks)

  1. The total EMF of the cells is 4.5 V (three cells × 1.5 V each), and this voltage is applied across both bulbs / the parallel combination
  2. In a parallel circuit, each branch has the same potential difference across it, so the remaining working bulb still has the full 4.5 V across it
  3. Because the voltage across the remaining bulb is unchanged, it glows at the same brightness as before
  4. When one bulb breaks, there is one fewer parallel branch, so the total resistance of the circuit increases
  5. Because the total resistance increases and the EMF is unchanged, the total current drawn from the cells decreases

Key terms in this question

electromotive force (EMF) · current

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