A string of decorative lights is wired in series. A homeowner replaces the string with a new set wired in parallel. Describe two differences in how the current behaves in the parallel circuit compared with the series circuit.

AQA GCSE Physics (8463) — 4.2.2 Series and parallel circuits · Describe · 4 marks · View as Markdown

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

A string of decorative lights contains six identical bulbs. In the original series circuit, when one bulb breaks the entire string goes out. The homeowner replaces it with a parallel circuit containing the same six bulbs.

Model answer (4 marks)

1. In the series string the same current flows through every bulb, so if one bulb breaks the current stops and all bulbs go out.
2. In the parallel string the current splits at the junctions; each bulb has its own branch and the total current from the supply equals the sum of the currents through the six branches.
3. Because the current can take several paths, if one bulb burns out the other five continue to light.
4. Each bulb in the parallel circuit has the full supply voltage across it, whereas in series the supply voltage is divided between the bulbs.

Examiner tips

  • Use the word ‘current’ and ‘potential difference’ explicitly; mention that current splits and that the supply voltage is the same across each branch.
  • Show that the total current is the sum of branch currents – this is a key point for full marks.

Common mistakes

  • Confusing voltage with current – students may say the voltage is the same in series. Failing to state that the current splits at the junctions. Not mentioning that the broken bulb does not affect the others in parallel.

Mark scheme (4 marks)

  1. In a series circuit the current is the same through every component / through the whole circuit; in a parallel circuit the current splits at each junction / branch
  2. In the parallel circuit the total current from the source equals the sum of the currents through each branch
  3. In the parallel circuit each bulb / branch has the same potential difference across it (equal to the supply potential difference), whereas in the series circuit the potential difference is shared between the bulbs
  4. Because current can take more than one path in the parallel circuit, if one bulb breaks the others continue to work / the circuit remains complete for the other branches

Key terms in this question

series · parallel · current

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