A heating engineer installs two identical resistive heating elements in a domestic boiler. In one design, the two elements are connected in series. In a second design, the same two elements are connected in parallel across the same power supply. Explain why the parallel design transfers energy to the water at a greater rate than the series design.

OCR A-Level Physics B: Advancing Physics (H557) — 3.1 Motion · Explain · 5 marks · View as Markdown

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

A domestic boiler can be wired so that its two heating elements are connected either in series or in parallel across the same mains supply voltage.

Model answer (5 marks)

In a series circuit the total resistance is the sum of the two element resistances, so the total resistance is greater than that of a single element. In a parallel circuit the total resistance is less than the resistance of either element. Both designs are connected to the same mains voltage, so the lower resistance of the parallel circuit gives a greater current. Power transferred is P = IV; with the same voltage a greater current means more power. Therefore the parallel design transfers energy to the water at a greater rate than the series design.

Examiner tips

  • State the resistance relationship for series and parallel circuits
  • Explain that the same voltage is applied in both designs
  • Show that lower resistance gives higher current
  • Use P = IV to link current to power

Common mistakes

  • Confusing series and parallel resistance relationships
  • Forgetting that the voltage is the same in both designs
  • Using P = I²R instead of P = IV without justification

Mark scheme (5 marks)

  1. In a series circuit the total resistance is equal to the sum of the individual resistances, so total resistance is greater than in the parallel circuit
  2. In a parallel circuit the total resistance is less than the resistance of either individual element
  3. Both designs are connected to the same potential difference (voltage), so the lower resistance in the parallel design results in a greater current
  4. Power transferred is equal to current multiplied by voltage (P = IV), so a greater current at the same voltage means more power is transferred
  5. Therefore the parallel design heats the water more quickly / transfers energy at a greater rate than the series design

Key terms in this question

series · parallel · power

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