A building uses two types of electric heaters in its entrance hall. Heater X is connected in a series circuit and Heater Y is connected in a parallel circuit with an identical heater. Both circuits are powered by the same mains supply. Explain how connecting Heater Y in parallel with an identical heater affects the total resistance of that circuit and, as a result, the power transferred by Heater Y compared to Heater X operating alone.

OCR A-Level Physics B: Advancing Physics (H557) — 3.3 Work, energy and power · Explain · 5 marks · View as Markdown

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

Power is the rate at which an appliance transfers energy. For a circuit component, power is equal to the product of the current passing through it and the potential difference across it. When resistors are connected in parallel, the total resistance is less than the lowest individual resistance.

Model answer (5 marks)

1. Heater Y and its identical twin are connected in parallel, so both share the same potential difference from the mains.
2. The equivalent resistance of two equal resistors in parallel is R/2, which is less than the resistance of a single heater.
3. A lower resistance means a larger total current is drawn from the supply (I=V/R).
4. Because each heater still experiences the full supply voltage, the power of each heater is P=VI and is therefore the same as the power of Heater X.
5. The total power of the parallel pair is the sum of the two equal powers, so it is greater than the power of Heater X alone.

Examiner tips

  • Use the formula P=VI and explain why V is unchanged in parallel. Show that R_eq=R/2 for two equal resistors. Link lower R to higher I. State that each heater’s P is unchanged but total P doubles.
  • common_mistakes
  • :
  • Saying the power of Heater Y is lower because of lower resistance. Confusing the voltage across each heater with the total voltage. Forgetting to double the power when two heaters operate simultaneously.

Mark scheme (5 marks)

  1. Connecting Heater Y in parallel means both heaters share the same potential difference (from the supply)
  2. The total resistance of the parallel circuit is less than the resistance of a single heater alone
  3. Lower total resistance means a greater total current is drawn from the supply
  4. The potential difference across Heater Y is the same as across Heater X (full supply voltage), so the power transferred by Heater Y is the same as Heater X
  5. The total power transferred by the parallel circuit is greater than that of Heater X alone, because both heaters transfer power simultaneously

Key terms in this question

parallel · total resistance · power

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