A hospital uses two types of medical warming blankets to keep patients comfortable during recovery. Blanket X has a power rating of 60 W and Blanket Y has a power rating of 90 W. Both blankets are connected to the mains supply, which provides the same potential difference across each blanket. Explain why Blanket Y transfers more energy to a patient than Blanket X when both are switched on for the same amount of time, and describe what this means for the current flowing through each blanket.

OCR A-Level Physics A (H556) — 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).

Both blankets are connected in parallel to the same mains supply and operate for the same duration.

Model answer (5 marks)

Blanket Y transfers more energy because power is the rate of energy transfer. With the same supply voltage, the higher power rating of 90 W means more energy is delivered per unit time than the 60 W blanket. Energy transferred = power × time, so for the same operating time Y gives greater energy.

Since both blankets are in parallel, the potential difference across each is the same. Power = I V, and with V equal, the blanket with the higher power must draw a larger current. Thus Blanket Y carries a greater current than Blanket X.

Examiner tips

  • State that power is the rate of energy transfer and use the formula E = P t.
  • Explain that the same voltage in parallel means P = I V, so higher P → higher I.
  • Mention that greater energy transfer means a higher current for the same voltage.

Common mistakes

  • Confusing power with energy; writing that the blanket with higher power has more energy without linking to time.
  • Assuming the blankets have the same resistance and ignoring the current‑voltage relationship.
  • Using the wrong units or omitting the fact that the blankets are in parallel.

Mark scheme (5 marks)

  1. Power is the rate at which a device transfers energy
  2. Blanket Y has a higher power rating so it transfers more energy in the same time
  3. Energy transferred equals power multiplied by time, so greater power gives greater energy for the same time
  4. Both blankets have the same potential difference across them (connected in parallel to the same supply)
  5. Since power = current × voltage and voltage is the same, Blanket Y must carry a greater current than Blanket X

Key terms in this question

power · potential difference · current · energy

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