A sports centre has two electric water pumps, Pump A and Pump B, that move water around a heating system. Pump A has a potential difference of 230 V across it and a current of 4 A flowing through it. Pump B operates at the same potential difference but has twice the current of Pump A. Explain which pump transfers energy at a greater rate and why a higher current leads to a greater rate of energy transfer.

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 pumps are connected to the same mains supply of 230 V and operate continuously. Pump B has a current of 8 A flowing through it.

Model answer (5 marks)

Power is the rate at which energy is transferred (energy per second). For an electric device the power is P = IV, where I is the current and V the potential difference. Both pumps have the same V = 230 V, but Pump B has twice the current of Pump A (8 A vs. 4 A). Therefore Pump B has a greater power: P_B = 230 V × 8 A = 1840 W, whereas P_A = 230 V × 4 A = 920 W. The higher current means more charge flows each second (Q = It). With the same potential difference, a larger charge per second carries more energy per second, so Pump B transfers energy at a greater rate.

Examiner tips

  • State that power is energy per second and give the formula P=IV.
  • Show the numerical calculation for each pump to demonstrate the difference.
  • Explain that higher current = more charge per second, leading to higher power.

Common mistakes

  • Confusing power with voltage or current alone.
  • Using the wrong formula (e.g., P=V^2/R instead of P=IV).
  • Failing to show that both pumps have the same voltage before comparing currents.

Mark scheme (5 marks)

  1. Power is the rate at which energy is transferred (accept: power is energy transferred per second)
  2. Power equals current multiplied by potential difference / P = IV
  3. Pump B has a greater power / transfers energy at a greater rate because it has a higher current (at the same potential difference)
  4. A higher current means more charge flows per second / greater rate of flow of charge
  5. More charge flowing per second through the same potential difference means more energy transferred per second / greater power output

Key terms in this question

potential difference · current · energy

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