A engineer is designing a circuit for a new electric vehicle charging station. One charging port uses a motor with a high resistance, and a second charging port uses a motor with a lower resistance. Both motors are connected in parallel to the same power supply, so the potential difference across each motor is identical. Explain why the motor with the lower resistance transfers energy at a greater rate than the motor with the higher resistance, when both are connected to the same potential difference.

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 motors are connected in parallel to a power supply, so the potential difference across each motor is the same. The two motors have different resistances.

Model answer (5 marks)

Power is the rate at which energy is transferred (P = IV). With the same potential difference across each motor, the motor of lower resistance allows a greater current to flow, because I = V/R. A larger current at the same voltage therefore gives a larger power output, since P = IV. Consequently the lower‑resistance motor transfers energy at a greater rate – it delivers more power. Energy transferred over a given time is E = Pt, so the higher power means more energy is transferred in the same time.

Examiner tips

  • Use the formula P = IV to link voltage, current and power; show I = V/R for each motor; explain that higher I gives higher P; mention E = Pt to tie back to energy transfer.
  • Structure answer: definition of power, effect of resistance on current, link to power, conclusion on energy transfer. Keep sentences short and use exact terminology.

Mark scheme (5 marks)

  1. Power is the rate at which energy is transferred (power = current × voltage / P = IV)
  2. With the same potential difference, the motor with lower resistance allows a greater current to flow
  3. A greater current means greater power is transferred (linking larger current to larger power at the same voltage)
  4. Therefore the lower-resistance motor transfers more energy per second / has a higher power output
  5. Energy transferred also equals charge × potential difference (E = QV) or energy = power × time (E = Pt), linking back to why more current means more energy transferred over the same time

Key terms in this question

power · potential difference · resistance · energy

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