A rechargeable electric bicycle motor transfers energy to the wheels. The motor operates at a potential difference of 36 V and draws a current of 15 A. Explain how the concepts of power and energy transfer apply to this motor, and describe what happens to the rate of energy transfer if the current drawn by the motor doubles while the potential difference stays the same.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric bicycle uses a rechargeable battery to power a motor. The motor is connected to the battery and drives the rear wheel. The motor operates at a potential difference of 36 V and draws a current of 15 A during normal riding.
Model answer (5 marks)
Power is the rate at which the motor transfers energy to the wheels.
Power equals current multiplied by potential difference: P = IV.
With a 36 V supply and 15 A current, the motor’s power is 36 V × 15 A = 540 W.
If the current doubles to 30 A while the voltage remains 36 V, the power becomes 36 V × 30 A = 1080 W.
Thus the energy transferred to the wheels per second doubles – the wheels receive twice as much energy each second.
Power equals current multiplied by potential difference: P = IV.
With a 36 V supply and 15 A current, the motor’s power is 36 V × 15 A = 540 W.
If the current doubles to 30 A while the voltage remains 36 V, the power becomes 36 V × 30 A = 1080 W.
Thus the energy transferred to the wheels per second doubles – the wheels receive twice as much energy each second.
Examiner tips
- Show the formula P = IV and plug in the numbers for the 540 W calculation.
- Explain that power is energy per unit time and that doubling current doubles power at constant voltage.
- Mention the consequence – faster energy transfer to the wheels.
Common mistakes
- Using the wrong formula (e.g. P = V²/R) instead of P = IV.
- Forgetting to state that power is energy per second.
- Not showing the calculation or units for the 540 W and 1080 W values.
Mark scheme (5 marks)
- Power is the rate at which the motor transfers energy
- Power equals current multiplied by potential difference (P = IV)
- The motor's power is 540 W (correct calculation: 15 × 36)
- If the current doubles (to 30 A), the power also doubles (to 1080 W)
- Greater power means energy is transferred to the wheels at a faster rate / more energy transferred each second
Key terms in this question
power · potential difference · current · energy · rate
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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