A student is investigating two electric heaters, Heater A and Heater B, connected to the same mains supply. Heater A has a greater resistance than Heater B. The student says: 'The heater with the greater resistance will always transfer energy at a faster rate.' Explain whether the student is correct, referring to how resistance affects current and power in this circuit.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two electric heaters, Heater A and Heater B, are each connected separately to the same mains potential difference. Heater A has a greater resistance than Heater B.
Model answer (5 marks)
The student is incorrect.
1. Power is the rate of energy transfer: P = IV.
2. With the same mains voltage, a higher resistance reduces the current (I = V/R).
3. Therefore Heater A (higher R) draws less current.
4. Lower current gives lower power: P_A < P_B.
5. Heater B, having lower resistance, draws more current and so transfers energy faster.
1. Power is the rate of energy transfer: P = IV.
2. With the same mains voltage, a higher resistance reduces the current (I = V/R).
3. Therefore Heater A (higher R) draws less current.
4. Lower current gives lower power: P_A < P_B.
5. Heater B, having lower resistance, draws more current and so transfers energy faster.
Examiner tips
- State the power formula P=IV first.
- Explain the effect of R on I using Ohm’s law.
- Show the comparison of P_A and P_B clearly.
- Use the word ‘therefore’ to link reasoning to conclusion.
Common mistakes
- Confusing resistance with power; writing P = I²R instead of P = IV.
- Assuming higher resistance means higher power without considering the voltage.
- Failing to mention that the mains voltage is the same for both heaters.
Mark scheme (5 marks)
- The student is incorrect
- Power (rate of energy transfer) depends on both current and voltage / P = IV
- A greater resistance causes a smaller current to flow (at the same potential difference)
- Heater A (greater resistance) therefore has a lower power / transfers energy at a slower rate than Heater B
- Heater B (lower resistance) draws a larger current and therefore has a greater power / transfers energy at a faster rate
Key terms in this question
power · resistance · current
Related
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- Decode the mark scheme abbreviations →
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