A student is investigating two electric heaters, Heater A and Heater B, which are connected to the same mains supply. Heater A has a greater resistance than Heater B. Both heaters are ohmic conductors operating at constant temperature. Explain which heater transfers energy to the surroundings at a greater rate, and describe what happens to the rate of energy transfer for both heaters if the potential difference of the mains supply is increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two electric heaters, Heater A and Heater B, are connected to the same mains potential difference. Heater A has a greater resistance than Heater B. Both heaters behave as ohmic conductors at constant temperature.
Model answer (5 marks)
1. Power is the rate at which energy is transferred to the surroundings.
2. With the same potential difference, the heater with the lower resistance draws the greater current.
3. Heater B has the lower resistance, so it carries a larger current and therefore has a higher power.
4. Increasing the mains potential difference raises the current through each heater (Ohm’s law, I = V/R).
5. Because power P = IV, both current and voltage increase, so the rate of energy transfer (power) rises for both heaters.
2. With the same potential difference, the heater with the lower resistance draws the greater current.
3. Heater B has the lower resistance, so it carries a larger current and therefore has a higher power.
4. Increasing the mains potential difference raises the current through each heater (Ohm’s law, I = V/R).
5. Because power P = IV, both current and voltage increase, so the rate of energy transfer (power) rises for both heaters.
Examiner tips
- Use the definition of power first, then relate current to resistance via Ohm’s law, and finally apply P = IV to show the effect of changing V.
- Show the logical sequence: same V → lower R → higher I → higher P.
- Mention that both I and V increase together, so P increases for both heaters.
Common mistakes
- Confusing resistance with power – stating that higher resistance gives higher power.
- Forgetting to mention that both current and voltage increase when V is increased.
- Using the wrong formula (e.g. P = I²R) without explaining the change in I and V.
Mark scheme (5 marks)
- Power is the rate at which energy is transferred (by an appliance / to the surroundings)
- Because they share the same potential difference, the heater with lower resistance draws greater current (Heater B has lower resistance so carries greater current)
- Heater B transfers energy at a greater rate (has greater power) because it has lower resistance and therefore higher current
- Increasing the potential difference increases the current through each heater (since they are ohmic conductors / current is proportional to potential difference)
- The rate of energy transfer (power) increases for both heaters when the potential difference is increased, because power depends on both current and potential difference (P = IV, both of which increase)
Key terms in this question
potential difference · resistance · ohmic conductor
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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