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A2 Daily A Level Physics question

2026-06-18 OCR A Applied contexts: transformers/grid efficiency OCR-A 6.3.3 Alternating currents and transformers (qual.) OCR-A 6.3.4 Transmission of electrical power: National Grid efficiency and I^2R losses

Which statement must be true when a step-up transformer is used at a power station to send the same power over long transmission lines with fixed resistance to a distant city?

  1. A Heating losses in the lines decrease because the current in the lines is reduced. (correct)
  2. B Heating losses in the lines increase because the higher voltage forces more current through the lines.
  3. C Heating losses in the lines are unchanged because energy conservation fixes the total power.
  4. D Heating losses in the lines are unchanged because they depend only on the wire resistance, not on current or voltage.

Answer

The correct answer is A.

Correct: A — Heating losses in the lines decrease because the current in the lines is reduced. A: Correct — at fixed transmitted power, stepping up the voltage lowers the line current, so resistive heating (proportional to current squared times resistance) falls. B: Incorrect — at fixed power a higher transmission voltage means a lower current, not more; it is the current that sets I^2R heating. C: Incorrect — while total energy is conserved, the share lost as heating depends on the current in the lines, which changes with transmission voltage. D: Incorrect — resistive heating depends on both the resistance and the current; resistance alone does not determine the loss.

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