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A Levels Physics (9702)•9702/12/M/J/18
Question 32 from 9702/12/M/J/18

Explanation

Power loss in supply wires reduces voltage and power to the kettle

Steps:

  • Supply current: I = P/V = 2400 W / 240 V = 10 A.
  • Total wire resistance: 0.50 Ω + 0.50 Ω = 1.0 Ω.
  • Voltage drop in wires: V_drop = I × R = 10 A × 1.0 Ω = 10 V; kettle p.d. = 240 V - 10 V = 230 V.
  • Kettle power: P_k = V_k × I = 230 V × 10 A = 2300 W = 2.3 kW.

Why A is correct:

  • Matches Ohm's law (V = IR) for voltage drop and P = VI for reduced power delivery.

Why the others are wrong:

  • B: Correct power but voltage drop is 10 V, not 5 V.
  • C: Ignores power loss in wires (I²R = 100 W) and underestimates voltage drop.
  • D: Same errors as C in power and voltage.

Final answer: A

Topic: Potential difference and power

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