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A2 Daily A Level Physics question

2026-01-23 OCR A Capacitance II: charging/discharging; time constant τ = RC; qualitative exponential behaviour OCR A Level Physics A (H556) Module 6: Fields and their consequences — 6.4 Capacitance: charging/discharging; time constant τ = RC; qualitative exponential behaviour

In a lab test, an initially uncharged 1000 µF capacitor is connected to a 9.0 V DC supply through a resistor and a switch. In Setup A the resistor is 1.0 kΩ; in Setup B it is 2.0 kΩ. The switch is closed at t = 0 in both setups. Compared with Setup A, what happens in Setup B to (i) the initial charging current just after t = 0 and (ii) the time taken to reach half of the final charge? Give a brief quantitative estimate where appropriate.

  1. A Initial current halves to about 4.5 mA; time to half-charge halves.
  2. B Initial current halves to about 4.5 mA; time to half-charge doubles. (correct)
  3. C Initial current stays about 9 mA; time to half-charge doubles.
  4. D Initial current doubles to about 18 mA; time to half-charge doubles.

Answer

The correct answer is B.

Correct: B — Initial current halves to about 4.5 mA; time to half-charge doubles. Right after switching on, the capacitor behaves like a short so the initial current is 9.0 V / R: 9 mA for 1.0 kΩ and 4.5 mA for 2.0 kΩ; the time to reach any fixed fraction of the final charge scales with RC, so doubling R doubles that time. A halves the initial current correctly but wrongly claims the process is faster when R is larger. B matches the correct initial-limit and timescale-scaling reasoning. C assumes the initial current is unaffected by R (or that the capacitor blocks current instantly), which contradicts the immediate t = 0 behaviour where current is set by the resistor. D reverses the initial-current effect of increasing resistance and so is inconsistent with 9.0 V / R.

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