A Level · A2 · multiple choice
A Level extension · Question 4
A Level extension · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- demanding
- Marks
- 1
- Topics
- 1
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multiple choice1 mark
A capacitor of capacitance 470 μF, charged to a potential difference V₀, is discharged through a resistor of resistance 22 kΩ. After what time has the charge on the capacitor fallen to 25% of its initial value?
- A
3.6 s
- B
7.2 s
- C
14 s
- D
21 s
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Answer overviewCorrect answer: C — 14 s
01
1 markAnswer and reasoning
How to approach it
Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.
- 1
The correct choice is C: 14 s
- 2
D is two time constants, which is the answer of a candidate who assumes the charge falls to 1/e² rather than to a quarter. The time constant is 10.3 s and t = RC ln 4 = 14.3 s.
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