CIE A Level Physics
CIE A Level Physics Capacitance Exam Questions
Free CIE A Level Physics exam questions for Capacitance, with a mark scheme for every pack. Covers capacitance, capacitors in series and parallel, energy stored, and exponential charging and discharging through a resistor.
Capacitance exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Physics Capacitance
Capacitance is Topic 19 of the Cambridge International AS & A Level Physics (9702) syllabus. It defines capacitance for isolated conductors and parallel plate capacitors, combines capacitors in series and parallel, and finds the energy stored from the area under a charge-voltage graph. You then analyse discharge through a resistor using the time constant RC and exponential equations. It draws on D.C. circuits and Electric fields. Examiners commonly ask for combined capacitance, energy stored, the time constant from a graph, and the remaining charge or voltage after a given time.
What you need to know
- Defining capacitance C = Q/V, measured in farads. For an isolated spherical conductor, C = 4πε0r.
- Combining capacitors Parallel: C = C1 + C2 + ...; series: 1/C = 1/C1 + 1/C2 + ... (the opposite of resistors).
- Energy stored Energy stored equals the area under a graph of potential difference against charge: W = ½QV = ½CV^2 = Q^2/2C.
- Discharging When a capacitor discharges through a resistor, Q, V and I all decrease exponentially: x = x0 e^(-t/RC).
- Time constant The time constant τ = RC is the time for the quantity to fall to 1/e (about 37%) of its initial value. A graph of ln V against t has gradient -1/RC.
Common mistakes in Capacitance
- Using the resistor formulae for capacitors. Capacitors in parallel add directly; in series use reciprocals.
- Using W = QV for energy stored. The energy stored is ½QV, because the p.d. rises from zero as charge is added.
- Forgetting to convert microfarads to farads before calculating.
- Saying the time constant is the time to discharge completely. In theory discharge never completes; τ is the time to fall to 37%.
- Using a linear decrease instead of the exponential equation for charge remaining.
Try three Capacitance exam-style questions
1. Calculate the energy stored in a 470 μF capacitor charged to 12 V. (2 marks)
Show the mark scheme answer
W = ½CV^2 = ½ × 470 × 10^-6 × 12^2; W = 0.034 J.
2. A 470 μF capacitor charged to 12 V discharges through a 10 kΩ resistor. Calculate the time constant and the p.d. after 2.0 s. (3 marks)
Show the mark scheme answer
τ = RC = 10 × 10^3 × 470 × 10^-6 = 4.7 s; V = 12 e^(-2.0/4.7); V = 7.8 V.
3. Calculate the combined capacitance of a 2.0 μF capacitor and a 4.0 μF capacitor connected (a) in parallel and (b) in series. (2 marks)
Show the mark scheme answer
(a) 2.0 + 4.0 = 6.0 μF; (b) 1/C = 1/2.0 + 1/4.0 so C = 1.3 μF.
Capacitance FAQs
What does Cambridge AS & A Level Physics Capacitance cover?
It covers the definition of capacitance, capacitors in series and parallel, energy stored in a capacitor and exponential discharge through a resistor with the time constant RC.
Which paper is Capacitance examined in?
It is an A Level topic assessed in Paper 4, and discharge experiments with log graphs are common in Paper 5.
How do I find the time constant from experimental data?
Plot ln V against t; the gradient is -1/RC, so the time constant is -1/gradient. Alternatively, read the time for V to fall to 37% of its initial value.
