CCEA A Level Physics
CCEA A Level Physics 5.4 Capacitors Exam Questions
Free CCEA A Level Physics revision for 5.4 Capacitors: what you need to know, common mistakes and exam-style questions with answers. Covers capacitance, energy stored, capacitors in series and parallel, charging and discharging, the time constant CR and exponential decay.
Practise 5.4 Capacitors with past papers
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About CCEA A Level Physics 5.4 Capacitors
Capacitors sits in Unit A2 2: Fields, Capacitors and Particle Physics and links electric fields to circuits. You need to define capacitance, calculate the energy stored, combine capacitors in series and parallel, and describe experiments to charge and discharge a capacitor through a resistor. Discharge follows an exponential decay with time constant τ = CR, which you analyse using graphs of current, charge or p.d. against time and a log graph. You should also know applications such as camera flash units and back-up power. Expect calculations with microfarads and kilohms, a graph-based determination of the time constant, and an energy stored question.
What you need to know
- Capacitance C = Q/V, the charge stored per unit p.d., unit farad (F). Practical values are usually microfarads (μF) or nanofarads (nF).
- Energy stored Energy = 1/2 QV = 1/2 CV^2 = Q^2/2C, equal to the area under a graph of V against Q.
- Series and parallel Parallel: C = C1 + C2 + ... Series: 1/C = 1/C1 + 1/C2 + ... (the opposite pattern to resistors).
- Discharge Through a resistor: Q = Q0 e^(-t/CR), and the same form for V and I. The time constant τ = CR is the time for the charge to fall to 1/e (about 37 percent) of its initial value.
- Charge and discharge experiments Use a capacitor, resistor, voltmeter or data logger and a stopwatch. Record V or I at regular times, then plot ln V against t: a straight line with gradient -1/CR.
- Applications Camera flash (rapid energy release), back-up supplies for memory, smoothing rectified a.c., and timing circuits using the time constant.
Common mistakes in 5.4 Capacitors
- Using the series formula for capacitors in parallel; capacitors combine the opposite way to resistors.
- Saying a capacitor fully discharges in one time constant; it falls to about 37 percent of its starting value.
- Using 1/2 QV with Q in microcoulombs without converting, giving answers a million times too large.
- Confusing the half-time (0.693CR) with the time constant (CR).
- Thinking current flows through the dielectric; charge builds up on the plates and current flows in the external circuit.
Try three 5.4 Capacitors exam-style questions
1. A 470 μF capacitor is charged to 12 V. Calculate the charge and energy stored. (3 marks)
Show the mark scheme answer
Q = CV = 470 x 10^-6 x 12 = 5.6 x 10^-3 C; energy = 1/2 CV^2 = 0.5 x 470 x 10^-6 x 144; = 0.034 J.
2. The capacitor discharges through a 10 kΩ resistor. Calculate the time constant and the p.d. after 4.7 s. (3 marks)
Show the mark scheme answer
τ = CR = 470 x 10^-6 x 10 x 10^3 = 4.7 s; after one time constant V = 12 e^(-1); = 4.4 V.
3. Describe how a graph can be used to find the time constant of a discharging capacitor. (3 marks)
Show the mark scheme answer
Record p.d. across the capacitor at regular time intervals during discharge; plot ln V against t; gradient = -1/CR so time constant = -1/gradient.
5.4 Capacitors FAQs
What does CCEA A Level Physics 5.4 Capacitors cover?
Capacitance, energy stored, series and parallel combinations, charge and discharge experiments, the time constant CR, exponential decay and applications.
Which unit is 5.4 examined in?
It is in Unit A2 2: Fields, Capacitors and Particle Physics.
What equations do I need for capacitors?
C = Q/V, energy = 1/2 QV = 1/2 CV^2, series and parallel combination rules, τ = CR and Q = Q0 e^(-t/CR).
