Edexcel A-Level Physics Capacitor Energy

Practise Edexcel A-Level Physics capacitor energy by using equivalent formulas, Q-V graph areas and stored-energy comparisons.

Exam points

calculate stored capacitor energy using half QV, half CV squared or Q squared over 2C
derive or identify capacitor energy from the triangular area under a charge-voltage graph
compare stored energies when capacitance, voltage tolerance or capacitor type changes

[Maximum number: 5]

A student carried out an investigation to compare the energy stored in a mechanical mouse trap with the energy stored in an electronic mouse trap.

The student tested the capacitor by charging it until the p.d. across the capacitor was 600 V and recording the p.d. as it was discharged through a resistor of resistance $3000 \Omega$ .

The following graph was obtained.

Figure for Question 4.4.101 - Energy stored by a capacitor question 1 — Edexcel A-Level Physics A2

Determine whether the mechanical or the electric mouse trap stores the most energy.

Read the time constant from the graph: $RC=4.5\,\mathrm{s}$ . With $R=3000\,\Omega$ , $C=1.5\times10^{-3}\,\mathrm{F}$ . Therefore $W=\tfrac12CV^2=\tfrac12(1.5\times10^{-3})(600)^2=270\,\mathrm{J}$ , much greater than $0.47\,\mathrm{J}$ , so the electronic trap stores more energy.

Figure for Answer to Question 4.4.101 - Energy stored by a capacitor question 1 — Edexcel A-Level Physics A2

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4.4.101 - Energy stored by a capacitor question 1