A-LevelPhysicsCapacitance9702/41 · May/June 2019 · Q67 Marks

State two different functions of capacitors in electrical circuits.

(a)State two different functions of capacitors in electrical circuits.[2]
(b)Three uncharged capacitors of capacitances C₁, C₂ and C₃ are connected in series with a battery of electromotive force (e.m.f.) E and a switch, as shown in Fig. 6.1. [Figure 6.1] When the switch is closed, there is a charge + q on plate P of the capacitor of capacitance C₁. Show that the combined capacitance C of the three capacitors is given by the expression 1/C = 1/C₁ + 1/C₂ + 1/C₃.[3]
(c)(i)A student has available four capacitors, each of capacitance 20 µF. Draw circuit diagrams, one in each case, to show how the student may connect some or all of the capacitors to produce a combined capacitance of: 60 µF[1]
(c)(ii)15 µF.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[2] Any valid two points e.g.: • to store (electrical) energy • smoothing/reduce ripple (on direct voltages/currents) to block d.c. • timing/time delay (circuits) • in oscillator (circuits) in tuning (circuits) • to prevent arcing/sparks
[1] clear indication of equal charge on each capacitor
[1] E = V1 + V2 + V3 and V = Q/C
[1] completion of algebra leading to 1 / C = 1 / C1 + 1 / C2 + 1/C3
[1] three capacitors connected in parallel
[1] parallel combination of three capacitors connected in series with one capacitor

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.