Q#4 (Past Exam Paper – June 2016 Paper 42 Q7)
(a) State two uses of capacitors in electrical circuits, other than for the smoothing of direct current. [2]
(b) The combined capacitance between terminals A and B of the arrangement shown in Fig. 7.1 is 4.0 μF.
Fig. 7.1
Two capacitors each have capacitance C and the remaining capacitors each have
capacitance 3.0 μF.
The potential difference (p.d.) between terminals A and B is 12 V.
(i) Determine the capacitance C. [2]
(ii) Calculate the magnitude of the total positive charge transferred to the arrangement. [2]
(iii) Use your answer in (ii) to state the magnitude of the charge on one plate of
1. a capacitor of capacitance C,
2. a capacitor of capacitance 3.0 μF.
[2]
[Total: 8]
Solution:
(a) e.g.
storing energy
blocking d.c.
in oscillator circuits
in tuning circuits
in timing circuits
(b)
(i)
{For the capacitors in parallel, combined capacitance = 3 + 3 = 6.0 μF
The 2 capacitors C are connected in series to the parallel combination.
Combined capacitance between A and B = 4.0 μF
1 / 4 = 1/6 + 1/C + 1/C }
1/6 + 1/C + 1/C = 1/4
C = 24 μF
(ii)
{The total charge is determined using the total capacitance and the applied potential difference.
Combined capacitance between A and B = 4.0 μF
p.d. between A and B = 12 V}
Q = CV
Q = 4.0×10-6 × 12
Q = 48 μC
(iii)
1.
{Current is the flow of charge. So, we need to identify the flow of current on the capacitors.
Current flows into the two capacitors C. So, the total charge appears on C.
The total charge is received by the capacitors in series.}
48 μC
2.
{At the junction of the parallel combination, the current splits equally. So, the charge is shared by the capacitors in parallel.}
24 μC
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