Antonine Education A2 Physics
Self-Test 2 Capacitors
Examination Time - 20 minutes
1 (a) Write the symbols for:
(i) a non-electrolytic capacitor
(ii) an electrolytic capacitor
(i) (ii)
- +
(2)


(b) What is the unit for capacitance?
Farads
2 (a) A capacitor is connected in series with a bulb. They are then connected to a battery. The bulb lights momentarily then goes out. Then the bulb and capacitor are connected to an a.c. supply. The bulb lights up. Explain these observations
There is a current as the charge flows onto the plates.
With dc, the plates charge up and stay charged.
In ac, the charge is flowing on and off all the time
(b) State one advantage and one disadvantage of an electrolytic capacitor compared with the non-electrolytic.
Advantage:
Capacitor has a larger capacity for a given size.
Disadvantage:
Polarity is important
3 A capacitor is connected to a battery
(a) Sketch the graph that shows how charge and voltage are related.
(1)

(2)





(2)
(2)
(b)
Q Straight Line 
Axes labelled appropriately 

On the graph above, show how energy contained in the capacitor can be worked out from the graph
V
(1)
Turn over

Antonine Education A2 Physics
4 A 470

F capacitor is charged up to a potential difference of 16 V.
(a) Calculate:
(i) the charge held by the capacitor.
(ii) the energy held by the capacitor.
(i) Q = 470 × 10 -6 × 16 = 7.52 × 10 -3 C
(ii) E = ½ × 470 × 10 -6 × 16 2 = 0.0602 J
(b) The capacitor is made of two plates with a dielectric 2
 m thick. What is the electric field strength?
E = V/d = 16 ÷ 2 × 10 -6
= 8 × 10 6 V m -1
(2)




(2)
5 When a capacitor is discharged through a resistor, the voltage falls as shown in the graph:
V
(a) What is this graph called? t
(1)
Exponential
(b) The capacitor has a capacitance in Farads and resistance in ohms. What is the unit for the product of resistance and capacitance, RC ?
Seconds
(c) In the data sheet, the formula for this discharge is voltage formula is of the same pattern, V
Q

V
0 e
 t / RC

Q
0 e
 t / RC
. Show that the
Q = CV  CV = CV e -t/RC . C cancels out  V = V e -t/RC
(d) A 470 
F capacitor is discharged through a 100 k

resistor. The initial voltage
16 V. How long does it take the capacitor to discharge to 6 V? Explain whether



(1)
(1)
(3) your answer would be different if the discharge was from 8 V to 3 V.
6 = 16 e -t/100 000 × 0.000470
6 ÷ 16 = e -t/47  t = ln(6 ÷ 16) × -47 = -0.980 × -47 = 46 s
No, because the voltage ratio is the same



Total = 20 marks
End of examination. Now go back and check your work