State Examinations Commission – Physics Higher Level, 2004.
Question 8
Define (i) potential difference, (ii) capacitance. (12)
Describe an experiment to demonstrate that a capacitor can store energy. (12)
The circuit diagram shows a 50 µF capacitor
connected in series with a 47 k resistor, a 6
V battery and a switch. When the switch is
closed the capacitor starts to charge and the
current flowing at a particular instant in the
circuit is 80 µA.
Calculate
(i) the potential difference across the resistor and hence the potential difference across the
capacitor when the current is 80 µA;
(ii) the charge on the capacitor at this instant;
(iii) the energy stored in the capacitor when it is fully charged. (27)
Describe what happens in the circuit when the 6 V d.c. supply is replaced with a 6 V a.c. supply. (5)
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Define (i) potential difference, (ii) capacitance. (12)
Textbook
Describe an experiment to demonstrate that a capacitor can store energy. (12)
Connect battery to capacitor for 10 seconds.
Remove battery from circuit and insert bulb in its place.
It is observed that the bulb lights for several seconds. Hence the
capacitor must be acting as an energy store.
The circuit diagram shows a 50 µF capacitor
connected in series with a 47 k resistor, a 6
V battery and a switch. When the switch is
closed the capacitor starts to charge and the
current flowing at a particular instant in the
circuit is 80 µA.
Calculate
(i) the potential difference across the resistor and hence the potential difference across the
capacitor when the current is 80 µA;
The potential difference across resistor is
V = IR
V = 8.0 x 10-5 x 4.7 x 104
V = 3.8 V
Therefore pd across capacitor is 6 – 3.8 = 2.2 V
(ii) the charge on the capacitor at this instant;
C = Q/V
=> Q = CV = 5.0 x 10-5 x 2.2 = 1.1 x 10-4 C.
Chris Garvey 2004
(iii) the energy stored in the capacitor when it is fully charged. (27)
Energy stored
= ½CV2
= ½(5.0 x 10-5 )(6)2 … (when fully charged the pd across it is the same as that of the
dc source)
= 9 x 10-4 J
Describe what happens in the circuit when the 6 V d.c. supply is replaced with a 6 V a.c. supply. (5)
Current flow continuously in the circuit because the capacitor
charges in one direction and then the other with the frequency of
the ac supply, i.e. there is a continuous flow of charge through the
circuit. In this particular circuit there would be a continuous
reversing of the pd across the resistor.
Chris Garvey 2004