AQA, Edexcel
A Level
A Level Physics
Capacitors (Answers)
2022
Total Marks:/30
, 1. George wishes to explore the rules for multiple capacitors. To do this, he constructs a circuit using three:
C1 is connected in series with C2; these are both connected in parallel with C3. All are connected to a
6 V power supply. C 1 = 10 µF, C 2 = 20 µF andC 3 = 50 µF .
Total for Question 1: 13
(a) Explain in terms of the flow of electrons how a potential difference is built up across a capacitor. [3]
Solution: Initially, electrons flow from the cell. They cannot travel between the plates of the [1]
capacitor because of the dielectric insulator. However, the initial current removes electrons
from the ’positive’ side of the capacitor and builds up a surfeit them on the ’negative’ side.
Since current must be the same everywhere, the charge on one side is of equal magnitude but
opposite polarity to that on the other side. Thus, there is a potential difference across the
capacitor. The current will fall to zero when the PD equals the EMF.
(b) Which of the following is correct? For two capacitors in series, irrespective of their capacitances,
the charge stored by the first will be...
i. Half of that stored by the second.
ii. The same as that stored by the second.
iii. Twice that stored by the second.
Solution: 2
(c) In any circuit, charge is conserved. Use this, in combination with Kirchoff’s laws, to show that [3]
1
the total capacitance of two capacitors in series is given by = 1+1.
Ctotal C1 C2
Solution: According to Kirchoff’s second law, Vtotal = V1 + V2. Charge
stored by each Qis theQ same.
Since Q = V C, = + Q.
CtotalC1C2
This simplifies to give the required
result.
A Level
A Level Physics
Capacitors (Answers)
2022
Total Marks:/30
, 1. George wishes to explore the rules for multiple capacitors. To do this, he constructs a circuit using three:
C1 is connected in series with C2; these are both connected in parallel with C3. All are connected to a
6 V power supply. C 1 = 10 µF, C 2 = 20 µF andC 3 = 50 µF .
Total for Question 1: 13
(a) Explain in terms of the flow of electrons how a potential difference is built up across a capacitor. [3]
Solution: Initially, electrons flow from the cell. They cannot travel between the plates of the [1]
capacitor because of the dielectric insulator. However, the initial current removes electrons
from the ’positive’ side of the capacitor and builds up a surfeit them on the ’negative’ side.
Since current must be the same everywhere, the charge on one side is of equal magnitude but
opposite polarity to that on the other side. Thus, there is a potential difference across the
capacitor. The current will fall to zero when the PD equals the EMF.
(b) Which of the following is correct? For two capacitors in series, irrespective of their capacitances,
the charge stored by the first will be...
i. Half of that stored by the second.
ii. The same as that stored by the second.
iii. Twice that stored by the second.
Solution: 2
(c) In any circuit, charge is conserved. Use this, in combination with Kirchoff’s laws, to show that [3]
1
the total capacitance of two capacitors in series is given by = 1+1.
Ctotal C1 C2
Solution: According to Kirchoff’s second law, Vtotal = V1 + V2. Charge
stored by each Qis theQ same.
Since Q = V C, = + Q.
CtotalC1C2
This simplifies to give the required
result.
