Page 9 - PRE-U STPM PHYSICS TERM 2

Physics Term 2 STPM Chapter 13 Capacitors

(ii) Potential difference
V
V’ = —
ε
r
1 × 10 4
= —–——
5
= 2.0 × 10 V
3
13 13

(iii) Capacitance C’ = ε C
r
–9
= 5 × 3.54 × 10
–8
= 1.77 × 10 F
Quick Check 3

1. A parallel-plate capacitor is charged in air. It 2. The electric field between the plates of an

is then electrically isolated and lowered into isolated air-spaced parallel-plate capacitor
a liquid dielectric. Which of the following sets is E. Without disconnecting the charging
of changes is correct? voltage, an insulator of relative permittivity
A Both the capacitance and the charges on 2 is inserted in between the plates. What is

the plates increase. the final electric field between the plates?

1
B Both the capacitance and the charges on A ––E C 2E
the plates decrease. 2
C The capacitance increases and the B E D 2E
potential difference across the plates 3. A rolled paper capacitor is made from strips
decreases. of metal foil of dimensions 2 cm × 40 cm
D The capacitance decreases and the separated by paper of relative permittivity
potential difference across the plates 2 and thickness 0.002 cm. Estimate its
increases. capacitance.

Capacitors in Series and in Parallel
13.4
13.4 Capacitors in Series and in Parallel
Learning Outcome 2012/P1/Q24, 2013/P2/Q3, 2014/P2/Q3, 2017/P2/Q4

Students should be able to:
• derive and use the formulae for effective capacitance of capacitors in series and in parallel

1. Figure 13.7 shows three capacitors of capacitance C , C and C 3

1
2
connected in parallel to a battery. Q 1 C 1
2. Charges flow into the capacitors until the potential difference

Q 2 C 2
across each capacitor is equal to the charging voltage V. Hence
the potential differences across the capacitors are the same, V.
Q 3 C 3
3. The charge in each capacitor is given by
Q = C V, Q = C V, Q = C V

1 1 2 2 3 3 V
The total charge, Q = Q + Q + Q
1 2 3
= C V + C V + C V
1 2 3 Battery
= (C + C + C )V
1 2 3
Figure 13.7
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