Page 12 - PRE-U STPM PHYSICS TERM 2

Physics Term 2 STPM Chapter 13 Capacitors

Four of the capacitors have one plate each connected to one common terminal, and the other plate
to another common terminal. Hence these four capacitors are in parallel. The charge on the fifth

capacitor 4Q = sum of charges on the four capacitors.

Hence the fifth capacitor is in series with the four capacitors.
A
C
.
The area of each small plate = — Hence capacitance of each small capacitor = — .
3 3
( 3)
13 The equivalent capacitance of 4 such capacitors in parallel = 4 × — = —C 13
4
C
The effective capacitance of the arrangement C’ is given by 3
1 1 1
— = —— + ——
C’
4
1
—C —C
3 3
3 + 12
= ———
4C
4
C’ = –—C
15
Example 5
Two capacitors of capacitance 2 µF and 4 µF, initially uncharged are connected in series with a
12 V battery. Calculate
(a) the equivalent capacitance,
(b) the charge on each capacitor,
(c) the potential difference across each capacitor.
Solution:
(a) The equivalent capacitance C is given by 12 V
1 1 1
— = — + —
C 2 4 2 µF 4 µF
3
= —
4 V 1 V 2
4
C = — µF
3
(b) Charges on both capacitors are equal
= charge on equivalent capacitor
( 3 )
4
= — µF × 12 V
= 16 µC
(c) Potential difference across 2 µF capacitor
Q
V = —
1 C
1
16 µC
= ———
2 µF
= 8 V
Potential difference across 4 µF capacitor
V = V – V
2 1
= (12 – 8) V
= 4 V

52

13 Physic T2.indd 52 10/18/18 3:18 PM