Page 10 - PRE-U STPM PHYSICS TERM 2

Physics Term 2 STPM Chapter 13 Capacitors

The three capacitors can be replaced by a single capacitor of capacitance C, if the charge in the
equivalent capacitor is
Q = CV
Hence CV = (C + C + C )V
1 2 3 Exam Tips
Equivalent capacitance, C = C + C + C 3
1
2
For capacitors in parallel
13 This formula for the equivalent capacitance can be • same potential difference across all capacitors 13
extended for any number of capacitors connected • charge in each capacitor ∝ capacitance.
in parallel. • equivalent capacitance, C = sum of capacitance

Capacitors in Series

1. Figure 13.8 shows the three capacitors of capacitance C , +Q C 1 –Q +Q C 2 –Q +Q C 3 –Q
1
C , and C connected in series to a charging voltage V. A B M N X Y
2 3
2. Electrons flow from the negative terminal of the charging
voltage and charge plate Y of the capacitor C negative. V 1 V 2 V 3
3
3. The charge –Q on plate Y induced a charge +Q on the V
plate X which in turn causes the plate N to be charged
–Q and so on.
Hence the charges on all the capacitors are the same,
i.e. Q. Figure 13.8
Q
Q
Q
4. The potential difference across each capacitor is given by V = — , V = — , V = —
1 C 2 C 3 C
1 2 3
The total potential difference, V = V + V + V
1 2 3
Q Q Q
= — + — + —
C C C
1 2 3
C )
1 1 1
= Q — + — + —
( C C
1 2 3
5. Since only a charge of Q is transferred from the charging voltage to the capacitors, the equivalent
capacitor would have a charge of Q when connected to the same charging voltage V, and
Q
V = —
C
C = equivalent capacitance
C )
Q 1 1 1
Hence, — = Q — + — + —
C ( C C
1 2 3
1 1 1 1
— = — + — + — Exam Tips
C C C C
1 2 3
When two capacitors C and C are connected in series,
1
2
• the charges in both capacitors, and in the equivalent capacitor
are equal.
1 ( C + C )

• potential difference across C : V = C 2 V
1
1 2
2 ( C + C )
potential difference across C : V = C 1 V
2
1 2
50

13 Physic T2.indd 50 10/18/18 3:18 PM