PH 112 Capacitance January 27, 2005 MJM
Name _____________________ Box _______
Capacitance is defined as C = q/V. A capacitor can be any two conductors where we may have equal and
opposite charges q, and a potential difference V between the conductors.
When two capacitors C1 and C2 are attached in series, the equivalent capacitance C is given by
1/C = 1/C1 + 1/C2.
+q
C1
V1
+q
C
The sketch at the right shows two capacitors in
series and their equivalent capacitance C.
Two features are very important
1) each capacitor has the same charge q
2) the potential difference V =V1 + V2
V
+q
C2
V2
Use the relations V = V1 + V2 and q/V1 = C1, etc.
to show that 1/C = 1/C1 + 1/C2.
For capacitors in parallel, V1 = V2 = V and q = q1 + q2. Use these relations to show C = C1 + C2.
+q1
C1
V1
Find the equivalent capacitance
in F of the three capacitors
shown in the sketch.
+q2
C2
+q
C
V2
V
a
3 F
6 F
Cab =
b
5 F