AP Physics C E&M
HW 3.2
1. A parallel plate capacitor’s plates are initially a distance d apart and have a surface area A. They are
connected to a battery and charged to a potential difference V. The battery is disconnected and the
plates are pulled apart to a distance 2d.
a. What is the charge on the plates after they are pulled apart?
b. What is the new potential difference between the plates?
c. How much work was done as the plates were separated?
2. A 100µF capacitor is charged to 12V. It is then disconnected from the voltage source and connected
in parallel to a second capacitor, which is initially discharged. If the voltage drops to 9V when the
second capacitor is connected, what is the capacitance of the second capacitor?
3. A capacitor with capacitance C0 is connected to a battery and charged to a potential V0. It is then
disconnected from the battery and connected to a second capacitor (initially discharged, with
capacitance C1).
a. What is the new value of potential difference across the two capacitors?
b. What is the initial energy stored in the capacitor C0?
c. What is the final total energy stored in both capacitors?
d. Where did the energy go?
AP Physics C E&M
HW 3.2
4. The plates in a parallel plate capacitor are initially separated by 1.80mm. The capacitor is connected
to a battery and charged so that it stores 12J of energy. What happens to the charge and energy stored
in the capacitor under the following conditions:
a. While still connected to the battery, the plate separation is reduced to 1.20mm.
b. The capacitor is disconnected from the battery, then the separation is reduced to 1.20mm.
5. Find the equivalent capacitance of the following, assuming all capacitors are 100µF.
a.
C2
V0
C1
C3
b.
C1
V0
C2
C3
AP Physics C E&M
HW 3.2
c.
C1
C2
C3
V0
6. In the figure at right, capacitor C1 has a capacitance of 2.0µF and C2 has a
capacitance of 5.0µF. Both capacitors are charged to 120V and then
disconnected from their batteries. They are then connected together with
switches, but with opposite polarities as shown. When the switches are closed,
a. What is the new potential difference across the two capacitors?
b. What is the charge on C1 after the switches are closed?
c. What is the charge on C2 after the switches are closed?
C1
C2
0
