A

1
Parallel Plate Capacitor

• device to store charge
• (also stores energy)
• connect capacitor to battery (V)
• plates become oppositely charged Q/-Q

Q
-Q
A
charge ? potential difference
d
Q C V
-

• constant of proportionality is called the
  capacitance C

12 V
2
The 750mF capacitor in a camera flash attachment
is charged up by a 6V battery. What Q does it
store?
QCV (6 volts)(750?10-6 coul/volt)
4.5?10-3 coul
3
ConcepTest
A one millifarad capacitor has is has a voltage
of 100 V across it. What is the total charge on
the capacitor? 1) Zero 2) 0.1 C 3)
10 C 4) 100,000 C
4
ConcepTest
A one millifarad capacitor has is has a voltage
of 100 V across it. What is the total charge on
the capacitor? 1) Zero 2) 0.1 C 3)
10 C 4) 100,000 C
What is the charge on the positive plate of the
capacitor? 1) Zero 2) 0.05 C 3) 0.1
C 4) 0.2 C
5
Capacitors

• Capacitance depends only on geometry
• Example parallel-plate capacitor
• A area of plate
• d separation between plates
• e0 8.85 x 10-12 C2/N-m2

(permittivity of free space)
6
How big is 1 Farad?

• choose d 1 mm
• find the area A for such a capacitor

A C (d/e0) (1 F) (0.001 m) /
(8.85x10-12) 1.1 x 108 m2 110 km2
This capacitor is as big as a city!!!
7

• Capacitor C1 is connected across a battery of 5
  V. An identical capacitor C2 is connected across
  a battery of 10 V. Which one has the most charge?

1) C1 2) C2 3) both have the same charge 4)
it depends on other factors
8
What must be done to a capacitor in order to
increase the amount of charge it can hold (for a
constant voltage)?

• 1) increase the area of the plates
• 2) increase the separation between the plates
• 3) decrease the separation between the plates
• 4) decrease the area of the plates
• 5) either (1) or (2)
• either (1) or (3)
• either (2) or (4)
• either (3) or (4)

9

• Increasing the separation, d, of the two charged
  plates of a capacitor, after it has been
  disconnected from its battery, will
• (1) Increase the charge on the plates.
• (2) Increase the capacitance of the plates.
• (3) Increase the voltage across the plates.
• (4) None of the above.

10

• A parallel-plate capacitor initially has a
  potential difference of 400 V and is then
  disconnected from the charging battery. If the
  plate spacing is now doubled (without changing
  Q), what is the new value of the voltage?

1) 400 V 2) 200 V 3) 100 V
4) 800 V 5) 1600 V
11

• Inserting a dielectric material between two
  charged parallel conducting plates, originally
  separated by air and now disconnected from the
  battery, will
• (1) Increase the charge on the plates.
• (2) Increase the capacitance of the plates.
• (3) Increase the voltage across the plates.
• (4) None of the above.

12
Answers to Multiple Choice Questions
2) 0.1 C 3) 0.1 C 2) C2 6) either (1) or
(3) (3) Increase the voltage across the plates.
4) 800 V (2) Increase the capacitance of the
plates.