Capacitance

1
Capacitance
A capacitor consists of two conductors that are
close but not touching. A capacitor has the
ability to store electric charge.
2
Capacitance
Parallel-plate capacitor connected to battery.
(b) is a circuit diagram.
3
Capacitance
When a capacitor is connected to a battery, the
charge on its plates is proportional to the
voltage
The quantity C is called the capacitance. Unit of
capacitance the farad (F) 1 F 1 C/V
4
Capacitance
The capacitance does not depend on the voltage
it is a function of the geometry and materials of
the capacitor. For a parallel-plate capacitor
5
A parallelplate capacitor has a capacitance Co.
A second parallelplate capacitor has plates with
twice the area and twice the separation. The
capacitance of the second capacitor is most
nearly (A) ¼Co (B) ½Co (C) Co
(D) 2Co (E) 4Co
6
A parallelplate capacitor has a capacitance Co.
A second parallelplate capacitor has plates with
twice the area and twice the separation. The
capacitance of the second capacitor is most
nearly (A) ¼Co (B) ½Co (C) Co
(D) 2Co (E) 4Co
7
Two square parallelplate capacitors of
capacitances C1 and C2 have the dimensions shown
in the diagrams above. The ratio of C1 to C2 is
(A) 1 to 4 (B) 1 to 2 (C) 1 to 1
(D) 2 to 1 (E) 4 to 1
8
Two square parallelplate capacitors of
capacitances C1 and C2 have the dimensions shown
in the diagrams above. The ratio of C1 to C2 is
(A) 1 to 4 (B) 1 to 2 (C) 1 to 1
(D) 2 to 1 (E) 4 to 1
9
Dielectrics
A dielectric is an insulator, and is
characterized by a dielectric constant
K. Capacitance of a parallel-plate capacitor
filled with dielectric
10
Dielectrics
Dielectric strength is the maximum field a
dielectric can experience without breaking down.
11
Dielectrics
The molecules in a dielectric tend to become
oriented in a way that reduces the external field.
12
Dielectrics
This means that the electric field within the
dielectric is less than it would be in air,
allowing more charge to be stored for the same
potential.
13

• A sheet of mica is inserted between the plates of
  an isolated charged parallelplate capacitor.
  Which of the following statements is true?
• The capacitance decreases.
• The potential difference across the capacitor
• decreases.
• (C) The energy of the capacitor does not change.
• (D) The charge on the capacitor plates decreases
• (E) The electric field between the capacitor
  plates increases.

14

• A sheet of mica is inserted between the plates of
  an isolated charged parallelplate capacitor.
  Which of the following statements is true?
• The capacitance decreases.
• The potential difference across the capacitor
• decreases.
• (C) The energy of the capacitor does not change.
• (D) The charge on the capacitor plates decreases
• (E) The electric field between the capacitor
  plates increases.

Since the capacitor is isolated, Q remains
constant. Filling the place with oil (a
dielectric) will increase the capacitance,
causing the potential (V Q/C) to decrease.
15
Which of the following capacitors, each of which
has plates of area A, would store the most charge
on the top plate for a given potential difference
V?
16
Which of the following capacitors, each of which
has plates of area A, would store the most charge
on the top plate for a given potential difference
V?
The capacitor with the largest capacitance will
store the most charge. C ??0A/d where ?glass gt
?air and ?vacuum so E
17
Storage of Electric Energy
A charged capacitor stores electric energy the
energy stored is equal to the work done to charge
the capacitor.
18

• A 20 µF parallelplate capacitor is fully charged
  to 30 V. The energy stored in the capacitor is
  most nearly
• 9 103 J (B) 9 103 J (C) 6 104 J
  
• (D) 2 104 J (E) 2 107 J

19

• A 20 µF parallelplate capacitor is fully charged
  to 30 V. The energy stored in the capacitor is
  most nearly
• 9 103 J (B) 9 103 J (C) 6 104 J
  
• (D) 2 104 J (E) 2 107 J

UC ½ CV2
20
A potential difference V is maintained between
two large, parallel conducting plates. An
electron starts from rest on the surface of one
plate and accelerates toward the other. Its speed
as it reaches the second plate is proportional
to (A) 1/V (B) (C) vV (D) V (E)
V2
21
A potential difference V is maintained between
two large, parallel conducting plates. An
electron starts from rest on the surface of one
plate and accelerates toward the other. Its speed
as it reaches the second plate is proportional
to (A) 1/V (B) (C) vV (D) V (E)
V2
W K q?V and K ½ mv2