Home Work Solutions 4/5
1. Figure 1 shows a variable “air gap” capacitor for manual tuning. Alternate plates are connected together; one group of plates is fixed in position, and the other group is capable of rotation.
Consider a capacitor of n = 8 plates of alternating polarity, each plate having area A = 1.25 cm2 and separated from adjacent plates by distance d = 3.40 mm. What is the maximum capacitance of the device?
Figure 1 Sol
For maximum capacitance the two groups of plates must face each other with maximum area. In this case the whole capacitor consists of (n – 1) identical single capacitors connected in parallel.
Each capacitor has surface area A and plate separation d so its capacitance is given by C0 = 0A/d.
Thus, the total capacitance of the combination is
0 12 2 2 4 2 120 3
1 (8 1)(8.85 10 C /N m )(1.25 10 m )
1 2.28 10 F.
3.40 10 m
n A
C n C
d
2. Figure 2 displays a 12.0 V battery and 3 uncharged capacitors of capacitances C1 = 4.00 μF, C2 = 6.00 μF, and C3 = 3.00 μF. The switch is thrown to the left side until capacitor 1 is fully charged.
Then the switch is thrown to the right. What is the final charge on (a) capacitor 1, (b) capacitor 2, and (c) capacitor 3?
Figure 2 Sol
The charges on capacitors 2 and 3 are the same, so these capacitors may be replaced by an equivalent capacitance determined from
1 1 1
2 3
2 3
2 3
C C C
C C
eq C C
.
Thus, Ceq = C2C3/(C2 + C3). The charge on the equivalent capacitor is the same as the charge on either of the two capacitors in the combination, and the potential difference across the equivalent capacitor is given by q2/Ceq. The potential difference across capacitor 1 is q1/C1, where q1 is the charge on this capacitor. The potential difference across the combination of capacitors 2 and 3 must be the same as the potential difference across capacitor 1, so q1/C1 = q2/Ceq. Now some of
2.72 x 10-12 F
the charge originally on capacitor 1 flows to the combination of 2 and 3. If q0 is the original charge, conservation of charge yields q1 + q2 = q0 = C1 V0, where V0 is the original potential difference across capacitor 1.
(a) Solving the two equations
1 2
1 eq
1 2 1 0
q q C C q q C V
for q1 and q2, we obtain
2 2 2
1 2 3 0
1 0 1 0
1
2 3
eq 1 1 2 1 3 2 3
1
2 3
C C C V .
C V C V
q C C C C C C C C C C C C C
With V0 = 16.0 V, C1= 4.00 F, C2= 6.00 F and C3 =3.00 F, we find Ceq = 2.00 F and q1 = 42.7 C.
(b) The charge on capacitors 2 is
2 1 0 1 (4.00 F)(16.0 V) 42.7 C 21.3 C
q C V q .
(c) The charge on capacitor 3 is the same as that on capacitor 2:
3 1 0 1 (4.00 F)(16.0 V) 42.7 C 21.3 C
q C V q .
3. The parallel plates in a capacitor, with a plate area of 8.50 cm2 and an air-filled separation of 3.00 mm, are charged by a 6.00 V battery. They are then disconnected from the battery and pulled apart (without discharge) to a separation of 8.00 mm. Neglecting fringing, find (a) the potential
difference between the plates, (b) the initial stored energy, (c) the final stored energy, and (d) the work required to separate the plates.
Sol
(a) Let q be the charge on the positive plate. Since the capacitance of a parallel-plate capacitor is given by 0 A di, the charge is q CV 0 AV di i. After the plates are pulled apart, their separation is dfand the potential difference is Vf. Then 𝑞 = ϵ0𝐴𝑉𝑓/𝑑𝑓 and
0
0 0
f f f .
f i i
i i
d d A d
V q V V
A A d d
With di3.00 10 m 3 , Vi6.00 V,anddf 8.00 10 m 3 , we have Vf 16.0 V. (b) The initial energy stored in the capacitor is
2 12 2 2 4 2 2
2 0 11
3
1 (8.85 10 C /N m )(8.50 10 m )(6.00 V)
4.51 10 J.
2 2 2(3.00 10 m)
i
i i
i
U CV AV
d
(c) The final energy stored is
2 2
0 2 0 0
1 1
2 2 .
f f i f
f f i i
f f i i i i
d d d
A A AV
U V V U
d d d d d d
With df /di8.00 / 3.00, we have Uf 1.20 10 10 J.
(d) The work done to pull the plates apart is the difference in the energy:
W = Uf – Ui = 7.52 10 11 J.
4. For the arrangement of Fig. 3, suppose that the battery remains connected while the dielectric slab is being introduced. Calculate (a) the capacitance, (b) the charge on the capacitor plates, (c) the electric field in the gap, and (d) the electric field in the slab, after the slab is in place.
Fig. 3 Sol
(a) The electric field E1 in the free space between the two plates is E1 = q/0A while that inside the slab is E2 = E1/ = q/0A. Thus,
V E d b E b q
A d b b
0 1 2
F
H G I
K J F
H G I
K J b g
0
, and the capacitance is
12 2 2 4 2
0
0
8.85 10 C /N m 115 10 m 2.61
13.4pF.
2.61 0.0124m 0.00780m 0.00780m A
C q
V d b b
(b) q = CV = (13.4 10–12 F)(85.5 V) = 1.15 nC.
(c) The magnitude of the electric field in the gap is
9
4
1 12 2 2 4 2
0
1.15 10 C
1.13 10 N C.
8.85 10 C /N m 115 10 m E q
A
(d) Using Eq. 25-34, we obtain
E E
2 1
4
113 10 3
2 61 4 33 10
.
. N C . .
N C
5. A slab of copper of thickness b = 2.00 mm is thrust into a parallel-plate capacitor of plate area A = 2.40 cm2 and plate separation d = 5.00 mm, as shown in Fig. 4; the slab is exactly halfway between the plates. (a) What is the capacitance after the slab is introduced? (b) If a charge q = 3.40 μC is maintained on the plates, what is the ratio of the stored energy before to that after the slab is inserted? (c) How much work is done on the slab as it is inserted? (d) Is the slab sucked in or must it be pushed in?
Fig. 4 Sol
(a) The length d is effectively shortened by b so C' = 0A/(d – b) = 1.06 pF.
(b) The energy before, divided by the energy after inserting the slab is
2
0 2
0
/( )
/ 2 5.00
2.50.
/ 2 / 5.00 3.00
A d b
U q C C d
U q C C A d d b
(c) The work done is
2 2 2
0 0
1 1
( ) 5.44 J.
2 2 2
q q q b
W U U U d b d
C C A A
(d) Since W < 0, the slab is sucked in.
-8.16 J
