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
C   n 1 C0 
 n 1  0 A  (8 1)(8.851012 C2 /N  m2 )(1.25 104 m2 )  2.28 1012-12
F.
d
3.40 103 m
2.72 x 10
F
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 C2  C3



.
Ceq C2 C3
C2 C3
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
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
q1 q2

C1 Ceq
q1  q2  C1V0
for q1 and q2, we obtain
C12  C2  C3 V0
C12V0
C12V0
q1 


.
C2C3
Ceq  C1
 C1 C1C2  C1C3  C2C3
C2  C3
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
q2  C1V0  q1  (4.00 F)(16.0 V)  42.7 C  21.3 C .
(c) The charge on capacitor 3 is the same as that on capacitor 2:
q3  C1V0  q1  (4.00  F)(16.0 V)  42.7 C  21.3 C .
2
3. The parallel plates in a capacitor, with a plate area of 8.50 cm 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 d i , the charge is q  CV   0 AVi d i . After the plates are pulled apart, their
separation is d f and the potential difference is Vf. Then 𝑞 = ϵ0 𝐴𝑉𝑓 /𝑑𝑓 and
Vf 
df
0 A
q
d f 0 A
d
Vi  f Vi .
 0 A di
di
With di  3.00 103 m , Vi  6.00 V, and d f  8.00 103 m , we have V f  16.0 V .
(b) The initial energy stored in the capacitor is
 AV 2 (8.85 1012 C2 /N  m2 )(8.50 104 m 2 )(6.00 V) 2
1
U i  CVi 2  0 i 
 4.511011 J.
3
2
2d i
2(3.00 10 m)
(c) The final energy stored is
2
1 A
1  A  d f  d f   AV 2  d f
U f  0 V f2  0  Vi    0 i   U i .
2 df
2 d f  di 
di  di  di
With d f / di  8.00 / 3.00 , we have U f  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,
b g
V0  E1 d  b  E2b 
F
q IF
bI
d b  J
G
G
J
H AKH  K,
0
and the capacitance is
8.85 1012 C2 /N  m2 115 104 m2   2.61

 0 A
q
C 

 13.4pF.
V0   d  b   b  2.61 0.0124m  0.00780m    0.00780m 
(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
E1 
q
1.15 109 C

 1.13 104 N C.
12
2
2
4
2
 0 A  8.85 10 C /N  m 115 10 m 
(d) Using Eq. 25-34, we obtain
113
.  104 N C
 4.33  103 N C .

2.61
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?
E2 
E1

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
U
q 2 / 2C C   0 A /(d  b)
d
5.00
 2




 2.50.
U  q / 2C  C
0 A / d
d  b 5.00  3.00
(c) The work done is
W  U  U   U 
q2  1 1 
q2
q 2b


(
d

b

d
)


 5.44
-8.16J.J


2  C  C  2 0 A
2 0 A
(d) Since W < 0, the slab is sucked in.