# SOLUTIONS-TO-CONCEPTS

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```							                                                CHAPTER – 31
CAPACITOR

1.   Given that
12
Number of electron = 1 × 10
12         –19       –7
Net charge Q       = 1 × 10 × 1.6 × 10 = 1.6 × 10 C
 The net potential difference = 10 L.
q   1.6  10 7           –8
 Capacitance – C =        =             = 1.6 × 10 F.
v       10
2           2                                                                  5 cm
2.   A = r = 25 cm
d = 0.1 cm
0.1 cm
0 A    8.854  10 12  25  3.14            –5
c=         =                            = 6.95 × 10 F.
d                 0 .1
3.   Let the radius of the disc = R
2
Area = R
C = 1
–3
D = 1 mm = 10 m
 A                                                                             1 mm
C= 0
d
8.85  10 12  r 2    2 10 3  1012    10 9
1=                         r =              =        = 5998.5 m = 6 Km
10  3               8.85        27.784
2             –3   2
4.   A = 25 cm = 2.5 × 10 cm
d = 1 mm = 0.01 m
V = 6V      Q=?
0 A   8.854  10 12  2.5  10 3
C=        =
d                0.01
8.854  10 12  2.5  10 3                   –10
Q = CV =                                × 6 = 1.32810 × 10 C
0.01
–10              –10
W = Q × V = 1.32810 × 10 × 6 = 8 × 10 J.
2             –3
5.   Plate area A = 25 cm = 2.5 × 10 m
Separation d = 2 mm = 2 × 10–3 m
Potential v = 12 v
0 A   8.85  10 12  2.5  10 3            –12
(a) We know C =            =                3
= 11.06 ×10 F
d             2  10
q              –12   q
C=      11.06 ×10 =
v                   12
–10
 q1 = 1.32 × 10 C.
(b) Then d = decreased to 1 mm
–3
 d = 1 mm = 1 × 10 m
0 A   q   8.85  10 12  2.5  10 3    2
C=        =   =                             =
d     v           1 10  3             12
–12            –10
 q2 = 8.85 × 2.5 × 12 × 10 = 2.65 × 10 C.
–10            –10
 The extra charge given to plate = (2.65 – 1.32) × 10 = 1.33 × 10 C.
6.   C1 = 2 F,         C2 = 4 F ,
C1   V   C2      C3
C3 = 6 F          V = 12 V
–6
cq = C1 + C2 + C3 = 2 + 4 + 6 = 12 F = 12 × 10 F
q1 = 12 × 2 = 24 C,        q2 = 12 × 4 = 48 C,    q3 = 12 × 6 = 72 C
31.1
Capacitor
7.              20 F        30 F   40 F

V = 12 V

 The equivalent capacity.
C1C 2C3                 20  30  40           24000
C=                          =                             =       = 9.23 F
C 2C3  C1C3  C1C 2   30  40  20  40  20  30    2600
(a) Let Equivalent charge at the capacitor = q
q
C=       q = C × V = 9.23 × 12 = 110 C on each.
V
As this is a series combination, the charge on each capacitor is same as the equivalent charge
which is 110 C.
(b) Let the work done by the battery = W
W                         –6          –3
V=            W = Vq = 110 × 12 × 10 = 1.33 × 10 J.
q
8.   C1 = 8 F,                    C2 = 4 F ,                 C3 = 4 F                          8 F
A
(C2  C3 )  C1
Ceq =                                                                                                    B           C
C1  C 2  C3                                                          12 V       4 F
4 F
88
=        = 4 F
16
Since B & C are parallel & are in series with A
So, q1 = 8 × 6 = 48 C               q2 = 4 × 6 = 24 C                q3 = 4 × 6 = 24 C
9.   (a)
C1            C1

A
B
C1 = 4
C2 = 6
C2            C2

 C1, C1 are series & C2, C2 are series as the V is same at p & q. So no current pass through p & q.
1   1   1   1   1 1
         
C C1 C 2    C C1C 2
C1     4
Cp=      =     = 2 F
2     2
C      6
And Cq = 2 =         = 3 F
2     2                                                                         C1                  C1
p
 C = Cp + Cq = 2 + 3 = 5 F
(b) C1 = 4 F,              C2 = 6 F,
C2                 C2
In case of p & q, q = 0
C      4                                                                                    q
 Cp = 1 =        = 2 F                                                            A                                   B
2     2
C2     6                                                                              C1       R          C1
Cq =      =    = 3 F
2      2
& C = 2 + 3 = 5 F
C2                  C2
C & C = 5 F
S
 The equation of capacitor C = C + C = 5 + 5 = 10 F
31.2
Capacitor
A              B          A5
10. V = 10 v                                                                           5 F   6 F
Ceq = C1 +C2                 [ They are parallel]
= 5 + 6 = 11 F
10 V
q = CV = 11 × 10 110 C 
11. The capacitance of the outer sphere = 2.2 F                                                             B6
C = 2.2 F
Potential, V = 10 v
Let the charge given to individual cylinder = q.
q
C=
V
 q = CV = 2.2 × 10 = 22 F
 The total charge given to the inner cylinder = 22 + 22 = 44 F
q              Kq
12. C =      , Now V =
V              R
q         R
So, C1 =            = 1 = 4 0R1
Kq / R1  K
Similarly c2 = 4 0R2
The combination is necessarily parallel.
Hence Ceq = 4 0R1 +4 0R2 = 4 0(R1 + R2)
13.                  C    C      C

A       C    C      C        B

C    C      C

C = 2 F
 In this system the capacitance are arranged in series. Then the capacitance is parallel to each other.
(a)  The equation of capacitance in one row
C
C=
3
C
(b) and three capacitance of capacity      are connected in parallel
3
 The equation of capacitance
C C C
C=            = C = 2 F
3 3 3
As the volt capacitance on each row are same and the individual is
Total           60
=                        =    = 20 V
No. of capaci tan ce   3
14. Let there are ‘x’ no of capacitors in series ie in a row
So, x × 50 = 200
 x = 4 capacitors.
10
Effective capacitance in a row =
4
Now, let there are ‘y’ such rows,
10
So,     × y = 10
4
 y = 4 capacitor.
So, the combinations of four rows each of 4 capacitors.

31.3
Capacitor
15.                                                                        C
4 F   C    8 F                        4 F               8 F
4 F           C       8 F
A                      B
A
B              3 F           6 F                  3 F                   6 F
D
3 F     D     6 F                                      D

50                                      50

48   8
(a) Capacitor =            =   
48   3
63
and        = 2 F
63
8
(i)       The charge on the capacitance             F
3
8        400
Q=           × 50 =     
3         3
400   100
 The potential at 4 F =               =
34    3
400   100
at 8 F =          =
38    6
100 100       50
The Potential difference =             =     V
3     6      3
(ii) Hence the effective charge at 2 F = 50 × 2 = 100 F
100                        100
 Potential at 3 F =   ; Potential at 6 F =
3                         6
100 100     50
Difference =         =     V
3    6      3
50
 The potential at C & D is              V
3
P R         1    1
(b)        =    =     = It is balanced. So from it is cleared that the wheat star bridge balanced. So
q S        2    2
the potential at the point C & D are same. So no current flow through the point C & D. So if we connect
another capacitor at the point C & D the charge on the capacitor is zero.
16. Ceq between a & b                                                                              C1C2/C1+C2
C2        C2
C1C 2         CC
=              C3  1 2                                                             a    C3     b              a               C3       b
C1  C2       C1  C 2
2C1C 2                                                                  C1        C1
= C3                (The three are parallel)
C1  C2                                                                                                    C1C2/C1+C2
17. In the figure the three capacitors are arranged in parallel.                                              A
A
All have same surface area = a =                                                                                                 a
3
b
0 A
First capacitance C1 =                                                                                              a
3d                                                                                  b
nd                     0 A
2 capacitance C2 =                                                                                          a
3(b  d)
rd                           0 A                                                              d
3 capacitance C3 =
3(2b  d)
Ceq = C1 + C2 +C3                                                                                   E           B           C        D

31.4
Capacitor

0 A     0 A       0 A       A 1   1     1 
=        +          +           = 0                
3d    3(b  d)   3(2b  d)    3  d b  d 2b  d 
 0 A  (b  d)(2b  d)  (2b  d)d  (b  d)d 
=                                                 
3             d(b  d)(2b  d)             


=

 0 A 3d2  6bd  2b 2   
3d(b  d)(2b  d)
2 0L        e  3.14  8.85  10 2  10 1
18. (a) C =                      =                                      [In2 = 0.6932]
In(R 2 / R1 )                In2
–13
= 80.17 × 10  8 PF
(b) Same as R2/R1 will be same.
19. Given that
–12                                 –12
C = 100 PF = 100 × 10 F                Ccq = 20 PF = 20 × 10 F
–12       –10
V = 24 V                               q = 24 × 100 × 10 = 24 × 10
q2 = ?
Let q1 = The new charge 100 PF V1 = The Voltage.
Let the new potential is V1
After the flow of charge, potential is same in the two capacitor
q2   q
V1 =        = 1
C2  C1
q  q1   q
=          = 1
C2     C1

24  10 10  q1       q1
=                    =
24  10 12      100  10 12

– q1 = q1
–10
= 24 × 10
5
–10
= 6q1 = 120 × 10
120    –10       –10
= q1 =        ×10 = 20 × 10
6

 V1 = q1            20  10 10 = 20 V
=
C1           100  10 12
20.                                    S/

A

Initially when ‘s’ is not connected,
2C       2C          5
 50 =  10  4 = 1.66 × 10 C
–4
Ceff =      q =
3        3          2
After the switch is made on,
–5
Then Ceff = 2C = 10
–5               –4
Q = 10 × 50 = 5 × 10
Now, the initial charge will remain stored in the stored in the short capacitor
Hence net charge flowing
–4            –4          –4
= 5 × 10 – 1.66 × 10 = 3.3 × 10 C.

31.5
Capacitor
21.
0.04 F      P

V

0.04 F

Given that mass of particle m = 10 mg
Charge 1 = – 0.01 C
2
A = 100 cm         Let potential = V
0.04
The Equation capacitance C =          = 0.02 F
2
The particle may be in equilibrium, so that the wt. of the particle acting down ward, must be balanced by
the electric force acting up ward.
 qE = Mg
V
Electric force = qE = q                       where V – Potential, d – separation of both the plates.
d
VC                      A                        0 A
= q                          C= 0                     d=
0 A                    q                          C
qE = mg
QVC
=        = mg
0 A
0.01  0.02  V
=                             = 0.1 × 980
8.85  10 12  100
0.1 980  8.85  10 10
V=                           = 0.00043 = 43 MV
0.0002
22. Let mass of electron = 
Charge electron = e
We know, ‘q’
For a charged particle to be projected in side to plates of a parallel plate capacitor with electric field E,
2
1qE  x                                                                                       a
y=            
2m   
 
A

d1
where y – Vertical distance covered or                                                       me
x – Horizontal distance covered
 – Initial velocity                                                                                        B

From the given data,                                                                                              d2
d1              V      qd1           q
y=          ,     E=       =             =        ,   x = a,          =?
2               R    0a 2  d1    0a 2                                                       b

For capacitor A –
q     qd1          a2
V1 =          =      as C1 = 0
C1   0a2          d1
Here q = chare on capacitor.
 0a 2
q = C × V where C = Equivalent capacitance of the total arrangement =
d1  d2
0a2
So, q =                 ×V
d1  d2

31.6
Capacitor

q             0a2  V                  V
Hence E =                 =                      =
 0a   2
(d1  d2 ) 0 a2        (d1  d2 )

d1  1    e V     a2
Substituting the data in the known equation, we get,                          =             2
2   2 (d1  d2 )m u
1/ 2
Vea 2                         Vea 2    
u =
2
u=                         
d1m(d1  d2 )                 d m(d  d ) 
 1    1   2 

qeme                                                         +       2 cm
–
23. The acceleration of electron ae =
Me                                                          +        qe                      –
+                ep              –
qpe                                                                       e–x            x
The acceleration of proton =                   = ap                                                           +                                –
Mp                                                                 +            E    qp E           –
+            E                   –
1   2
The distance travelled by proton X = apt        …(1)
2
The distance travelled by electron              …(2)
1               1 2
From (1) and (2)  2 – X = act2           x = act
2               2
 qpE 
      
x    ap      Mp 
                =       
2  x ac       qc F 
M 
      
 c 
x   M     9.1 10 31      9 .1
 10  4 = 5.449 × 10
–4
=         c =            27
=
2  x Mp   1.67  10        1.67
–4                     –4
 x = 10.898 × 10              – 5.449 × 10 x
10.898  10 4
x=                      = 0.001089226
1.0005449
24. (a)               1 F                 3 F
1    3

A                                          B              A                  B
5 F

2 F             6 F                               2     6

1 F                   3 F
As the bridge in balanced there is no current through the 5 F capacitor
So, it reduces to
5 F
similar in the case of (b) & (c)
as ‘b’ can also be written as.
2 F                           6 F
1 3 2  6    3 12         6  12
Ceq =             =           =         = 2.25 F
1 3 2  6   48 8            8
25. (a) By loop method application in the closed circuit ABCabDA
2Q  Q   Q                                                                      B      2 F         C       2 F
–12 +         1  1 =0                               …(1)                                                                              a
2F 2F 4F                                                                    Q                Q
In the close circuit ABCDA                                                                                        4 F
(Q – Q1)                  4 F
Q    Q  Q1
–12 +                 =0               …(2)                                                A                 D                         b
2F    4F
From (1) and (2) 2Q + 3Q1 = 48    …(3)
And 3Q – q1 = 48 and subtracting Q = 4Q1, and substitution in equation

31.7
Capacitor
48
2Q + 3Q1 = 48  8 Q1 + 3Q1 = 48  11Q1 = 48, q1 =
11
Q1     48     12
Vab =         =       =    V
4F   11 4   11
(b)                       12 V                                  12 V              4 F
a                                         a
2 F
2 F                           4 F
b
b                                                               24 V
24 V

The potential = 24 – 12 = 12
(2  0  12  4)   48
Potential difference V =                                  =    =8V
2 4           6
 The Va – Vb = – 8 V
Left      Right
(c)
2 V                      2 V
A                  B                        C
a

2 F       b           2 F
D
F                       E

From the figure it is cleared that the left and right branch are symmetry and reversed, so the current go
towards BE from BAFEB same as the current from EDCBE.
Q   0
The net charge Q = 0                                  V=        =   =0                    Vab = 0
C   C
 The potential at K is zero.
(d)                6 V
4 F

12 V          2 F
a                                               b
24 V            1 F

Net ch arg e      24  24  24   72
The net potential =                                 =              =    10.3 V
Net capaci tan ce        7         7
Va – Vb = – 10.3 V
26. (a)                 3/8

1 F
3 F
12/8
4 F

3/8
4/8                                                    3/8

3 F              1 F                 4/8       12/8
1/2           3/2

By star Delta conversion
3 F          1 F
     1  3 
 3      1                                                                           3 F         1 F
3       2 2        3 35   9  35   11
Ceff =                      =    =        =    F
8       1  3       8 24     24     6
  3      1 
     2   2 

31.8
Capacitor
(b)
1 F       3 F
2 F              4 F
2 F

4 F

3 F        1 F
by star Delta convensor

3/8 f
3/8 f

               4/8 f
12/8 f                                       1/2 f             3/2 f

       2 f                2 f

4/8 f                                     2 f               2 f
12/8 f

3/2 f             1/2 f
3/8 f
3/8 f
3/8 f

4 f            4 f

3 16 3  11
=       =    f
8 8 8   4
3/8 f

2 F                      4 F                                                  4/3 F
(c)

8/3 F
5 F
4 F                       8 F

4 F                                                                4 F

4 8
Cef =      4 = 8 F
3 3
(d)                          2 f                         4 f                                         2 f   4 f

6 f

4 f                          8 f                                        4 f    8 f
6 f
4 f                           8 f                                       4 f     8 f

6 f

2 f                         4 f                                         2 f   4 f

8/6 f

32/12 f

32/12 f

3 32 32 8  16  32
Cef =               =         = 8 f
8/6 f                             8 12 12 6     6
31.9
Capacitor
27.        2 f      2 f       2 f     2 f
A
1         2          3        4

5          6          7        8
B

= C5 and C1 are in series
22
Ceq =        =1
22
This is parallel to C6 = 1 + 1 = 2
22
Which is series to C2 =        =1
22
Which is parallel to C7 = 1 + 1 = 2
22
Which is series to C3 =         =1
22
Which is parallel to C8 = 1 + 1 = 2
22
This is series to C4 =                   =1
22
28.     A
A           2 f   Fig – 
1 F     C
Fig - 
B                                                                                    B

Let the equivalent capacitance be C. Since it is an infinite series. So, there will be negligible change if
the arrangement is done an in Fig – 
2C              2C  2  C
Ceq =        +1C=
2C                2C
 (2 + C) × C = 3C + 2
2
C –C–2=0
 (C –2) (C + 1) = 0
C = –1 (Impossible)
So, C = 2 F
29.
4 f      4 f       4 f                                   4 f
A                                                       A

C       c
2 f           C
2 f       2 f       2 f
B                                                     B

= C and 4 f are in series
4C
So, C1 =
4C
Then C1 and 2 f are parallel
C = C1 + 2 f
4C         4C  8  2C
        2                =C
4C             4C
2     2
 4C + 8 + 2C = 4C + C = C – 2C – 8 = 0
2  4  4  1 8     2  36   26
C=                    =         =
2                2      2
26
C=        = 4 f
2
 The value of C is 4 f

31.10
Capacitor
–8                                   –8
30. q1 = +2.0 × 10 c          q2 = –1.0 × 10                    c
–3           –9
C = 1.2 × 10 F = 1.2 × 10 F
q1  q2   3.0  10 8
net q =           =
2           2
q    3  10 8        1
V=      =                         = 12.5 V
c        2       1.2  10  9
31.  Given that
+20c
Capacitance = 10 F                                                                        + +              -    -
Charge = 20 c                                                                             + +              -    - x
x 10 + + 10           -    -
20  0                                                         +         x 10 -
 The effective charge =              = 10 F                                              + + –0           -    -
2                                                        + +              -    -
+ +             -     -
q            q     10
C=        V=          =       = 1 V
V            C     10
–6                             –7
32. q1 = 1 C = 1 × 10 C            C = 0.1 F = 1 × 10 F
–6
q2 = 2 C = 2 × 10 C
q1  q2   (1  2)  10 6             –6
net q =           =                 = – 0.5 × 10 C
2             2
q    1 10 7
Potential ‘V’ =   =              =–5V
c    5  10  7
But potential can never be (–)ve. So, V = 5 V 
33. Here three capacitors are formed
And each of
96
A=       10 12 f.m.
0
-   -   +    -      +    +         -     +
–3                                                    -    -   +    -      +    +        -      +
d = 4 mm = 4 × 10 m                                                          -    -   +    -      +    +        -      +
Capacitance of a capacitor                                                  -    -   +    -      +    +        -      +
-    -   +    -      +    +        -      +
96  10 12                                               -    -   +    -      +    +        -      +
0                                                             -    -   +    -      +    +        -      +
0 A              0                    –9
C=        =                      = 24 × 10        F.
d            4  10  3
 As three capacitor are arranged is series
C   24  10 9         –9
So, Ceq =       =            = 8 × 10
q       3
–9               –8
 The total charge to a capacitor = 8 × 10 × 10 = 8 × 10 c
–8          –8            –6
 The charge of a single Plate = 2 × 8 × 10 = 16 × 10 = 0.16 × 10 = 0.16 c.
34. (a) When charge of 1 c is introduced to the B plate, we also get 0.5 c
charge on the upper surface of Plate ‘A’.                                      0.5 C
–9          –8
A
(b) Given C = 50 F = 50 × 10 F = 5 × 10 F                                     – 0.5 C
–6                                                    +++++++++
Now charge = 0.5 × 10 C                                                    B
+++++++++             1.0 C
q    5  10 7 C
V=     =               = 10 V                                                 C
C    5  10  8 F
35. Here given,                                                                                    1 C   0.5 C
Capacitance of each capacitor, C = 50 f = 0.05 f
0.5 C
Charge Q = 1 F which is given to upper plate = 0.5 c charge appear on outer                          0.5 C
and inner side of upper plate and 0.5 c of charge also see on the middle.
(a) Charge of each plate = 0.5 c                                                                      0.5 C
0.5 C
Capacitance = 0.5 f
0.5 C

31.11
Capacitor
q        q     0 .5
C=       V=       =       = 10 v
V         C    0.05
(b) The charge on lower plate also = 0.5 c
Capacitance = 0.5 F
q          q      0 .5
C=       V=        =        = 10 V
V          C     0.05
 The potential in 10 V
–12                                 –12
36. C1 = 20 PF = 20 × 10 F,              C2 = 50 PF = 50 × 10 F
C1C 2     2  10 11  5  10 11             –11
Effective C =               =                         = 1.428 × 10 F
C1  C 2   2  10 11  5  10 11
–11                        –11
Charge ‘q’ = 1.428 × 10           × 6 = 8.568 × 10           C
11
q    8.568  10
V1 =      =                     = 4.284 V
C1     2  10 11
q    8.568  10 11
V2 =      =                = 1.71 V
C2     5  10 11
Energy stored in each capacitor
2                –11          2           –11
E1 = (1/2) C1V1 = (1/2) × 2 × 10 × (4.284) = 18.35 × 10 ≈ 184 PJ
2                –11        2         –11
E2 = (1/2) C2V2 = (1/2) × 5 × 10 × (1.71) = 7.35 × 10 ≈ 73.5 PJ
37. C1 = 4 F,         C2 = 6 F,      V = 20 V
C1C 2      46                                                 4 f               6 f
Eq. capacitor Ceq =           =       = 2.4
C1  C 2    46
The Eq Capacitance Ceq = 2.5 F
 The energy supplied by the battery to each plate
2                2
E = (1/2) CV = (1/2) × 2.4 × 20 = 480 J                                                        20 V
 The energy supplies by the battery to capacitor = 2 × 480 = 960 J
–6
38. C = 10 F = 10 × 10 F
For a & d
–4                                                           a       b               c
q = 4 × 10 C
–5                                                                                                 cd
c = 10 F                                                                    c               c

E=
1 q2
=

1 4  10 4    
2
= 8 × 10
–3
J = 8 mJ
c

2 c     2 10  5
For b & c
–4
q = 4 × 10 c                                                                            100 V
–5
Ceq = 2c = 2 × 10 F
4  10 4
V=              = 20 V
2  10  5
2          –5      2       –3
E = (1/2) cv = (1/2) × 10 × (20) = 2 × 10 J = 2 mJ
39. Stored energy of capacitor C1 = 4.0 J
1 q2
=        = 4.0 J
2 c2
When then connected, the charge shared
2         2
1 q1     1 q2
2
      2
 q 1 = q2
2 c      2 c
So that the energy should divided.
 The total energy stored in the two capacitors each is 2 J.

31.12
Capacitor
–6                 –6
40. Initial charge stored = C × V = 12 × 2 × 10 = 24 × 10 c
Let the charges on 2 & 4 capacitors be q1 & q2 respectively
q1   q   q  q
There, V =         = 2  1 = 2  q2 = 2q1 .
C1 C 2   2   4
–6
or q1 + q2 = 24 × 10 C
–6
 q1 = 8 × 10 c
–6        –6
q2 = 2q1 = 2 × 8 × 10 = 16 × 10 c
2
2              8
E1 = (1/2) × C1 × V1 = (1/2) × 2 ×   = 16 J
2
2
2              8
E2 = (1/2) × C2 × V2 = (1/2) × 4 ×   = 8 J
4
41. Charge = Q
 Capacitance of the sphere = C = 40R                                                                                     Q
2                   2            2
1Q    1 Q         Q                                                                                              R
Energy =           =          =       
2 C   2 4 0R   80R
42. Q = CV = 40R × V
1 q2
E=                                            [ ‘C’ in a spherical shell = 4 0R]
2 C
2
1 16 2  0  R 2  V 2          2
E=                              = 2 0RV                                                [‘C’ of bigger shell = 4 0R]
2     40  2R
–4        2
43.  = 1 × 10 c/m
–2                                                             3       –6
a = 1 cm = 1 × 10 m                                                            a = 10        m
4 2
1      2
1 (1 10 )        10 4
The energy stored in the plane =                           =             12
=      = 564.97
2 0   2 8.85  10       17.7
The necessary electro static energy stored in a cubical volume of edge 1 cm infront of the plane
1 2 3                             –6                 –4
=        a          = 265 × 10              = 5.65 × 10               J
2 0
2               –2   2
44. area = a = 20 cm = 2 × 10 m
–3
d = separation = 1 mm = 10 m
 0  2  10 3                                                    0  2  10 3
Ci =                             = 20                     = 0
C =
10  3                              2  10  3
qi = 24 0            q = 12 0                So, q flown out 12 0. ie, qi – qf.
–12              –12               –10
(a) So, q = 12 × 8.85 × 10 = 106.2 × 10 C = 1.06 × 10 C
(b) Energy absorbed by battery during the process
–10                    –10
= q × v = 1.06 × 10 C × 12 = 12.7 × 10 J
(c) Before the process
2                          –12                   –10
Ei = (1/2) × Ci × v = (1/2) × 2 × 8.85 × 10 × 144 = 12.7 × 10 J
After the force
2                    –12                     –10
Ei = (1/2) × Cf × v = (1/2) × 8.85 × 10 × 144 = 6.35 × 10 J
(d) Workdone = Force × Distance
1 q2           3                                         1 12  12   0   0  10 3
=           = 1 × 10                                  =         
2 0 A                                                   2        0  2  10  3
(e) From (c) and (d) we have calculated, the energy loss by the separation of plates is equal to the
work done by the man on plate. Hence no heat is produced in transformer.

31.13
Capacitor
45. (a) Before reconnection
C = 100 f              V = 24 V
q = CV = 2400 c (Before reconnection)
After connection
When C = 100 f         V = 12 V
q = CV = 1200 c (After connection)
(b) C = 100,                V = 12 V
 q = CV = 1200 v
W
(c) We know V =
q
W = vq = 12 × 1200 = 14400 J = 14.4 mJ
The work done on the battery.
2
(d) Initial electrostatic field energy Ui = (1/2) CV1
2
Final Electrostatic field energy U = (1/2) CV2
Decrease in Electrostatic
2           2
Field energy = (1/2) CV1 – (1/2) CV2
2      2
= (1/2) C(V1 – V2 ) = (1/2) × 100(576 –144) = 21600J
 Energy = 21600 j = 21.6 mJ
(e)After reconnection
C = 100 c,                  V = 12 v
2
 The energy appeared = (1/2) CV = (1/2) × 100 × 144 = 7200 J = 7.2 mJ
This amount of energy is developed as heat when the charge flow through the capacitor.
46. (a) Since the switch was open for a long time, hence the charge flown must be due to the both, when
the switch is closed.
Cef = C/2                                                                             C      C
EC
So q =
2
EC      E 2C
(b) Workdone = q × v =           E =
2       2
E
1 C       E 2C
(c) Ei =      E2 =
2 2        4
2   E 2C
Ef = (1/2) × C × E =
2
E 2C
Ei – Ef =
4
(d) The net charge in the energy is wasted as heat.
47. C1 = 5 f          V1 = 24 V
q1 = C1V1 = 5 × 24 = 120 c
and C2 = 6 f               V2 = R
q2 = C2V2 = 6 × 12 = 72
 Energy stored on first capacitor
2
1 q1   1 (120)2
Ei =         =        = 1440 J = 1.44 mJ
2 C1   2    2
nd
Energy stored on 2        capacitor
2
1 q2  1 (72)2
E2 =        =       = 432 J = 4.32 mJ
2 C2  2   6

31.14
Capacitor
(b) C1V1                                    C2V2
Let the effective potential = V                                                      5 f 24 v             6 f 12 v
+       –             +     –
C1V1  C 2 V2   120  72
V=                  =          = 4.36
C1  C 2        56
The new charge C1V = 5 × 4.36 = 21.8 c
and C2V = 6 × 4.36 = 26.2 c
2
(c) U1 = (1/2) C1V
2
U2 = (1/2) C2V
2                        2                    –6
Uf = (1/2) V (C1 + C2) = (1/2) (4.36) (5 + 6) = 104.5 × 10 J = 0.1045 mJ
But Ui = 1.44 + 0.433 = 1.873
 The loss in KE = 1.873 – 0.1045 = 1.7687 = 1.77 mJ
48.               +     –                         +      –

(i)
(ii)

+     –

When the capacitor is connected to the battery, a charge Q = CE appears on one plate and –Q on
the other. When the polarity is reversed, a charge –Q appears on the first plate and +Q on the
second. A charge 2Q, therefore passes through the battery from the negative to the positive
terminal.
The battery does a work.
2
W = Q × E = 2QE = 2CE
In this process. The energy stored in the capacitor is the same in the two cases. Thus the workdone
by battery appears as heat in the connecting wires. The heat produced is therefore,
2             –6                –5
2CE = 2 ×5 × 10 × 144 = 144 × 10 J = 1.44 mJ              [have C = 5 f V = E = 12V]
–2
49. A = 20 cm × 20 cm = 4 × 10 m
–3
d = 1 m = 1 × 10 m
k=4                  t=d
0 A         0 A      Ak
C=             =           = 0                                                                    20 cm
t            d     d
dt        dd
k            k
1 mm
8.85  10 12  4  10 2  4                  –9
=                                   = 141.6 × 10 F = 1.42 nf                                      20 cm
1 10  3
50. Dielectric const. = 4
F = 1.42 nf,              V=6V
–9               –9
Charge supplied = q = CV = 1.42 × 10 × 6 = 8.52 × 10 C
–9                      –9
Charge Induced = q(1 – 1/k) = 8.52 × 10 × (1 – 0.25) = 6.39 × 10 = 6.4 nc
8.52c
Net charge appearing on one coated surface =                       = 2.13 nc
4
51. Here
2      –2  2
Plate area = 100 cm = 10 m
–3                                                                               A = 100 cm2 
Separation d = .5 cm = 5 × 10 m
–3
Thickness of metal t = .4 cm = 4 × 10 m                                          d = 0.5 cm            t = 0.4 cm
12         2
0 A   A     8.585  10       10
C=             = 0 =                           = 88 pF
dt 
t   dt         (5  4)  10  3
k
Here the capacitance is independent of the position of metal. At any position the net separation is d – t.
As d is the separation and t is the thickness.

31.15
Capacitor
52. Initial charge stored = 50 c
50 c
Let the dielectric constant of the material induced be ‘k’.
Now, when the extra charge flown through battery is 100.
So, net charge stored in capacitor = 150 c
 A        q      A
Now C1 = 0           or 1  0                  …(1)
d          V      d
 Ak             q       Ak
C2 = 0               or, 2  0                 …(2)
d               V       d
q    1
Deviding (1) and (2) we get 1 
q2 k
50    1
         k=3
150 k
–3
53. C = 5 f             V=6V             d = 2 mm = 2 × 10 m.
(a) the charge on the +ve plate
q = CV = 5 f × 6 V = 30 c
V        6V               3
(b) E =      =              = 3 × 10 V/M
d    2  10  3 m
–3
(c) d = 2 × 10 m
–3
t = 1 × 10 m
0 A                               8.85  A  10 12                    10 4
k = 5 or C =                    5 × 10
–6
=                3
 10  9  A =
d                                     2  10                           8.85
When the dielectric placed on it
10 4
8.85  10 12      12
0 A                     8.85 = 10      10 4  5 5
C1 =             =              3
=  10  5 = 0.00000833 = 8.33 F.
dt
t              10             6  10  3      6
10  3 
k               5
–6
(d) C = 5 × 10 f.             V=6V
–5
 Q = CV = 3 × 10 f = 30 f
–6
C = 8.3 × 10 f
V=6V
–6
 Q = CV = 8.3 × 10 × 6 ≈ 50 F
 charge flown = Q – Q = 20 F
C1C 2
54. Let the capacitances be C1 & C2 net capacitance ‘C’ =
C1  C 2               1 cm2               +
 Ak                           Ak
Now C1 = 0 1                       C2 = 0 2
d1                            d2                                                    C1              6 mm

 0 Ak 1  0 Ak 2             k k 
                 0 A 1 2 
d d                                                       C2 k=4            4 mm
d1       d2                 1 2         8.85  10 12  10 2  24
C=                   =                       =
 0 Ak 1  0 Ak 2         k d  k 2 d1    6  4  10  3  4  6  10  3                        –
            0 A 1 2          
d1       d2                d1d2     
              
–11
= 4.425 × 10 C = 44.25 pc.
2        –2 2
55. A = 400 cm = 4 × 10 m
–3
d = 1 cm = 1× 10 m
V = 160 V
–4
t = 0.5 = 5 × 10 m
k=5

31.16
Capacitor

0 A             8.85  10 12  4  10 2                 35.4  10 4
C=                     =                                      4
=
dt 
t                               5  10              10  3  0.5
10  3  5  10  4 
k                                   5
56. (a) Area = A
Separation = d
 Ak                                         0 Ak 2                                  K1
C1 = 0 1                                 C2 =
d/2                                          d/2
K2
2 0 Ak 1 2 0 Ak 2     (2 0 A )2 k 1k 2

C1C 2               d         d                 d2             2k 1k 2  0 A
C=                      =                     =                        =
C1  C 2         2 0 Ak 1 2 0 Ak 2             k 1d  k 2 d   d(k 1  k 2 )
            (2 0 A )
d         d                     d2
(b) similarly
1   1   1   1       1         1           1
         =                    
C C1 C 2 C 3    3 0 Ak 1 3 0 Ak 2   3 0 Ak 3
d         d           d
d 1        1   1       d  k 2k 3  k 1k 3  k 1k 2 
=                        =                               
3 0 A  k 1 k 2 k 3    3 0 A      k 1k 2k 3         
3 0 Ak 1k 2k 3
C=
d(k 1k 2  k 2k 3  k 1k 3 )
(c) C = C1 + C2
A      A
0 k1 0 k 2
2      2     A
=             = 0 (k 1  k 2 )
d      d     2d
57.                                  dx
A

k1 d            dc
d – x tan
d
x tan
dc1   k2

B
X

Consider an elemental capacitor of with dx our at a distance ‘x’ from one end. It is constituted of two
capacitor elements of dielectric constants k1 and k2 with plate separation xtan and d –xtan
respectively in series
1   1    1          x tan      d  x tan 
                                    
dcR dc 1 dc 2      0k 2 (bdx )  0k 1(bdx )
 0bdx
dcR =
x tan  (d  x tan )

k2          k1
dx
or CR = 0bk1k2             k d  (k
2         1  k 2 )x tan 

 0bk 1k 2
=                   [logek2d+ (k 1 –k2) x tan]a
tan (k 1  k 2 )
 0bk 1k 2
=                     [logek2d+ (k 1 –k2) a tan – loge k2d]
tan (k 1  k 2 )
d
tan  =         and A = a × a
a
31.17
Capacitor

 0 ak 1k 2                                k 1 
CR =                                         loge   
k 
d                                     
      2  
(k 1  k 2 )
a
 0 a 2k 1k 2                               k 1 
CR =                                         loge   
k 
d(k 1  k 2 )                         
      2  
 0 a 2k 1k 2              k1
CR =                       In
d(k 1  k 2 )              k2
58.
/   s

V               C                   C

I. Initially when switch ‘s’ is closed
2        2     2
Total Initial Energy = (1/2) CV + (1/2) CV = CV        …(1)
II. When switch is open the capacitance in each of capacitors varies, hence the energy also varies.
i.e. in case of ‘B’, the charge remains
Same i.e. cv
Ceff = 3C
1 q2      1 c 2v 2   cv 2
E=          =          =
2 c       2   3c      6
In case of ‘A’
Ceff = 3c
1           1             3
E =  C eff v 2 = × 3c × v2 = cv2
2           2             2
cv 2 3cv 2   10cv 2
Total final energy =                     =
6     2       6
Initial Energy      cv 2
Now,                 =           =3
Final Energy      10cv 2
6
59. Before inserting
 A                             AV
C= 0 C                       Q= 0         C
d                              d                                              +                  –
After inserting
 A       Ak                    Ak                                            +         K        –
C= 0 = 0                     Q1 = 0       V
d        d                      d
k
+                 –
The charge flown through the power supply
Q = Q1 – Q
 AkV  0 AV         AV
= 0                = 0     (k  1)
d         d        d
Workdone = Charge in emf
02 A 2 V 2
(k  1)2
1q   2
1              d2                 AV 2
=     =                                = 0      (k  1)
2 C   2              0 A                2d
(k  1)
d

31.18
Capacitor
–4
60. Capacitance = 100 F = 10 F
P.d = 30 V
–4            –3
(a) q = CV = 10 × 50 = 5 × 10 c = 5 mc
Dielectric constant = 2.5
–4
(b) New C = C = 2.5 × C = 2.5 × 10 F
q
New p.d =                        [’q’ remains same after disconnection of battery]
c1
5  10 3
=              = 20 V.
2.5  10  4
(c) In the absence of the dielectric slab, the charge that must have produced
–4              –3
C × V = 10 × 20 = 2 × 10 c = 2 mc
(d) Charge induced at a surface of the dielectric slab
= q (1 –1/k)      (where k = dielectric constant, q = charge of plate)
–3         1            –3 3         –3
= 5 × 10        1        = 5 × 10 ×   = 3 × 10 = 3 mc.
    2 .5             5
61. Here we should consider a capacitor cac and cabc in series
4 0 ack
Cac =
k(c  a )
40bc                                                           C                   C
Cbc =                                                                                     a
(b  c )
1   1   1                                                                                    b
   
C Cac Cbc                                                            b
(c  a )   (b  c )   b(c  a)  ka(b  c )
=                       =
4 0 ack 40 bc          k 4 0 abc                                        C               b

40kabc                                                            a
C=                                                                                                C
ka(b  c )  b(c  a)
62. These three metallic hollow spheres form two spherical capacitors, which are connected in series.
Solving them individually, for (1) and (2)
4 0 ab
C1 =           ( for a spherical capacitor formed by two spheres of radii R2 > R1 )
ba
4 0R 2R1
C=
R2  R2
Similarly for (2) and (3)
4 0bc
C2 =
c b
( 40 )2 ab 2 c
C1C 2                (b  a)(c  a)
Ceff =
C1  C 2            ab(c  b)  bc(b  a) 
4 0                        
     (b  a)(c  b)    
4 0 ab 2 c              4 0 ab 2 c       4 0 ac
=                                =                  =
2       2
abc  ab  b c  abc             b 2 (c  a )        ca
63. Here we should consider two spherical capacitor of capacitance
cab and cbc in series
40 abk                  4 0bc
Cab =                     Cbc =
(b  a)                  (c  b )

31.19
Capacitor
1   1   1    (b  a)   (c  b)   c(b  a)  ka(c  b)
      =                   =
C Cab Cbc   4 0 abk 4 0bc        k 4 0 abc
40kabc
C=
c(b  a)  ka(c  b)
64. Q = 12 c
V = 1200 V
v         10–6 v
=3×
d              m
V         1200           –4
d=          =          = 4 × 10 m
( v / d) 3  10  6
Q     12  10 6     –8
c=    =             = 10 f
v      1200
 A        –8
 C = 0 = 10 f
d
10 8  d   10 8  4  10 4        2
A=                 =                   0.45 m
0        8.854  10  4
2       –2        2
65. A = 100 cm = 10               m
–2
d = 1 cm = 10 m
V = 24 V0
0 A   8.85  10 12  10 2            –12
 The capacitance C =                    =                       = 8.85 × 10
d            10  2
2             –12        2             –12
 The energy stored C1 = (1/2) CV = (1/2) × 10                    × (24) = 2548.8 × 10
C1   2548 .8  10 12            –7
 The forced attraction between the plates =                 =                  = 2.54 × 10 N.
d        10  2
66.
d                               K

M

We knows
 0bV 2 (k  1)
In this particular case the electricfield attracts the dielectric into the capacitor with a force
2d
Where      b – Width of plates
k – Dielectric constant
d – Separation between plates
V = E = Potential difference.
Hence in this case the surfaces are frictionless, this force is counteracted by the weight.
 0bE 2 (k  1)
So,                   = Mg
2d
 0bE 2 (k  1)
M=
2dg

31.20
Capacitor
67.                l1
l2

K1              K2

n                             n

(a) Consider the left side
The plate area of the part with the dielectric is by its capacitance
k  bx                                 b(L  x )
C1 = 1 0     and with out dielectric C2 = 0 1
d                                        d
These are connected in parallel
 b
C = C1 + C2 = 0 [L1  x(k 1  1)]
d
Let the potential V1
2
 0bv 1
U = (1/2) CV1 =
2
L1  x(k  1) …(1)
2d
Suppose dielectric slab is attracted by electric field and an external force F consider the part dx
which makes inside further, As the potential difference remains constant at V.
The charge supply, dq = (dc) v to the capacitor
2
The work done by the battery is dwb = v.dq = (dc) v
The external force F does a work dwe = (–f.dx)
during a small displacement
2
The total work done in the capacitor is dwb + dwe = (dc) v – fdx
This should be equal to the increase dv in the stored energy.
Thus (1/2) (dk)v2 = (dc) v2 – fdx
1 2 dc
f=    v
2   dx
from equation (1)
 0bv 2
F=           (k 1  1)
2d
2           F  2d                   F  2d
 V1 =                        V1 =
 0b(k 1  1)             0b(k 1  1)

F  2d
For the right side, V2 =
 0b(k 2  1)

F  2d
V1          0b(k 1  1)
=
V2            F  2d
 0b(k 2  1)

V1          k2  1
       =
V2          k1  1

k2  1
 The ratio of the emf of the left battery to the right battery =
k1  1

31.21
Capacitor
68. Capacitance of the portion with dielectrics,
l
k 0 A                                                                       l
C1 =
d
Capacitance of the portion without dielectrics,
 0 (   a )A
C2 =                                                                                      K
d                                                              E
0 A
 Net capacitance C = C1 + C2 =                        ka  (  a)
d
0 A
C=           a(k  1)                                                             a
d
Consider the motion of dielectric in the capacitor.
Let it further move a distance dx, which causes an increase of capacitance by dc
dQ = (dc) E
2
The work done by the battery dw = Vdg = E (dc) E = E dc
Let force acting on it be f
 Work done by the force during the displacement, dx = fdx
 Increase in energy stored in the capacitor
2        2
 (1/2) (dc) E = (dc) E – fdx
2              1 E 2 dc
 fdx = (1/2) (dc) E  f =
2 dx
0 A
C=            a(k  1)                           (here x = a)
d
 d  0 A

dc
=              a(k  1)
da   da  d



0 A           dc
        (k  1) =
d            dx
1 2 dc  1  A          
f=        E    = E 2  0 (k  1)
2   dx  2  d          
f   E 2  0 A(k  1)                                      1
m        2dm                                             2
2(  a)          2(  a)2dm               4md(  a)
t=                 =         2
=
ad              E  0 A(k  1)              0 AE 2 (k  1)

8md(  a)
 Time period = 2t =
 0 AE 2 (k  1)



31.22

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