Chapter Two
ELECTROSTATIC
POTENTIAL AND
CAPACITANCE
MCQ I
2.1 A capacitor of 4 μ F is connected as shown in the circuit (Fig. 2.1). The internal resistance of the battery is 0.5 Ω . The amount of charge on the capacitor plates will be
(a)
0
(b)
4 μ C
(c)
16 μ C
(d)
8 μ C 2n
Fig. 2.1
2.2 A positively charged particle is released from rest in an uniform electric field. The electric potential energy of the charge
(a)
remains a constant because the electric field is uniform.
(b)
increases because the charge moves along the electric field.
(c)
decreases because the charge moves along the electric field.
(d)
decreases because the charge moves opposite to the electric field.
2.3 Figure 2.2 shows some equipotential lines distributed in space.
A charged object is moved from point A to point B.
30V 40V 30V
A B A
B A
B
10V 20V 30V 40V 50V 10V 20V 50V 10V 20V 40V 50V
Fig I Fig II Fig III
(i) (ii) (iii)
Fig. 2.2
(a) The work done in Fig. (i) is the greatest.
(b) The work done in Fig. (ii) is least.
(c) The work done is the same in Fig. (i), Fig. (ii) and Fig. (iii).
(d) The work done in Fig. (iii) is greater than Fig. (ii)but equal to
that in Fig. (i).
2.4 The electrostatic potential on the surface of a charged conducting
sphere is 100V. Two statments are made in this regard:
S1 : At any point inside the sphere, electric intensity is zero.
S2 : At any point inside the sphere, the electrostatic potential is
100V.
Which of the following is a correct statement?
(a) S1 is true but S2 is false.
(b) Both S1 & S2 are false.
(c) S1 is true, S2 is also true and S1 is the cause of S2.
(d) S1 is true, S2 is also true but the statements are independant.
2.5 Equipotentials at a great distance from a collection of charges whose
total sum is not zero are approximately
(a) spheres.
(b) planes.
(c) paraboloids
(d) ellipsoids.
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2.6 A parallel plate capacitor is made of two dielectric blocks in series. One of the blocks has thickness d1 and dielectric constant k1 and the other has thickness d2 and dielectric constant k2as shown in Fig. 2.3. This arrangement can be thought as a dielectric slab of thickness d(= d1+d2) and effective dielectric constant k. The kis
kd + kd kd + kd ( + d)2kkd kk
11 22 1122 1212 12
Fig. 2.3 (a) b () d() c ()
d + d2 k + k2 (1 + kd 2) k + k2
1 1 kd 12 1
MCQ II
2.7 Consider a uniform electric field in the zˆ direction. The potential is a constant
(a)
in all space.
(b)
for any xfor a given z.
(c)
for any yfor a given z.
(d)
on the x-yplane for a given z.
2.8 Equipotential surfaces
(a)
are closer in regions of large electric fields compared to regions of lower electric fields.
(b)
will be more crowded near sharp edges of a conductor.
(c)
will be more crowded near regions of large charge densities.
(d)
will always be equally spaced.
2.9 The work done to move a charge along an equipotential from A to B
B
(a) cannot be defined as –∫ E.dl
A B
(b) must be defined as –∫ E.dl
A
(c)
is zero.
(d)
can have a non-zero value.
2.10 In a region of constant potential
(a)
the electric field is uniform
(b)
the electric field is zero
(c)
there can be no charge inside the region.
(d)
the electric field shall necessarily change if a charge is placed outside the region.
2.11 In the circuit shown in Fig. 2.4. initially key K1 is closed and key
12
K2 is open. Then K1 is opened and K2 is closed (order is important). [Take Q′ and Q′ as charges on C and C and Vand Vas voltage
1 2 1212
respectively.]
K1 K2
() ()
C1 C2
E
Fig. 2.4
Then
(a)
charge on C1 gets redistributed such that V1 = V2
(b)
charge on C1 gets redistributed such that Q1 ′ = Q2 ′
(c)
charge on C gets redistributed such that CV + CV = CE
(d)
charge on C1 gets redistributed such that Q1 ′ + Q2 ′ = Q
1 11221
2.12 If a conductor has a potential V ≠0 and there are no charges anywhere else outside, then
(a)
there must be charges on the surface or inside itself.
(b)
there cannot be any charge in the body of the conductor.
(c)
there must be charges only on the surface.
(d)
there must be charges inside the surface.
2.13 A parallel plate capacitor is connected to a battery as shown in Fig. 2.5. Consider two situations:
K
A: Key K is kept closed and plates of capacitors are moved apart using insulating handle.
B: Key K is opened and plates of capacitors are moved apart using
insulating handle.
Choose the correct option(s).
(a) In A : Q remains same but C changes.
(b)In B : V remains same but C changes.
(c) In A : V remains same and hence Q changes.
(d)In B : Q remains same and hence V changes.
VSA
2.14 Consider two conducting spheres of radii Rand R with R> R. If
1 21 2
the two are at the same potential, the larger sphere has more charge than the smaller sphere. State whether the charge density of the
13
Fig. 2.6
smaller sphere is more or less than that of the larger one.
2.15 Do free electrons travel to region of higher potential or lower potential?
2.16 Can there be a potential difference between two adjacent conductors carrying the same charge?
Can the potential function have a maximum or minimum in free space?
A test charge q is made to move in the electric field of a point charge Q along two different closed paths (Fig. 2.6). First path has sections along and perpendicular to lines of electric field. Second path is a rectangular loop of the same area as the first loop. How does the work done compare in the two cases?
SA
2.19 Prove that a closed equipotential surface with no charge within itself must enclose an equipotential volume.
2.20 A capacitor has some dielectric between its plates, and the capacitor is connected to a DC source. The battery is now disconnected and then the dielectric is removed. State whether the capacitance, the energy stored in it, electric field, charge stored and the voltage will increase, decrease or remain constant.
2.21 Prove that, if an insulated, uncharged conductor is placed near a charged conductor and no other conductors are present, the uncharged body must be intermediate in potential between that of the charged body and that of infinity.
2.22 Calculate potential energy of a point charge –q placed along the axis due to a charge +Q uniformly distributed along a ring of radius
R. Sketch P.E. as a function of axial distance z from the centre of the ring. Looking at graph, can you see what would happen if -q is displaced slightly from the centre of the ring (along the axis)?
2.23 Calculate potential on the axis of a ring due to charge Q uniformly distributed along the ring of radius R.
LA
2.24 Find the equation of the equipotentials for an infinite cylinder of radius r0, carrying charge of linear density λ.
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2.25 Two point charges of magnitude +q and -q are placed at (-d/2, 0,0) and (d/2, 0,0), respectively. Find the equation of the equipoential surface where the potential is zero.
2.26 A parallel plate capacitor is filled by a dielectric whose relative permittivity varies with the applied voltage (U ) as ε = αU where α = 2V–1 .A similar capacitor with no dielectric is charged to U0 = 78 V. It is then connected to the uncharged capacitor with the dielectric. Find the final voltage on the capacitors.
2.27 A capacitor is made of two circular plates of radius R each, separated by a distance d<