Question bank Class 12 Physics Chapter 8. Electrostatics
Standard:- 12th
Question Bank
Subject:- Physics
Chapter 8. Electrostatics
MCQ’s ( 1 Mark Each)
1) A metal foil of negligible thickness is introduced between two plates of a capacitor at
the centre. The capacitance of capacitor will be
a) Half
b) Double
c) Same
d) K times
Ans: c) Same
2) Capacitance (in F) of a spherical conductor of radius 1m is
a) 1.1 x 10-10
b) 9 x 10-9
c) 10-6
d) 10-3
Ans: a) 1.1 x 10-10
3) A dipole is placed in a uniform electric field, its potential energy will be minimum when
the angle between its axis and field is
a) 2π
b) π
c)
d) Zero
Ans: d) Zero
4) If an electron is brought towards another electron, the electric potential energy of the
system
a) decreases
b) increases
c) Becomes zero
d) Remains same
Ans: b) increases
5) The work done in carrying a charge Q once round a circle of radius r with charge q at
the centre of the circle is
a)
b)
c) Zero
d)
Ans: c) Zero
6) You are given a number of capacitors labelled as 8μF - 250V. Find the number of
capacitors needed to get an arrangement equivalent of 16μF - 1000V.
a) 4
b) 16
c) 32
d) 64
Ans: c) 32
7) A parallel plate capacitor with oil between the plates (dielectric constant of oil, k = 2)
has a capacitance C. If the oil is removed, then the capacitance of the capacitor becomes
a) 2C
b) 𝐶√2
c)
d)
Ans: d)
Very Short Answer (VSA) ( 1 MARK Each )
1. What do you mean by dielectric polarization?
2. Which physical quantity has its unit as J/C? Is it a scalar or vector?
3. What are linear isotropic dielectrics?
4. What happens to the energy stored in a capacitor, if the plates of a charged capacitor
are drawn apart, the battery remaining connected?
5. The mean free path of electrons in a metal is 4 x 10-8m. Find the electric field , in units
of V/m, which can give on an average 2 eV energy to an electron in the metal.
(Ans: E = V / d = 5 x 107 V/m)
6. Find the electric potential at the surface of an atomic nucleus(Z=50) of radius 9ⅹ10-13
cm.
(Ans: V = 8 x 106 V)
7. The capacity of a parallel plate capacitor is 10 μF when the distance between its plates
is 9 cm. What will be its capacity if the distance between the plates is reduced by 6 cm.
(Ans: C2 = 30μF)
Short Answer I (SA1) ( 2 MARKS Each )
1) Explain the principle of a capacitor.
2) Obtain an expression for the electric field intensity at a point outside uniformly
charged infinite plane sheet.
3) What are polar and non polar dielectrics.
4) Two capacitors each of capacity 2 μF are connected in parallel. This system is
connected in series with a third capacitor of 12μF capacity. Find the equivalent
capacity of the system. ( Ans: 3μF)
5) Two spheres A and B of radius a and b respectively are at the same potential. Find the
ratio of the surface charge densities of A and B.
(Ans: = b/a )
6) A molecule with a dipole moment p is placed in an electric field of strength E. Initially
the dipole is aligned parallel to the field. If the dipole is to be rotated to be anti-parallel
to the field,find the work required to be done by an external agent.
(Ans: W = 2pE)
Short Answer II (SA2) ( 3 MARKS Each )
1) Derive an expression for the effective capacitance of three parallel plate capacitors
connected in series.
2) Obtain an expression for the energy stored in a charged condenser . Express it in
different forms.
3) Obtain an expression for the capacitance of a parallel plate capacitor without a
dielectric .
4) Two parallel plate capacitors X and Y have the same area of the plates and same
separation between them, are connected in series to a battery of 15 V. X has air
between the plates while Y contains a dielectric of constant k = 2.
i) Calculate the capacitance of each capacitor if equivalent capacitance of the
combination is 2 μF. ii) Calculate the potential difference between the plates of X and
Y. iii)What is the ratio of the electrostatic energy stored in X and Y?
(Ans: i) Cx = 3μF Cy= 6μF, ii) Vx = 10 V Vy = 5 V
iii) Energy stored (x) / Energy stored (y) = 2 : 1 )
5) Find the amount of work done in rotating an electric dipole of dipole moment 3.2 x 10-
8Cm from its position of stable equilibrium to the position of unstable equilibrium in a
uniform electric field if intensity 104 N / C. (Ans: 6.4 x 10 -4 J)
6) Three point charges +q, +2q and Q are placed at the three vertices of an equilateral
triangle. Find the value of charge Q (in terms of q), so that electric potential energy of
the system is zero.
( Ans: Q = -2q / 3 )
Long Answer ( LA) ( 4 marks Each)
1) Obtain an expression for the potential energy of a dipole in an external field.
2) Find the capacitance of a parallel plate capacitor with dielectric slab between the plates.
3) Derive an expression for the electric potential due to an electric dipole.
