Important Question for CBSE class 12 Physics Unit 1 – Electrostatics


1.Which of the following is not an insulator?

(a) Glass

(b) Rubber

(c) Ebonite

(d) Human body

2. What is not true?

(a) It is not possible to create or destroy net charge carried by any isolated system.

(b) Charges can be created or destroyed in equal and unlike pairs only.

(c) Excess of electrons over protons in a body is responsible for positive charge of the body.

(d) Proper signs have to be used while adding the charge in a system

3.The electric field at a point is

(a) Always continuous

(b) Discontinuous only if there is a negative charge at that point

(c) Discontinuous if there is a charge at that point

(d) Continuous if there is no charge at that point

4. A positively charged particle is released from rest in a uniform electrical field. The electrical potential energy of a charge

(a) Remains a constant because the electric field is uniform.

(b) Decrease because the charge moves.

(c) Decreases because the charge moves opposite to the electric field.

(d) Increases because the charged moves along the electric field.

5.Equipotentials at a great distance from a collection of charges whose total sum is not zero are approximately.

(a) Planes

(b) Paraboloids

(c) Ellipsoids

(d) Spheres

6.Which of the following is not correct about Equipotential surfaces

(a) Are closer in regions of large electric fields compared to regions of lower electric fields

(b) Will always be equally spaced

(c) Will be more crowded near regions of large charge densities

(d) Will be more crowded near sharp edges of a conductor

7.Which statement is correct about region of constant potential

(a) The electric field is uniform

(b) There can be no charge inside the region & The electric field is zero.

(c) The electric field is non zero & There can be charge inside the region

(d) The electric field shall necessarily change if a charge is placed outside the region

8.A capacitor is charged by a battery. The battery is removed and another identical uncharged capacitor is connected in parallel. The total electrostatic energy of resulting system is

(a) Increases by a factor of 4

(b) Decreases by a factor of 2

(c) Remains the same

(d) Increases by a factor of 2

9.Three charged particles are collinear and are in equilibrium then 

(a) All the charged particles have the same polarity

(b) The equilibrium is unstable 

(c) All the charged particles cannot have the same polarity 

(d) Both b & c are correct 

10.A point charge is brought in an electric field. The electric field at a nearby point

(a) Will increase if charge is positive  & may decrease if charge is negative 

(b) Will decrease if charge is negative & may increase if charge is positive 

(c) May increase if charge is positive & may decrease if charge is negative 

(d) Will increase if charge is positive  & Will decrease if charge is negative


1.Does the charge given to a metallic sphere depend on whether it is hallow or solid? Give reason for your answer.

Ans :- In case of metallic sphere, charge given to it is mostly resides on its surface. Therefore, there is no difference whether the sphere is hollow or solids. As in both the cases, the charge that will reside will be same.

2. For any charge configuration, equipotential surface through a point is a normal to the electric field. Justify.

Ans :- No work is done in moving the test charge from one point of an equipotential surface to the other.

WB – WA = 0 => E.dl = 0

Hence, For any charge configuration, equipotential surface through a point is a normal to the electric field.

3.Why are electric field lines perpendicular at a point on an equipotential surface of a Conductor?

Ans :- Electric field is always normal to the equipotential surface, because no work is done on the equipotential surface.

As W = q0 (VA – VB) => VA – VB = 0

Hence , W = 0

4.A hallow metal surface sphere of radius 5 cm is charged such that potential on its surface is 10V. What is the potential at the centre of the sphere?

Ans :- Electric field inside the hallow spherical charged conductor is zero. Thus, the potential is constant and equal to the value at the surface, i.e. equal to 10V its value at the surface.

5.Can two potential surface intersect each other? Justify your answer.

Ans :- No, two equipotential surfaces cannot intersect each other because two normal can be drawn at intersecting point on two surfaces which give two directions of E at the same point which is not possible.

6.Why an electric field line cannot have sudden breaks?

Ans :- In an electrostatic field , a charge experiences a continuous force and it moves continuously. As the charge does not jump from one point to another, the field line cannot have sudden break.

7.What is the nature of symmetry of the electric field due to a point charge and electric dipole?

Ans :- The electric field due to a point charge has spherical symmetric with point charge at the centre. It is so because, field intensity is equal at equal distance from the point charge.

The electric field due to dipole has a cylindrical symmetry. The axis of the cylinder passes through the dipole axis. It is so because, the electric field due to dipole will be same at every point on the surface of a right circular cylinder with electric dipole as the axis.

8. What do you mean by electrostatic shielding? Write its uses.

Ans :- It is a phenomena of protecting a certain region of space from external electric field. The electrostatic shielding has been used in protecting sensitive instruments from outside electrical influence. Electrostatic shielding has also been used in the design of T.V. Cables.

9.Describe the construction of VAN DE GRAAFF Electrostatic Generator.

Ans :-The essential parts of Van de graaff generator is a large spherical conducting shell of radius equal to a few meters. This is supported at a suitable over the insulating pillars. A long narrow belt of insulating material like, silk, rubber or rayon is wound around two pulleys. one is at the ground level and another one is at the centre of the large spherical conducting shell. The belt is keep moving continuously over the pulleys with the help of a motor. There are two sharply pointed metal combs one is called spray comb and another one is collecting comb.

The positive ions to be accelerated are produced in a discharge tube.

The ion source lies at the head of the tube inside the spherical shell. The other end of the tube carrying the target nucleus is earthed .

The generator is enclosed in a steel chamber filled with nitrogen or methane at high pressure is order to minimize leakage of charge from spherical conductor.

10.Why does a charged glass rod attract a piece of paper?

Ans :- Paper is a dielectric, When a positively charged glass rod is brought near it, atoms of paper get polarised, with centre of negative charge of atoms coming closer to glass rod. Therefore, force of attraction between glass rod and piece of paper becomes greater.

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