### Chapter 1 Electric Charges and Fields

FREQUENTLY ASKED QUESTIONS CBSE Board Exam

UNIT I-ELECTROSTATICS

2 MARKS

1. Force of attraction between two point charges placed at a distance of ‘d’ is ‘F’. What distance apart they are kept in the same medium, so that, the force between them is ‘F/3’?
2. Define electric field intensity. Write its S I unit. Write the magnitude and direction of electric field intensity due electric dipole of length 2a at the midpoint of the line joining the two charges.
3. Sketch the electric lines of force due to point charges q > 0, q < 0 and for uniform field.
4. Define electric flux. Give its S.I unit and dimensional formula.
5. Two point charges 4μc and -2μc are separated by a distance of 1 m in air. At what point on the line joining the charges is the electric potential zero?
6. Depict the equipotential surfaces for a system of two identical positive point charges placed at distance d apart.
7. Deduce the expression for the potential energy of a system of two point charges q1 and q2 brought from infinity to that points r1 and r2.

3 MARKS

1. Derive an expression for electric field intensity at a point on the axial line and on the equatorial line of an electric pole.
2. Derive an expression for torque acting on an electric dipole in a uniform electric filed.
3. Derive an expression for total work done in rotating an electric dipole through an angle ‘θ’ in uniform electric field.
4. A sphere ‘S1’ of radius ‘r1’ encloses a charge ‘Q’. If there is another concentric sphere S2 of the radius r2 (r2 > r1) and there be no additional charges between S1 and S2, find the ratio of electric flux through S1 and S2.
5. State Gauss’s Theorem in electrostatics. Using this theorem, find the electric field strength due to an infinite plane sheet of charge.
6. State Gauss' theorem. Apply this theorem to obtain the expression for the electric field intensity at a point due to an infinitely long, thin, uniformly charged straight wire.
7. Using Gauss’s theorem, show mathematically that for any point outside the shell, the field due to a uniformly charged thin spherical shell is the same as if the entire charge of the shell is concentrated at the centre. Why do you expect the electric field inside the shell to be zero according to this theorem?
8. Deduce an expression for the electric potential due to an electric dipole at any point on its axis. Mention one contrasting feature of electric of a dipole at a point as compared to that due to single charge.
9. Define dielectric constant in terms of the capacitance of a capacitor.

5 MARKS

1. Give the principle and working of a Van de Graff generator. With the help of a labelled diagram, describe its construction and working. How is the leakage of charge minimized from the generator?
2. Briefly explain the principle of a capacitor. Derive an expression for the capacitance of a parallel plate capacitor, whose plates are separated by a dielectric medium.
3. Derive an expression for the energy stored in a parallel plate capacitor with air between the plates. How does the stored energy change if air is replaced by a medium of dielectric constant ‘K’?
4. A parallel-plate capacitor is charged to a potential difference V by a dc source. The capacitor is then disconnected from the source. If the distance between the plates is doubled, state with reason how the following change.
5. Explain the underlying principle of working of a parallel plate capacitor. If two similar plates, each of area ‘A’ having surface charge densities ‘+ σ’ & ‘-σ’ are separated by a distance ‘d’ in air, write expressions for

(i) The electric field at points between the two plates,

(ii) The potential difference between the plates &

(iii) The capacity of the capacitor so formed

6. A parallel plate capacitor is charged by a battery and the battery remains connected, a dielectric slab is inserted in the space

between the plates. Explain what changes if any , occur in the values of

(i) potential difference between the plates

(ii) Electric field between the plates

(iii) Energy stored in the capacitor.