CBSE Class 12 Physics Electrostatics Frequently Asked Questions

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FREQUENTLY ASKED QUESTIONS

ELECTROSTATICS

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) Define electric field intensity. Write its S.I unit. Write the magnitude and direction of electric field intensity due to an electric dipole of length2a at the midpoint of the line joining the two charges.

4) Sketch the electric lines of force due to point charges q > 0, q < 0 and for uniform field

5) Define electric flux. Give its S.I unit and dimensional formula.

6) 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?

7) Depict the equipotential surfaces for a system of two identical positive point charges placed at distance d apart.

8) 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 .

9) Derive an expression for electric field intensity at a point on the axial line and on the equatorial line of an electric pole.

10) Derive an expression for torque acting on an electric dipole in a uniform electric filed.

11) Derive an expression for total work done in rotating an electric dipole through an angle ‘θ’ in uniform electric field.

12) 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 .

13) State Gauss’s Theorem in electrostatics. Using this theorem, find the electric field strength due to an infinite plane sheet of charge.

14) 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.

15) 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?

16) 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.

17) Define dielectric constant in terms of the capacitance of a capacitor.

18) 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.

19) 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’? ; Also show that the energy density of a capacitor is.

20) 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

(i) electric field between the plates

(ii) capacitance, and

(iii) rgy stored in the capacitor

21) Explain the underlying principle of working of a parallel plate capacitor. If two similar plates, each of area ‘A’ having surface charge densities ‘+ s’ & ‘-

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