Electrostatics (For CBSE Class 12)


Q1. What is the work done in moving a test charge ‘q’ through a distance of 1 cm along the equatorial axis of an electric dipole?     [1]

Hint : on equatorial line V=0

Q2. Why in Millikan’s Oil Drop experiment, the charge measured was always found to be of some discrete value and not any arbitrary value?    [1]

Ans: Because charge is always quantized ie., Q = n x e

Q3. What is meant by electrostatic shielding?   [1]

Ans.: Electric filed inside a cavity is zero.

Q4. Why an electric dipole placed in a uniform electric field does not undergoes acceleration?   [1]

Ans: Because the net force on the dipole is zero (Fnet = 0).

Q5. Why electric field lines
       (i) Can never intersect one another?
       (ii) Cannot for closed loops sometimes?
       (iii) Cannot have break in between?            [1]

Ans : Because
(i) Electric field has an unique direction at any given point
(ii) Monopoles or single isolated charges exist unlike magnetism
(iii) Start from +ve charges and terminate at –ve charges

Q6. Show that at a point where the electric field intensity is zero, electric potential need not be zero.    [2]

Ans: If E = 0; V= constant  (since, E = – dV/dr)

Q7. What is the electric flux through the surface S in Vaccum?     [2]

Ans.: q/ε0

Q8. Write the expression for the electric field, charge density for a uniformly charged thin spherical shell.


Ans.: E = kQ/r^2 ;  σ = Q/4πr^2


This image has an empty alt attribute; its file name is 2-1.png

Write the expression for the electric field in the regions I, II, III shown in the above figure.
Ans: E(I) =E(III) = 0 ; E(II) = σ/ε0

Q10. Two free protons are separated by a distance of 1 A0. if they are released, what is the kinetic energy of each proton when at infinite separation.   [2]

Hint : at inifinte distance, K.E. = e2/4πε0r

Q11. How does the electric flux, electric field enclosing a given charge vary when the area enclosed by the charge is doubled?    [2]

Ans: (a) Φ = constant (b) E is halved

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