CBSE Class 12 Physics Electrostatic Solved Examples

CBSE Class 12 Physics Elecctrostatic Solved Examples. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Ex.1 Two small balls having equal charges Q, are suspended from a hook with two insulating threads each of length L. This arrangement is carried in the space, where there is no gravitation. The tension in each string will be-

Note :- In the above problem, the angle between the 'threads will be 1800.' Because in the absence of gravity, the tension in the threads will be only due to columb-repulsion. Therefore the angle between the threads will be 1800.

Ex.2 A copper atom consists of copper nucleus surrounded by 29 electrons. The atomic weight of copper is 63.5 g/mole. Let us now take two pieces of copper each wieghing 10g. Let us transfer one electron from one piece to another for every 1000 atoms in that piece. What will be the coulomb force between the two pieces after the transfer of electron if they are 1cm. apart. [Avogadro number N = 6 × 10²³ /g. mole charge on an electron = –1.6 × 10^–19 coulomb]
Sol. 63.5 g copper contains N = 6 × 10²³ copper atoms. Therefore number of copper atoms in 10g 

copper = 6×10²³/ 63.56 × 10

As only one electron is transferred for every 1000 atoms, therefore the number of electron transferred

Ex.3 A thin conducting ring of radius r has an electric charge +Q. What would be the increase in the tension of wire, if a point charge +q is placed at the centre of the ring ?

Ex.4 (a) Two similar point charges q1 and q2 are placed at a distance 'r' apart in air. If a dielectric slab of thickness 't' (< r) and dielectric constant 'K' is placed between the charges, calculate the coulomb force of repulsion (b). If the thickness of slab covers half the distance, between the charges, the coulomb repulsive force is reduced in the ratio 9 : 4, calculate the dielectric constant of slab.
Sol. (a) The repulsive force between the charges

Ex.5 The bob of a pendulum carries an electric charge of 39.2 × 10^–10 coulomb in an electric field of 20 × 10³ v/m and it is at rest. The angle made by the pendulum with the vertical will be, if the mass of pendulum is 8 × 10^–6 kg and g = 9.8 -
(A) 27º (B) 45º (C) 87º (D) 127º

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