Q1 Explain the meaning of the statement "electric charge of a body is quantised." Why can we ignore quantization of electric charge when dealing with large charges? Solution

Q2 State and explain
(i) the law of conservation of charges, with examples,
(ii) the principle of superposition as applied to electrostatic forces on a charge due to a number of charges. Solution

(i) The law of conservation of charges states that,
" The conservation of charge is the principle that the electric charge in an isolated system never changes.In the net electric charges the amount of total positive and negative charge is always remains to conserve in the system"
we can understand it more and clearly with the following examples,
1. In a chemical reaction, the charges in a closed system remain the same it does not change the number of positive and negative charges that remain the same.
2. When we rub the scale with silk cloth then the scale becomes negatively charged and the cloth becomes positively charged.
3. During the radioactive decay, we observe the Proton undergoes decays into a positron and a neutron but no net charge production occurs.

(ii) The principle of superposition as applied to electrostatic forces on a charge due to a number of charges, The resultant force is the vector sum of all the forces on the body .

Q3 State Coulomb's law in electrostatics. Express it in SI units. Mention two similarities and two dissimilarities between electrostatic and gravitational interactions. Solution

According to this law 2 stationary point charges repel or attract each other with the force which is directly proportional to the product of the charges and inversely proportional to the square of the distance between them this force is along the line joining the 2 charges.

Q4 Express Coulomb's law in electrostatics in vector form. What is the importance of the vector form of the law? Solution

Q5 Derive an expression for the intensity of the electric field at a point distant r from a point charge q. Solution

Q6 Obtain an expression for intensity of electric field in end-on position, i.e., the axial position of an electric dipole. Solution

Q7 With the help of a labelled diagram, obtain an. expression for the electric field intensity E at any point P on the equatorial line (broad-side-on position) of an electric dipole. Solution

Q8 Define dipole moment of an electric dipole show mathematically that electric field intensity due to a short dipole at a distance r along its axis is twice the intensity at the same distance along the Equatorial axis Solution

Products of one charge and the distance between the charges is called the magnitude of the electric dipole moment p . Suppose the charges of dipole are -q and +q and the small distance between them is 2l. Then the electric dipole moment p^-> is p^->= q x 2l^->

Q9 An electric dipole is placed in a uniform external electric field E^-> ⃗ . Show that the torque on the dipole is given by τ^-> ⃗ =p^-> ×E^-> , where p is the dipole moment. Solution

Q10 An electric dipole of dipole moment p^-> is placed in a uniform electric field E^-> with its axis inclined → to the field. Write an expression for the torque t experienced by the dipole in vector form. Show diagrammatically how the dipole should be kept in the electric field so that torque acting on it is
(i) maximum,
(ii) zero
Solution

Q11 Draw a labelled diagram showing that electric dipole making an angle θ with a uniform electric field E^->, derive and expression for the torque experienced by the dipole. Solution