Define electric dipole moment. Write its S.I. unit.
Electric dipole moment is defined as the product of charge and the distance between the charges, and is directed from negative to positive charge.
The SI unit of electric dipole moment is coulomb metre (Cm).
A thin straight infinitely long conducting wire having charge density 
is enclosed by a cylindrical surface of radius r and length l, its axis coinciding with the length of the wire. Find the expression for the electric flux through the surface of the cylinder.
Given, charge density = 
Radius and length of the cylindrical surface are r and l respectively.
Charge enclosed by the cylindrical surface = 
l
Now, using Gauss theorem,
Electric flux, 
An electric dipole is held in a uniform electric field.
(i) Show that the net force acting on it is zero.
(ii) The dipole is aligned parallel to the field. Find the work done in rotating it through the angle of 180°.
i) Dipole moment of dipole is given by,
i) 
Force on –q at A = -qE
Force on +q at B = + qE
So,
Net force = qE – qE = 0
ii)
Work done on dipole is given by, 
The field lines of a negative point charge are as shown in the figure. Does the kinetic energy of a small negative charge increase or decrease in going from B to A? 
The electric field present due to the given point charge will be directed towards the centre. A negative charge always experiences a force in the direction opposite to that of the external electric field present; that is away from the centre. This will cause its motion to retard while moving from B to A. Hence, its kinetic energy will decrease in going from B to A.
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