COULOMB’S LAW

The force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them, In free space

In material medium

Where ε = ε₀     ε_r        ε_r =  relative permittivity of the medium
in vector form

1.    This is a fundamental law and is based on physical observation
2.    The force is an action - reaction pair.
3.    The direction of force is always along the line joining the two charges.
4.    Electrical force between two point charges is independent of presence or absence of other charges.

Example 1.     A particle ‘A’ having a charge of 2 × 10^-6C and a mass of 100g is fixed at the bottom of a smooth inclined plane of inclination 30°. Where should another particle B, having same charge and mass be placed on the incline so that it may remain in equilibrium ?

Solution :    First of all draw the F.B.D. of the masses.
            For equilibrium ∑F = 0
            N = mg cos30°

Electric Field Intensity    It is the force experienced by a unit positive charge placed at a point in an electric field

                   N/Coul or Volt/m

Example 2.:    Two particles A and B having charges 8 x10^-6 C and –2 x10^-6C respectively are held fixed with a separation of 20 cm. Where should a third charged particle be placed so that it does not experience a net electric force ?

Solution :     As the net electric force on C should be equal to zero, the force due to A and B must be opposite in direction. Hence, the particle should be placed on the line AB. As A and B have charges of opposite signs, C cannot be between A and B
    Also A has larger magnitude of charge than B. Hence, C should be placed closer to B than A. The situation is shown in figure. Suppose BC=x and the charge on C is Q

Superposition principle    
Force experienced by a given charge in the field of a number of point charges is the vector sum of all the forces.

Lines of Force
•    Lines of force originate from positive charges and terminate on negative charges
•    Lines of force originate or terminate perpendicular to the surface
•    Tangent to the lines of force at any point gives the direction of the electric field.
•    Lines of force do not intersect.