Does the coulomb force that one charge exerts on another charge changes, if other charge is brought nearly?

1

(a)

Yes, it changes as the distance becomes less

The test charge used to measure electric field at a point should be vanishingly small.Why?

1

(a)

In case, test charges is not vanishingly small, it will produces its own electric field and the measured value of electric field will be different from the actual value of an electric field at that point.

Why electrostatic field be normal to the surface at every point of a charged conductor?

1

(a)

For the condition of electrostatics,the electric field lines must be normal to the surface of the conductor,otherwise there would be a non-zero component of electric field along the surface of conductor and charges could not be at rest.

An electrostatic field line is continuous curve, i.e. a field line cannot have sudden breaks. Why not?

1

(a)

An electrostatic field line cannot be a discontinuous curve,i.e it cannot have breaks.If it has breaks,then it will indicate absence of electric field at the break points.But the electric field vanishes only at infinity.

Force experienced by an electron in an electric field is F newton. What will be the force experienced by a proton in the same field? take mass of a proton is 1836 times the mass of an electron.

1

(a)

The proton will experience the same force,F newton but in the oppposite direction.