Test: Coulomb's Law & Its Applications - JEE MCQ

Since the charges are unlike, the force will be attractive. Thus the force directs from 1C to -4C.

The ratio of a medium's permittivity to the permittivity of free space is known as the dielectric constant or relative permittivity. The equation for relative permittivity is κ = ϵ/ϵ0, where ϵ is the permittivity of the medium and ϵ0 is the permittivity of free space. Both permittivity and the permittivity of free space have the same unit, farads per meter (F/m), so the dielectric constant is dimensionless. 

Electrostatics means charges at rest, or relatively at rest so this force can be applicable only when charge produce electric field which is constant which is possible only when charges at rest or relatively at rest.

According to coulomb's law unlike charges always attract each other while like charges always repel each other so between two positive charges repulsion is working.

Since the charge –q is moving in elliptical orbit so to make its motion stable the total angular momentum of the charge is constant since it experience a centripetal force from the charge +Q so it follow the motion as the motion of earth around sun.

Coulombs law states the force between two charged particles is directly proportional to the product of charges and inversely proportional to the square of distance between them.
So if the distance is doubled then the force would decrease 4 times.
So the new force would be 120/4=30 Newton.