Two point charges and are separated by a distance in vacuum
PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Electric Force Field and Potential · Electric Force Field and Potential
Problem
Two point charges q1 and q2 are separated by a distance r in vacuum. They are then immersed in an isotropic homogeneous dielectric medium of relative permittivity εr without changing their separation.
(a) Derive an expression for the ratio of the magnitudes of the electrostatic forces in vacuum and in the medium.
(b) Show that the force in the medium becomes equal in magnitude to the force in vacuum when the separation is reduced to r/εr, keeping the medium unchanged. State clearly any assumptions made.
(c) Two charges q1=+5.0μC and q2=−5.0μC are placed 0.25m apart in kerosene, for which εr=2.0. Calculate the separation at which these charges must be kept in air so that the magnitude of the electrostatic force between them remains the same as in kerosene at 0.25m.