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The electric force acting between two point charges kept at a certain distance in vacuum is F. If the same two charges are kept at the same distance in a medium of dielectric constant K, the electric force between them is _______.

Asked in GSEB Board July 2016 · Coulomb's law and superposition

• (1) KF
• (2) F
• (3) K²F
• (4) F/K

Step-by-step solution

Why the other options are wrong

• (1) Multiplies by K instead of dividing. A dielectric medium weakens the force between the charges, and K>1, so the force must come out smaller than F, not larger.
• (2) Assumes the medium changes nothing. The permittivity of the medium, Kε₀, takes the place of ε₀ in Coulomb's law, so the force drops by the factor K.
• (3) No single slip gives K²F. The constant K enters Coulomb's law once, through ε=Kε₀ in the denominator, so the force becomes F/K.

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