# Constancy of gravitational constant

Hello,
We are being taught about coulomb's law, my teacher said that the constant in the law which we denoted by k, changes with change in medium because of permittivity of the medium.
But why does the constant changes unlike the gravitational constant which does not change with medium although the nature and formula of the force are similar?
according to the super position theory( which I read in fundamentals of physics - Resnick Halliday and walker) that the charged particles interact independently by coulombs force like the masses in gravitational force, then why the medium affects the coulombs force while not the gravitational force?
Sorry for my amateurish language.

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mfb
Mentor
There are positive and negative electric charges - this allows to get effects like polarization you cannot have in gravity. Using the superposition principle, the medium does not affect the force - but the medium gives an additional force.

but Mr mfb aren't we just calculating the force between two particles. what the medium has to do in it? for example 2 particles with charges q are at distance r then the force would surely be 1/(4 π ε) * q^2 /r^2 why the medium can "add up" to the force "between them" only? also the value of permittivity changes with medium if the medium causes addition of extra forces, then why the value of permittivity in vacuum is not zero? since vacuum doesn't have charges which can add up to the force between two different particles.

mfb
Mentor
The reduced force in a medium is an effective description of the total force on an object, coming from the other charge and the medium together.
why the medium can "add up" to the force "between them" only?
The medium can change the total force on the charges.
then why the value of permittivity in vacuum is not zero?
The value is defined to be 1 without any effects from a medium.