Effect of introducing a dielectric between two charged balls

Question

So let's suppose I have two charged metallic balls kept at a distance from each other and they experience some force.

If I introduce a dielectric between the balls,say plastic, then my book states that the force between the balls would decrease.

But the effect of medium on the field last only in the medium and the balls are outside the plastic(albeit touching it),then shouldn't the force between them be the same no matter what?

If yes, then can we possibly decrease the force between the two balls using a dielectric, i mean you can't put the balls in the dielectric medium,they will always remain in free space?

Answer 1

The effect of the medium on the field does not only exist in the medium!! The charges within the dielectric move to screen the penetrating field. The result is an image charge density which is weak because it’s a dielectric, not a metal. But nonetheless, the dielectric’s charge density extends out into free space and adds with the other fields just like any other charge density (superposition). So the fields at the location of the spheres change, and therefore the force on each sphere changes too.