-1
$\begingroup$

This is a common statement that the force between two charges changes when the intervening medium is changed however the gravitational force remains the same. But I have some problem with this.

When two charges are kept in a medium, their electric field polarises the medium and this in turn affects the strength of the electric field and hence the force on the other charge changes. But isn't this new force a result of the intervening particles of the medium ? I think this new force is the net resultant of all the forces due to those polarised particles.

"So does the force between them change ? Or is it the net force on each charge which has changed ?"

And also when two bodies are kept in a medium , the particles of the medium also exert gravitational force on the two bodies , so shouldn't we say that the gravitational force between them has changed ? (And hence define something like gravitational permittivity !!)

Improve this question
$\endgroup$
7
  • 1
    $\begingroup$ Since the net force is the only thing that's observable, where's the difference between saying "the force has changed" and "the force hasn't changed but now there's a different force adding to/subtracting from it"? $\endgroup$
    – ACuriousMind ♦
    Commented Mar 16, 2022 at 16:40
  • $\begingroup$ @ACuriousMind so shouldn't the universal law of gravitation have similar term because medium will affect the observable force here also !?? $\endgroup$
    – Ankit
    Commented Mar 16, 2022 at 17:00
  • $\begingroup$ How do you think it would? As long as there's equal amounts/density of the medium in all direction, why do you think the medium is at all relevant in terms of gravitation? $\endgroup$
    – ACuriousMind ♦
    Commented Mar 16, 2022 at 17:04
  • $\begingroup$ @ACuriousMind but there isn't uniform medium all the time .. having equal amount in all direction is I think a special case only.. $\endgroup$
    – Ankit
    Commented Mar 16, 2022 at 17:06
  • $\begingroup$ @Ankit a medium cannot be polarised by gravity because there are no negative masses i.e. no negative gravitational charges. Also gravity cannot be shielded by a medium. So it is fundamentally different from the situation with electromagnetism. $\endgroup$
    – John Rennie
    Commented Mar 16, 2022 at 17:06

1 Answer 1

Reset to default
1
$\begingroup$

When two charges are kept in a medium, their electric field polarises the medium and this in turn affects the strength of the electric field and hence the force on the other charge changes.

Yes.

But isn't this new force a result of the intervening particles of the medium ? I think this new force is the net resultant of all the forces due to those polarised particles.

Yes.

So does the force between them change ? Or is it the net force on each charge which has changed ?

It is the net force on each charge that changes, due to the polarization of the matter in between.

And also when two bodies are kept in a medium , the particles of the medium also exert gravitational force on the two bodies , so shouldn't we say that the gravitational force between them has changed ?

The matter between two particles contributes to the net gravitational force between those particles. However, the situation is somewhat different from that of electric forces.

Ordinary, electrically neutral, matter is neutral because it has a balance of positive and negative charges. However, ordinary matter is not "gravitationally neutral". There is no balance in ordinary matter between "positive" and "negative" mass. Thus, the gravitational effect of matter upon a test mass does not depend upon any "gravitational polarization", while the electrical effect of electrically neutral matter upon a test charge does depend upon electrical polarization.

So, when we place an mass between two particles, the gravitational force is increased, but we would expect that merely because the matter has mass.

When we place an electrically neutral dielectric or a conductor between two charged particles, the electric force is increased, but it seems to need explaining, because the dielectric or conductor is electrically neutral. And, we explain the effect by polarization.

Improve this answer
$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.