Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Two point charges a distance $d$ apart in free space exert a force of $1.4\times10^{-4}N$. When the free space is replaced by a homogeneous dielectric medium, the force becomes $0.9\times10^{-4}N$. What is the dielectric constant $\epsilon_r$ of the medium?

I have been staring at this problem for awhile. I can only think of using $E={F/Q}$. But the problem is this only applies to free space and I do not know $Q$. So I am not sure where to start to compare the free space and medium.

share|cite|improve this question
up vote 0 down vote accepted

The charges are still the same. Only the dielectric medium is varied. So, it's easier than you think...

Both the forces differ only by the relative permittivity $\epsilon_r$.

Hence the relation, $$\frac{F}{F_m}=\frac{\epsilon}{\epsilon_0}=\epsilon_r$$

share|cite|improve this answer
Oh wow! Thanks, as always I think to hard about it. – user22740 Apr 5 '13 at 1:56

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

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