# Homework Help: Electrostatics Problem

1. Jun 16, 2014

### sankalpmittal

1. The problem statement, all variables and given/known data

Two similar point charges q1 and q2 are placed at a distance r apart in the air. A dielectric slab of thickness t(<<r) having dielectric constant K is placed between the charges. Calculate the coulomb force of repulsion between the charges.

Now assume that a slab of thickness half the separation between the charges and the Coulomb's repulsive force is reduced in the ratio 9:4. Calculate K for such a slab.

2. Relevant equations

Coulomb's law :

F=q1q2/4πεr2

where ε=εoK

3. The attempt at a solution

I am not sure. For such a system how should I apply Coulomb's law ? We have a dielectric slab between the system. So how to make that law apply here ?

I just thought that I have to transform the given system to its equivalent vacuum one, but how should I do that ?

2. Jun 17, 2014

### sankalpmittal

Can someone help me please ? I am really clueless. :(

3. Jun 17, 2014

### Tanya Sharma

Hi Sankalp

You have the right idea .

First convert the slab width 't' into effective air separation . Can you do that ?

4. Jun 17, 2014

### sankalpmittal

Thanks..

If we hypothetically bring the two charges with the separation "t", then

q1q2/4πεoKt2 = q1q2/4πεox2

Solving we get x=√(K)t

Then in the actual system, effective separation is d-t+x ? Right ?

BTW, are you an IIT aspirant ?

5. Jun 17, 2014

### Tanya Sharma

Correct.

r-t+x

6. Jun 17, 2014

### sankalpmittal

Ya ya... I mistyped... I always do... d for distance...

Now with this information I solved both the parts completely....

Thanks a lot !!! The answer matches with the key. :)