# I Electric field inside a cavity

1. Jan 10, 2017

### jonjacson

It is a simple arrangement. Imagine that we have an empty cubic box. Outside the box the electric potential is infinite. Imagine that you put an electric charge at the geomectric center of the box. What is the field around this charge? In other words, the tool I have to calculate the field generated by a charge is the Coulomb law, and according to this law the only parameters that influence the field are the distance to the source and its charge, but I guess the shape of the box must have an influence too.

2. Jan 10, 2017

### Staff: Mentor

Is the box made out of some material?

Infinite potentials are unphysical, especially if you want to look at fields as their derivatives.

The field will be the superposition of the field by the charge and the fields of box and whatever is outside.

3. Jan 10, 2017

### jonjacson

Well, I want to mean that outside the box neither the particle nor its field can cross it. In other words, the entire universe for that charge is simply the empty box.

4. Jan 11, 2017

### jonjacson

Maybe a picture will help. In the black space neither the charge nor its field could penetrate.

The question is: Does the field around the charge follow the Coulomb law? If we change the shape of the square, Doesn't that have an influence on the field around the charge?

5. Jan 11, 2017

### Staff: Mentor

You'll need some new physics to describe "nothing can penetrate the box" in a consistent way, and your answer will depend on this description.

6. Jan 11, 2017

### jonjacson

:)

So your answer is that it may be different to Coulomb law, interesting.!