If a conducting hollow sphere is under the influence of an electric field, by electrostatic induction positive and negative charges should align as shown in the picture. But then, what happens inside the sphere?

By Gauss's law, the field inside should be zero? However, the positive charges at the inner wall should create a field pointing to the center? That contradiction confuses me, can anybody please explain what is my fault here?



1 Answer 1


The field outside the sphere has negative “divergence,” because more of the arrows point towards the sphere than away from it. The mathematical version of the previous sentence is

$$ \oint \mathrm d\vec A\cdot\vec E = Q_\text{inside} $$


  • $\mathrm d\vec A$ means “a little bit of some surface”
  • $\mathrm d\vec A \cdot \vec E$ means “does the field point toward the inside or the outside”
  • $\oint$ means “add up all the little bits until you have a closed shape”

and $Q_\text{inside}$ is the total electric charge inside the surface you’ve made. (Dear fellow pedants: I’m using units where there’s no annoying constant here.) The surface doesn’t have to be a material surface (but it can be); you can make any imaginary surface with your brain and this relationship should hold.

So, suppose you have arrows pointing away from the inner surface of the conductor into the void. What are they pointing towards? Either some of the arrows meet in the void somewhere (in which case there must be a negative charge at that location), or you have the same number of arrows entering the void as exiting.

Note also that for the field outside to have negative divergence, you should have drawn more $-$ than $+$ symbols. I haven’t counted yours. You are correct that, if you have a surface with net positive charge, the electric field should be different on the one side than on the other side. But that suggests that you just shouldn’t have drawn your inner ring of $+$ charges, or that you should have included your “extra” negative charge in the void someplace.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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