Why Does Potential Become Zero at 2a in Graph D?

[RoboNerd]

Homework Statement

Homework Equations

I know that potential from a point charge equals kq/r and that potential difference equals - (integral of)(E dot dl)

The Attempt at a Solution

I tried to approach this problem using the point charge approach by comparing the relative values of kq/r for each sphere and then guessing which graph works from there.

I understand why there is the same potential from 0 < r < a for the right answer D, but I do not understand why the potential becomes 0 at 2a as we at 2a get a negative potential from the outer sphere that is much larger than that of the potential contributed by the inner sphere as we get infinitely closer and closer to the outer sphere, meaning that the potential should have a vertical asymptote at 2a with it becoming more negative (such as in C).

Could anyone please explain why this is the case with D being the right answer? Thanks in advance!

 

[kuruman]

Is there an electric field in the region outside the two spheres? What does Gauss's Law have to say about this?

 

[RoboNerd]

Gausses law says that the net electric field with radius greater than 2a is greater than 0, so that explains why there is a zero net electric field.

I see now why D is the right answer, thanks very much