What potential reference for a cylinder inside a cylinder

1. Feb 17, 2015

Eric Peraza

The question is
A long metal cylinder with radius a is supported on an insulating stand on the axis of a long, hollow, metal tube with radius b. The positive charge per unit length on the inner cylinder is λ, and there is an equal negative charge per unit length on the outer cylinder.

Calculate the potential V(r) for r<a. (Hint: The net potential is the sum of the potentials due to the individual conductors.) Take V=0 at r=b.

i know how to do the problem, the only question i have is what do i use for the reference in the integral for change in voltage? i know how to do spheres and all and calculating e-fields is easy, but how do i know when to use infinity as a reference and when to use something else as a reference? and what would that something else be?

2. Relevant equations
Even though it's inside the first cylinder do i set the reference at infinity?

2. Feb 17, 2015

Dick

They tell you where to set the reference. They say "V=0 at r=b". Setting the reference at infinity is not very useful here since an infinite cylinder (which is what they mean when they say "long") has infinite potential at infinity.

3. Feb 17, 2015

Eric Peraza

ohhhh i feel stupid i did not see that part...thank you for the explanation though and pointing that out to me!