Electric field, surface charge density

[Cocoleia]

Homework Statement

Homework Equations

The Attempt at a Solution

I have the full solution, the first part being:

I don't understand how they came up with the expression for Vab. I know usually ΔV=-∫E dl, but I'm not sure how they found their expression. Can someone explain? Thanks.

 

[TSny]

I know usually ΔV=-∫E dl, but I'm not sure how they found their expression.

Yes, you can think of their expression as coming from ΔV=-∫E dl if you express E in terms of D. Use Gauss' law for D to obtain an explicit expression for D in terms of the free charge Q on the inner conductor.