Finding potential difference in a very long hollow cylinder

1. Mar 10, 2015

aldofbg

A very long hollow cylinder has inner radius A and outer radius B. It has a known unit charge per unit volume p. Find the difference in the electric potential a point on the axis and a point a distance A+D assuming A+D<B
The diagram is in the url. It is #3

http://people.physics.tamu.edu/etanya/P208/exam_2_2014.pdf

I used Gauss law to find the Electric field at a point 'R'. I got pV/E0 then I integrated this from 0 to A+D and I got the answer p[A+D]^2/E0 though this isn't the answer. Could someone explain what I did wrong?
The correct answer is -p/2E0 (4A^2/2 - A^2/2 - A^2ln2)

2. Mar 10, 2015