# Cylindrical shell- electric potential problem

1. Feb 28, 2012

### giacomh

A very long insulating cylindrical shell of radius 6.40 cm carries charge of linear density 8.90μC/m spread uniformly over its outer surface. What would a voltmeter read if it were connected between the surface of the cylinder and a point 4.00 cmabove the surface?

λ=dq/dr
V=k∫dq/r

V=λk*ln([r+x]/r)
V=(8.9e-6)*9e9*ln(.104/.064)
V=3.89e4

The actual answer is 7.78e4 (twice my answer). I can't figure out whats wrong with my integral to make the answer half of what it should be. Thanks!

2. Feb 29, 2012

### ehild

λ is the linear charge density, λ=dq/dL.
Check also the formulae for V. What is the electric field around a very long cylindrical shell according to Gauss' Law?

ehild