# Voltmeter probe Problem

1. Sep 26, 2008

### bjonesp

The problem statement, all variables and given/known data
A very long insulating cylinder of charge of radius 2.70cm carries a uniform linear density of 16.0 nC/m

Question: If you put one probe of a voltmeter at the surface, how far from the surface must the other probe be placed so that the voltmeter reads 195 ?

Relevant equations: The relevant equation for this problem is V=(lamda/2*pi*epsilon nought)*ln(R/r)

The attempt at a solution: For this problem I am trying to find little r in the above equation. 'Lambda' is given in the problem statement as (16*10^-9). To find little r, I first got the ln() function by itself by dividing each side of the equation by 'lambda/...' To save time I have denoted that function by A. So V/A=ln(R/r). I then took the exponential of each side to get ride of the ln() and I am left with r=R/[e^(V/A)]. I do not understand why this is not correct could someone please help me out. Thank you very much

2. Sep 26, 2008

### Astronuc

Staff Emeritus
Be careful about the reference point. If R and r are measured from the central axis of the cylinder, and one is being asked about the distance x from the surface, which is at R, then what is the relationship between r and x?