# Charge on a glass bead derived from potential difference between 2 points

1. Feb 24, 2010

### samdunhamss

1. The problem statement, all variables and given/known data
A 2.0 mm diameter glass bead is positively charged. The potential difference between a point 2.0 mm from the bead and a point 4.0 mm from the bead is 500V. What is the charge on the bead?

4.2*10^-10

I am unable to understand or even get near this answer. Any help is greatly appreciated.

2. Relevant equations
V = Kq/r

3. The attempt at a solution

V = kq[1/r -1/r']

500 V = 9x10^9 (q) [1/.004-1/.006]

500/((9x10^9)(1/.004-1.006)) = q

q= 2.23x10^-10

Last edited: Feb 24, 2010
2. Feb 24, 2010

### collinsmark

'Just worked it out. It's kind of silly. When the problem says "a point x mm from the bead", it does not mean from the center of the bead. It means from the edge of the bead. So the two points in question are actually 3 mm and 5 mm from the bead's center.

 By the way, in your original attempt you were using the bead's diameter instead of its radius, to get the distance to the center.

3. Feb 24, 2010

### samdunhamss

Thank you, your help is appreciated.