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

[samdunhamss]

Homework Statement

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?


Actual Answer
4.2*10^-10

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

Homework Equations

V = Kq/r

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

 

[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.

[Edit] 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.

 

[samdunhamss]

Thank you, your help is appreciated.