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

  1. Feb 24, 2010 #1
    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?

    Actual Answer

    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. jcsd
  3. Feb 24, 2010 #2


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    Homework Helper
    Gold Member

    '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.
  4. Feb 24, 2010 #3
    Thank you, your help is appreciated.
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