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Homework Help: Electric Field strength of sphere

  1. Mar 31, 2010 #1
    Hi everyone

    I just need some help with this question. Its a simple one I'm pretty sure.

    1. The problem statement, all variables and given/known data

    A solid sphere of radius 3 cm is made from a conductor and has a charge of
    + 4 uC placed upon it. What is the electric field strength 1 mm from the
    centre of the sphere?

    2. Relevant equations

    I think this is relevant equation to this.

    E = Q/(4pi eo r^2)

    3. The attempt at a solution

    Q is (4 X 10^-6)/ ( 4 * pi * (8.8 * 10^-12) * (r^2) )

    I am a bit confused about r. Is r just the radius in meter which will be 0.03m or do I have to include 1mm from the center in that case it would be 0.014m?

    Please help.

  2. jcsd
  3. Mar 31, 2010 #2
    well if the charge was placed on the outer surface then
    no electric field inside at any point
    due to electrostatic shielding
    in case it is placed inside, then r is the distance of the straight line joining the charge and the point at which to calculate the electric field
    again that point must lie inside in the case
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