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Homework Help: Does the electric field reach a value equal to half its maximum value?

  1. Jun 30, 2013 #1
    A non-conducting sphere of radius R = 7 cm carries a charge Q = 4 mC distributed uniformly throughout its volume. At what distance, measured from the center of the sphere does the electric field reach a value equal to half its maximum value?

    My attempt:

    Emax= q(4mC)/7
    r= 7/2 cm = 3.5

    is this correct? because one of my friend said 9.9... so idk if im right or are both values
  2. jcsd
  3. Jun 30, 2013 #2
    I'm not sure how exactly you determined the maximum of the electric field. It appears that all you did was divide the charge by the radius. This is not a correct formula and doesn't even give you the units of an electric field. My suggestion would be to use Gauss' Law to find the electric field as a function of radius. This will allow you to determine at what radius the maximum occurs (although with a bit of experience it should be fairly obvious). From there you can set the equation for the field equal to half that value and solve for r.

    Just a hint, there should actually be two radii at which the field is equal to half its maximum.
    Last edited: Jun 30, 2013
  4. Jun 30, 2013 #3
    I agree with your friend.
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