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Homework Help: Earth - Electric Field Problem

  1. Sep 26, 2008 #1
    1. The problem statement, all variables and given/known data

    Near the surface of the Earth there is an electric field with a magnitude of about 92 N/C directed vertically down. Assume this field is caused by a net free charge on the Earth.

    (a) What are the magnitude and sign of the charge on the total Earth that produces this field?

    2. Relevant equations

    I tried using mearth g / E but this not appear to yield the correct answer. :confused:

    3. The attempt at a solution

    mg / E
    (5.9736 x 1024 * -10) / 92
    -6.49 x 1023 C

    Thanks for the help! :wink:
  2. jcsd
  3. Sep 26, 2008 #2


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    Science Advisor

    I'm unsure as to why you're using the mass of the earth.

    There is an object (the earth) causing an electric field to be generated with a magnitude 92N/C at the surface (distance known) from the center.

    Perhaps try thinking about the problem as if the earth were merely a particle located at the center.
  4. Sep 26, 2008 #3


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    Homework Helper

    Why are you using mg here? This is for an electric field, not a gravitational field. The E-field is due entirely to the net charge and not the gravitational field. Just apply the formula for E-field due to a charge here, taking into account the radius of the Earth.
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