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Summing two electric fields

  1. Oct 13, 2011 #1
    The electrical system of typical thundercloud can be represented by a vertical dipole consisting of a charge of +40C at a height of 10km and a charge of -40C vertically below it at a height of 6km. What is the electric field at the ground directly beneath the thundercloud.

    I am simply adding the the field from each point charge at the ground together but get the wrong answer...any suggestions?

    Eground= 40/4*∏*ε0(1/10000^2 - 1/6000^2)

    Answer should be 12.8KV/m upwards
     
  2. jcsd
  3. Oct 13, 2011 #2

    SammyS

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    At the point on the ground, where you are calculating the E field:
    What is the direction of the E field due to a positive charge which is above?

    What is the direction of the E field due to a negative charge which is above?
     
  4. Oct 13, 2011 #3
    +ve into ground, -ve charge field up?
     
  5. Oct 13, 2011 #4

    SammyS

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    Yes. Now look at the signs you have in your calculation.
     
  6. Oct 13, 2011 #5
    A sign error isn't my problem - it's the magnitude I get wrong...
     
  7. Oct 13, 2011 #6

    SammyS

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    Place proper parentheses so the π and ε0 are in the denominator.
     
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