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Homework Help: Estimate the potential of the cloud relative to the ground

  1. Apr 30, 2005 #1
    Hello! here is one question which I still donno how to solve:

    A thunder cloud with a large flat square base measuring 3000m x 3000m passes over an area on level ground. Assuming the cloud carries a total charge of 40C which is spread evenly over its base and is at a height of 0.5 km above the ground, estimate the potential of the cloud relative to the ground.

    My solution: by V = 1/(4*pi*epsilon) Q/r^2
    Potential of the cloud relative to the ground = (9x10^9) 40/ 0.5x10^3 = 7.2 x10^8 V

    It is wrong...please help me anyway. Thanks in advance. :smile:
  2. jcsd
  3. Apr 30, 2005 #2

    Doc Al

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    Staff: Mentor

    That equation is for the electric field (field, not potential) of a point charge. Treat this as a plane of charge. What's the field from a plane of charge? (Don't neglect the induced charge on the earth.)

    Once you know the field, you can calculate the potential difference.
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