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Homework Help: Tell me whether I solved this electrostatics problem correctly

  1. Dec 31, 2006 #1
    Here's the problem:

    The passage of a thundercloud overhead caused the vertical electric field strength in the atmosphere, measured at the ground, to rise to 0.1 statvolt/cm [E_0].

    (a) How much charge didthe thundercloud contain, in esu per cm^2 of horizontal area?
    (b) Suppose there was enough water in the thundercloud in the form of 1-millimeter-diameter [convert to cm and divide by two, call this r] drops to make 0.25 cm of rainfall [d], and that it was those drops which carried the charge. How large was the electric field strength at the surface of one of those drops?

    Here's what I got:

    (a) E_0 = 2πσ, from which we get σ = E_0/(2π)
    (b) Here's where I am not sure whether I am doing the right thing or not.

    Consider some part of the water at the ground of area A at the top (assuming level water).

    Φ = 4πq and Φ = E_0 * A => q = AE_0/(4π)
    This is the total charge there is.

    On the other hand,
    Adρ = AE_0/(4π) <=> ρ = E_0/(4πd)

    Then the charge which is carried by one drop is:
    Q = (4/3)πρr^3 => Q = (r^3)E_0/(3d)

    Since the field at the surface is Q/R^2, we get:

    E = R*E_0/(3d)
  2. jcsd
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