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Lighting strike

  1. Feb 8, 2009 #1
    1. The problem statement, all variables and given/known data
    Lighting strike: Storm clouds build up large negative charges that dwell in a charge center. Suppose a storm cloud has -25 C in a charge center located 10 km above the ground. The negative charge center attracts an equal amount of positive charge that is spread on the ground below the cloud. The charge center and the ground function as a charged capacitor, with a potential difference of 4 x 10^8 V. The large electric field between these two electrodes may ionize the air leading to a conducting path between the cloud and the ground.

    a) What is the magnitude of the electric field between the charge center and the ground?
    b) What is the capacitance of the charge center ground system?
    c) If the cloud transfers all of its charge to the ground via several rapid lighting flashes lasting a total of 1 s what is the average power.

    2. Relevant equations

    3. The attempt at a solution

    I started off by using F=k(q'*q')/r^2

    -25=8.99*10^9*(4 *10^8)(q')/(10,000)^2

    Am I even on the right track?
  2. jcsd
  3. Feb 8, 2009 #2


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    Staff Emeritus
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    Homework Helper

    Uh, no, not really.

    You're plugging in 4 *10^8 for the charge, but that number refers to the potential, not the charge.
    You're plugging in -25 for the force, but that number refers to the charge, not the force.

    Hint for (a): the units of electric field are Volts per meter. You are given a voltage, and a distance in meters.

    (b): there should be an equation defining capacitance in your textbook. It involves voltage and charge, which are given in the problem statement.

    Get those, and then we'll work on (c).
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