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Homework Help: Need help asap with this electric field problem please

  1. Apr 14, 2009 #1
    In the air over a particular region at an altitude of 500 m above the ground, the electric field is 150 N/C directed downward. At 600 m above the ground, the electric field is 80 N/C downward. What is the average volume charge density in the layer of air between these two elevations?

    the answer is supposed to be in C/m^3. i have looked at this question for a long time now and can not find a way to solve it. i think this problem would be easier if the volume of the slab of air of 100m height can be calculated and then use Gauss' law. but other then that i can not think of any other way to solve it. can anyone help me?
  2. jcsd
  3. Apr 14, 2009 #2


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    Homework Helper

    Well that is what you want to do isn't it?

    Apply Gauss Law. Say you have a 100 x 1 cubic meter closed surface with field lines only going through 2 surfaces - top and bottom.

    Isn't the net flux the charge inside, and the charge per cubic meter would be 100th of that?
  4. Apr 14, 2009 #3
    so should i just kind of make up a volume since it apparently isn't important?
  5. Apr 14, 2009 #4


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    It does make a difference.

    The only information you are given are for the horizontal surfaces at 500m and 600m. You can ignore the vertical surfaces because the dot product of the E field and the vertical is 0.

    You have 150/m2 at the bottom and 80/m2 at the top.
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