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Homework Help: Electric and Gravitational Forces & Fields

  1. Oct 20, 2008 #1
    I'm getting pretty frustrated with this equation, and I'm not sure what I'm really doing. I've gone to the teacher for help, but as soon as I left school and came home, I've drawn a blank about what he told me..

    "At a point in Earth's atmosphere, the electric field is 150 N/C downward and the gravitational field is 9.80 N/kg downward.

    a) Determine the electric force on a proton (p+) placed at this point.

    b) Determine the gravitational force on the proton at this point. The proton has a mass of 1.67 x 10-27 kg."

    For a), I'm pretty sure you use the formula:
    E = Fe/q, and then manipulate it to find Fe, right?

    I tried doing this:
    Fe = q * E
    Fe = 9.80 N/kg * 150 N/C

    I get Fe = 1470 N, when in the back of the text book it says I should get 2.40 x 10-17 N
    So I'm not getting the answer that it says in the text book. I was wondering if I did something wrong with 'q', because isn't that the charge, which should be in coulombs? Am I supposed to use the gravitational field and the mass of the proton to find the charge?

    I also tried this:
    Fe = (1.67 x 10-27 * 9.80 N/kg) * 150 N/C
    and I got Fe = 2.50 x 10-24 N
    Still the wrong answer.
    Maybe I'm entering it wrong into my calculator?

    As for b), I was told to use:
    g = Fg/m, then manipulate it to this:
    Fg = mg
    Fg = 1.67 x 10-27 * 9.80 N/kg

    Fg = 1.64 x 10-26 N
    which is right, so I don't really know why I typed this all down! Haha
    Last edited: Oct 20, 2008
  2. jcsd
  3. Oct 20, 2008 #2


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    Homework Helper

    q here is charge. What is the charge of a proton? Don't worry about "g" for this part of the question. It doesn't have anything to do with the electric force.
  4. Oct 20, 2008 #3
    Oh, yeah. So instead of 1.67 x 10-27 N/kg, I use 1.60 x 10-19 C.

    Fe = 1.60 x 10-19 C * 150 N/C
    Fe = 2.40 x 10-17 N

    Thanks! I get so frustrated, I seem to just miss one little thing that screws up the answer! I can't believe I did that..
  5. Oct 20, 2008 #4


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    Homework Helper

    Yeah, it happens!
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