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Homework Help: Coulomb's Law calculation help

  1. Mar 31, 2009 #1
    1. The problem statement, all variables and given/known data
    Nobel laureate Richard Feynman once said that if 2 persons stood at arm's length from each other and each person had 1% more electrons than protons, the force of repulsion between them would be enough to lift a "weight" equal to that of the entire Earth. Carry out an order-of-magnitude calculation to substantiate this assertion.

    2. Relevant equations
    F = k |q1||q2| / r2

    3. The attempt at a solution
    I had no idea to do this so i used Coulombs law with q1 = q1 = -e
    I used -e because it said theres 1% more electrons so if there's 100 protons, there's 101 electroons and 100 of the electrons would cancel out the protons so the net charge is 1 electron (e). but then It didnt said anything about r so i just guessed and used r = 2 m and then again with r = 1.5m but i got an answer that was x 10^-28 and the answer is supposed to be ~1025

    Am I even using the right equation??
  2. jcsd
  3. Mar 31, 2009 #2
    you're using the right equation, but you need to make a better guess on how many electrons each person will have - how much charge to use.

    Try to find how many atoms or protons are in the average human body. Then use a charge equal to 1% of that value of protons. If you can only find atoms, use an educated guess as to how many protons a body has from that number of atoms.

    googled for you:

    "A 70 kg body would have approximately 7*1027 atoms. That is, 7 followed by 27 zeros:


    source: http://education.jlab.org/qa/mathatom_04.html (not too reputable but who cares. see if it gives you an appropriate answer)

    So for you rcharge, use .01 * 7 * 1027 electrons

    also, i measured my arm just now

    with hand: ~85 cm
    without hand: ~55cm

    your distance is way too high

    lol 2m = ~6ft. your arm is as tall as a person? :D
    Last edited: Mar 31, 2009
  4. Mar 31, 2009 #3
    Yeah okay I was thinking about that and that makes sense.

    And yeah I misunderstood the question. I thought they meant each person was an arms length away so the total distance between them was 2 arms lengths so i used 2(0.75) and 2(1.0)

    With that info, I got 6.10 x 10^61. Which is way too high
  5. Mar 31, 2009 #4


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    Did you convert the charge units into Coulombs?
  6. Apr 1, 2009 #5
    Yeah thats where the problem was.

    If I use # of electrons = 1.8 x 10^28, then take 1% of that, then multiply that by 1.602 x 10^-19 to convert to Coulombs, square that value, multiply by k, divide by r^2 (r = .85 m) and i get 1.03 x 10^25

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