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Coulomb's Law calculation help

  • Thread starter zandbera
  • Start date
1. Homework Statement
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. Homework 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??
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:
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


Staff Emeritus
Science Advisor
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Did you convert the charge units into Coulombs?
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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