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Homework Help: Help! The force between copper pennies.

  1. Sep 22, 2008 #1
    1. The problem statement, all variables and given/known data

    We know that, within the limits of measurement, the magnitudes of negative charge on the electron and the positive charge on the proton are equal. Suppose, however, that these magnitudes differed from each other by as little as 0.00010%. With what force would two copper pennies, placed 1.0m apart, then repel each other? What do you conclude? Assume a penny has a mass of 3.11g and is made of copper (atomic weight = 63.5 g/mol, and atomic number = 29)

    2. Relevant equations

    F = kq^2/r^2 and 1mol = 6.02*10^23

    3. The attempt at a solution

    There are many given conditions, but it seems some of them are non-use. I absolutely got confused. Shall I find the number of the atomic first? Like 3.11*6.02*10^23/63.5 but what is the difference between this and 29? And how to use 0.00010%? =.= Help Please~
  2. jcsd
  3. Sep 22, 2008 #2


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    Hi ZoroP! :smile:

    The atomic number is the number of protons per molecule. In this case, it will also be the number of electrons per molecule. :smile:
  4. Sep 22, 2008 #3
    Hello ZoroP,

    I think the crux of this problem is to explain WHY we know that the proton and electron each have the same charge. If they differed, as the problem suggest we pretend, then there would be a measureable force. The 0.00010% is the pretend difference in charge between the electron and the proton (it doesn't matter in this case which one is charged more, so just pick one) and see if you can determine the excess charge in a single proton-electron pair and see if you can extrapolate to an entire penny from there.
  5. Sep 22, 2008 #4
    Hi tiny or tim~ :smile: thanks very much~. But I'm sorry that I may get some misunderstanding with molecule, it is a number unit, right? Like 1 mol C atomic = 6.02×1023 C atomic? Then where does 29 come from and am I useing the right method for calculate the number (3.11*6.02*10^23/63.5)? Thank you~
  6. Sep 22, 2008 #5
    Thx, yay~ well, it seems u like chemistry very much! Thank you for helping my Phys~
    Yeah, i agree with you that the 0.00010% is the point of the question, but i sitll dont know how to use it and what its meanning is here. Maybe it will show the question "what do you conclude?" And your measureable force is an interesting idea/word~:smile:, so by your method the kq^2/r^2 should be changed like k(1+0.00010%)q^2/r^2?
  7. Sep 22, 2008 #6


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    That is a good start. What does that number represent?
  8. Sep 22, 2008 #7
    I need to know that how many atomic there are in the penny, so I use the mass of the penny divides the mass of an atomic. Thanks.
  9. Sep 22, 2008 #8


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    Next: how many protons and electrons are in a penny, if there are that many (expression shown in post #6) atoms in a penny?

    edit: signing off for the night ... good luck.
  10. Sep 22, 2008 #9
    I think i get the method, thanks a lot.
  11. Sep 23, 2008 #10
    Hello again,
    I think I should clarify what I was trying to say earlier with an example. Let's say an electron has 0.00010% more charge than a proton e.g. a proton has a charge of +1q and the electron has a charge of -1.0000010q. If I take a proton and an electron together (perhaps bind them together in an atom) the NET charge (e.g. sum of charges of electron and protons) is
    [tex]1q - 1.0000010q = 0.0000010q[/tex].
    That is, each proton-electron pair has an excesss charge of 0.0000010q. So for the penny problem, you must sum the total number of proton-electron pairs in a penny and that will give you the charge on each penny. And then you can use the force formula to calculate the force.
    Hope this helps more.
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