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Homework Help: Neutral electrons.

  1. Feb 12, 2007 #1
    Last question out of all the ones ive done has got me 100% stumped, went and asked the tutor and he confessed he has not coverd this at all but desided to leave it into the practice questions, so wondering if anyone here might be able to explain how this works.

    1. The problem statement, all variables and given/known data

    An aluminium nail has an excess charge of +3.2 ┬ÁC. How many electrons must be added to the nail to make it electrically neutral?

    2. Relevant equations

    as far as I know an electron is 1.6x10^-19

    the tutor scribbled down quick since he was heading home what I beleave was ment to be the equation to equal the answer.


    3. The attempt at a solution

    Im not even sure how he came to that conclusion, more than an aswer would anyone be able to explain how he got to where he is, or give me something to work on, to work it out.
  2. jcsd
  3. Feb 12, 2007 #2


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    Staff: Mentor

    You can use units to be sure you get the right answer.

    3.2 [uC] / 1.6x10^-19 [C/e-] = ?

    Be sure to remember that 1uC = 10-6C
  4. Feb 12, 2007 #3
    Thanks that helped some, and ive been trying to backwards work the equation. Based on the test questions the answer is 2.0x10^13

    The only way I can obtain that actuall answer is to make the equation.

    3.2x10^-6 / 1.6x10^19 = 2.0x10^13

    Is this pure chance or is there some step im not quite geting that allows for the units to change from positive to negative?
  5. Feb 12, 2007 #4


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    Staff: Mentor

    Not random at all. See how the units help you to figure out what to do?

    (total charge) / (charge per electron) = (number of electrons)
  6. Feb 12, 2007 #5
    not quite, I understand that if its a postive charge on the nail ill need to add a negative charge to counter it.

    and that the total charge devided by the charge per electron would show the number of electrons required, all I cant get is why 1.6x10^-19 needs to become 1.6x10^19 in order to give the corect answer.
  7. Feb 12, 2007 #6


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    Staff: Mentor

    I think you are using your calculator incorrectly. There is no way that 10^-6/10^19 = 10^13.

    Maybe try it again, and be careful with the exponent signs on the calculator?
  8. Feb 12, 2007 #7
    Perhaps there is an issue with my calculator then, because it keeps giving me 2x10^13 every time.

    Casio: fx-85ES.

    entered in as (3.2x10^-6) Devide (1.6x10^19) = 2x10^13

    Edit. Its done this to me once before, not sure why but had to remove batterys and replace them then just did it, and now calculation is working as (3.2x10^-6) Devide (1.6x10^-19) = 2x10^13

    The odd thing is tho, settings are as before. maby i got a duff calc.

    Thanks for all your help.
    Last edited: Feb 12, 2007
  9. Feb 12, 2007 #8


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    Staff: Mentor

    Then just do it by hand instead. 3.2/1.6 = 2.0 and 10^-6/10-19 = 10^(-6+19) = ?

    I have no idea what is going on with your calculator or the entry sequence. Try some simple stuff like 10^2/10^4 and 10^-2/10^-4 to figure out where the problem is.
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