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How to find the number of excess electrons?

  1. Apr 28, 2008 #1
    1. The problem statement, all variables and given/known data

    How many excess electrons are on a ball with a charge of -4.00*10^-17 C?

    2. Relevant equations
    I know that the charge per electron is 1.60 *10^-19C.


    3. The attempt at a solution

    My textbook does not explain how to do this, but I thought I would divide-->4.00*10^-17 C * 1 electron/-1.60*10^-19. I got -2.5*10^-36. The answer from the book is 2.5*10^2 electrons.
    I did some messing around and did this-->4.00*10^-17C*1 e/1.60*10^19C=2.5*10^2 electrons.
    What's the correct way to do this problem?
    Thanks in advance.
     
  2. jcsd
  3. Apr 28, 2008 #2

    dynamicsolo

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    Homework Helper

    Your set-up is right and the units will check. But how do you divide

    4.0 x 10^-17 / 1.6 x 10^-19 ?

    What is 1 / 1.6 x 10^-19 ?

    (In fact, your check is also incorrect. You may want to review how division works with powers of ten and what negative exponents mean. 10^-17 / 10^-19 = 100 ; 10^-17 / 10^19 = 10^-36 .)
     
  4. Apr 28, 2008 #3
    uhh, i'm confused.............????
     
  5. Apr 28, 2008 #4

    dynamicsolo

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    Homework Helper

    You're dividing by 1.6 x .0000000000000000001 . So 4 / 1.6 is 2.5 , but what is

    10^-17 / 10^-19 =

    0.00000000000000001 / 0.0000000000000000001 ?
     
  6. Apr 28, 2008 #5
    oh i see!
    10^-17/10^-19=100
    so therefore 2.5*10^2.
    thanks so much! :D
     
  7. Apr 28, 2008 #6
    Charge is quantized so the excess charge has to be a multiple of e (elementary charge)

    q=ne, where n is the number of electrons.

    That's why it works I believe since your textbook didn't explain it.
     
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