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Homework Help: Physics Question

  1. May 11, 2005 #1
    Hi; Could somebody please help me with this question: A 3.0g copper penny has a postive charge of 38uC. What fraction of its electrons has it lost?
    The method I tried was: (3g/63.55g) x 6.02E23 x A(what i'm solving for) = 38xE-6. Than I took my answer and divided it by 29. However this will not give me the correct answer, and the correct answer is 1/(3.5E9). Can somebody please help me. Thank you so much.
     
  2. jcsd
  3. May 11, 2005 #2

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    Look at your units. You have a dimensionless number on the left side and coulombs on the right side. You need to factor in the charge on an electron.
     
  4. May 11, 2005 #3
    Try a more systematic approach:

    - How many electrons does it take to make up [tex]-38\mbox{\mu C}[/tex] charge? (hint: use the electron charge)

    - How many electrons are in 3.0g copper (when neutral)? (29 per copper atom)
     
  5. May 11, 2005 #4
    Thanks...

    Thanks guys, I figured this one out.
     
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