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Homework Help: What is the net charge

  1. May 22, 2007 #1
    1. The problem statement, all variables and given/known data

    A metal sphere has a charge of +8.0 mirco coulomb. What is the net charge after 6.0x10^13 electrons have been placed on it.

    2. Relevant equations

    N=q/e or Coulombs Law F= k q1q2/r^2 e=1.6x10^-19

    3. The attempt at a solution

    Okay.well i know a mirco coulomb is 10^-6 So i figured you would multiply 8.0^-6 by 6.0^13 and then divide by electrons. But it doesn't seem to get the correct answer. The answer is -1.6 Mirco Coulombs but i don't know how to get that answer. It seems to be just mulitplication or divison, but i tired a 100 different ways, and i can't get -1.6Mirco Coulombs. Can someone help? Thank you!!!!

  2. jcsd
  3. May 22, 2007 #2

    Doc Al

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    How many microCoulombs of charge does this equal? What is the sign of this charge?
  4. May 22, 2007 #3
    if you divide 10^6/6.0x10^13 you get 1.67x10^<b>-8 </b>. It seems like this is so simple. I must be overlooking something. it means i'm still off my a little bit
    Last edited: May 22, 2007
  5. May 22, 2007 #4


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    How much charge do you have with 6.10^13 electrons? Do you know what the charge of a single electron is? The first step to this problem is figuring out what charge you are adding to the sphere.
  6. May 22, 2007 #5
    The charge of a single electron is 1.60x10^-19. I put that in the given of the problem. And it 6.0x10^13. So i would think you would have mutiply 6.0x10^13 electrons and the charge of a single electron and then divide by the sphere. But that still gives me the wrong answer. It gives me 1.2mircoC but then anwser should be -1.6mircoC
  7. May 22, 2007 #6


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    Why would you 'divide by the sphere'?? Don't you want to add?
  8. May 22, 2007 #7


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    Yes, you would multiply the the number of electrons by the charge of an electron to get the total charge of the electrons you're adding. I don't know why you would then try to divide by the charge already on the sphere. You are looking for the net charge. You have postive charge on the sphere, and are adding negative charge. So what charge would be left over?
  9. May 22, 2007 #8
    Ah okay. That makes sense. Thank you <b>So</b> much! One last thing. Why would the other charge be negative? i know that a single electron can either be postive or negative, but i don't understand why, because obviously in this case it the electron would have to be -1.6x10^-6 for it to be -9.6x10^l6 then add the sphere to get the correct answer.
  10. May 22, 2007 #9


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    Electrons always have a negative charge!

    Perhaps you are thinking of the positron, which is the anti-particle to the electron. THAT one has a positive charge.
    Last edited: May 22, 2007
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