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Homework Help: Finding out how many electrons have passed a point in the circuit

  1. Sep 7, 2007 #1
    1. The problem statement, all variables and given/known data

    Calculate the charge which has passed through a point in a circuit when there is a current of 6.3kA for 58 minutes. Calculate the number of electrons passing a point in the circuit.

    How to work out the part in blue.

    2. Relevant equations

    We're told the charge of one electon is "e = -1.6 x 10^-19 C"

    3. The attempt at a solution

    Q = I x t

    t = 58min = 3480 seconds.
    I = 6.3 x 10^3

    3480 x 6.3 x 10^3 = 2.2 x 10^7 (rounded up a bit as my college teacher told me to do).

    But it's the part in blue I get stuck on. I actually already know the answer (also rounded up slightly I believe) is 1.4*10^26 but it's how I get to that answer using the information "e = -1.6 x 10^-19 C" and "Q = 22 x 10^7"

    If someone could show me how to do it, so I can then practice doing so on the other questions I've got and learn it properly. Also, apologies if this is a really moronic question for some of you, browsing around you all seem to be quite intelligent ^^;
  2. jcsd
  3. Sep 7, 2007 #2


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    Take the total charge... 2.2*10^7C... and divide by charge/electron = 1.6*10^-19C/electron. That gives you the number of electrons.
  4. Sep 7, 2007 #3
    2.2*10^7 / 1.6*10^-19 gives me 1.375^-12

    I can see that 1.375 is rounded up to 1.4 but the power is still -12 and the answer the college teacher has given us is 1.4*10^26
    Last edited: Sep 7, 2007
  5. Sep 7, 2007 #4


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    The exponent is 1.6*10^-19. not +19.
  6. Sep 7, 2007 #5
    I made it from negative nineteen to positive nineteen and got the right answer she said we should get.

    So did she make a typo with multiply ten to the power negative nineteen?
    Last edited: Sep 7, 2007
  7. Sep 7, 2007 #6


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    No 1.6*10^-19 is right.

    [tex]\frac{(2.2*10^7)}{(1.6*10^{-19})}[/tex] should give you 1.375*10^26.

    Did you divide correcty?
  8. Sep 7, 2007 #7
    I'm either incredibly stupid or my calculator is playing mind games.

    I'm inclined to believe it's the first one.

    Thanks for all your help learningphysics. I've finally got it.
  9. Sep 7, 2007 #8


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    :wink: No prob. happens to all of us.
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