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Homework Help: Counting electrons: current and charge

  1. Feb 8, 2010 #1
    Hi everyone. I thought I understood this problem, but now I'm unsure. Everything is worked out step by step with the answers, but when I try to duplicate it, I get something different. Can anyone shed some light on it?

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

    Suppose there is a steady current of 0.50 A in a flashlight bulb lasting for 2.0 min. How much charge passes through the bulb during this time? How many electrons does this represent?

    The current and time elapsed are given; therefore, the definition of current allows the calculation of the charge q. Since each electron carries a charge of magnitude 1.6x10-19 C, q can be converted into the number of electrons.

    2. Relevant equations


    I = 0.50 A
    t = 2.0 min = 1.2x102 s

    I=q/t, so the magnitude of the charge is

    q = It = (0.50A)(1.2x102 s) = (0.50C/s)(1.2x102 s) = 60 C

    Solving for number of electrons from q = ne

    n = q/e = 60 C/1.6x10-19 C/electrons = 3.8x1020 electrons

    3. The attempt at a solution

    I understand the first part of the problem (everything to get the 60 C). I've done several of these problems before and this is the first one that's tripped me up. I'm using a calculator to do this and when I enter that into my computer, I come up with 3.75x10-18.

    I know that 1.6x10-19 C is the electric charge of an electron (although, earlier in the book it states that it's -1.6x10-19 C and that protons have an electric charge of 1.6.x10-19 C). The only way I can get the answer they came up with is if I use 1.6x1019 and that doesn't make any sense to me. What am I missing?
  2. jcsd
  3. Feb 8, 2010 #2
    Your answers look right to me, 60C and 3.8x10^20 electrons. You my not be operating the calculator right? Do simple math problems that you know the answer to to figure out the calculator or go online and get the manual for your calculator?

    Hope this helps.
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