Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Electric current? how many electrons?

  1. Jan 5, 2012 #1
    1. The problem statement, all variables and given/known data
    A net charge of 49 mC passes through the cross-sectional area of a wire in 12.0 s

    (a) What is the current in the wire?
    _4.1E-3_ A

    (b) How many electrons pass the cross-sectional area in 1.0 min?
    ______electrons


    2. Relevant equations

    I=Q/t


    3. The attempt at a solution
    part A was very simple, 49E-3/12 = 4.1E-3 amps

    however part b i do not understand.

    1 electron = 1.6E-19 and 1 coulumb = 6.24E18


    49E-3 x 6.24E18 = 3.06E17 x 60 = 1.8E19 incorrect

    49E-3 x 1.6E-19 = 7.84E-21 x 60 = 4.7E-19 incorrect
     
  2. jcsd
  3. Jan 5, 2012 #2
    You need to realize that 1amp means 1 coulomb per second.
    Current is the rate of flow of electric charge
     
  4. Jan 5, 2012 #3
    if i am sort of understanding what you are saying, you mean to use my answer for part a in order to get my answer for part b?

    well i have tried something like that, but i still must be off track because i still got it wrong

    4.1E-3 x 6.24E18 = 2.56E18 x 60 = 1.5E18 incorrect
     
  5. Jan 5, 2012 #4
    You have the current to be 4.1 x 10^-3A which means 4.1 x 10^-3 coulombs per second.
    The charge on 1 electron is 1.6 x 10^-19 coulombs so how many electrons per second.
    Be careful the question asks for how many in 1MINUTE
     
  6. Jan 5, 2012 #5
    if i have 4.1E-3 c/s then wouldnt i have to multiply that by the one electron?

    4.1E-3 x 1.6E-19 = 6.56E-22

    but like you said since it is one minute and there are 60 seconds in a minute i would multiply 6.56E-22 by 60 in order to get 3.9E-20

    im sorry for all this, but im just really confused haha :)
     
  7. Jan 5, 2012 #6
    To find how many electrons you do not multiply !!!!!!!!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook