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Homework Help: Electrostatics question, involving mass?

  1. May 2, 2013 #1
    1. The problem statement, all variables and given/known data
    One electron has a mass of 9.11x10^-31 KG. Find the force 1.0 gram of electrons would exert on another 1.0g of electrons from 1.0 kilometer away

    F = ?
    Melectron = 9.11x10^-31 kg --> (0.01kg of electron = 9.11x10^-33 kg)
    Electron Coloumb = 1.6x10^-19 C (ignoring the negative sign b/c it is a vector)
    R = 1000m

    2. Relevant equations

    Since they are the same mass

    F = kQ^2/R^2

    3. The attempt at a solution

    K = 9x10^9
    R = 1000^2

    What is the value for Q? I am not sure because any value I try to enter is invalid.

    Answer is 2.8x10^20N and I am getting a negative power answer when I try. Can someone steer me in the right direction please?
  2. jcsd
  3. May 2, 2013 #2


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    What is the number of electrons need so that you have a mass of 1 gram of electrons?
  4. May 2, 2013 #3
    1/(9.11x10^-28) = 1.1x10^27 electrons per gram.

    if one electron is 1.6x10^-19c

    (1.1x10^27)(1.6x10^-19) = 1.76x10^8 is one charge of a gram of electrons

    Plugging it in...


    answer = 1.584x10^12 CORRECTED! --> 2.78784x10^20 which is correct

    Still off :|

    Edit: NVM! Correct answer. Stupid Casio fx-300ES calculator got it mixed up somewhere. Hate how I have to keep pressing Shift to get the answer in decimals -.- Thank you!
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