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Homework Help: Got challenged by teacher, Coulomb's Law.

  1. Feb 17, 2009 #1
    This may not be a challenge for some of you, but it is hard for me as I just got introduced to the topic, your help will be appreciated.
    It is about Electric Force. Coulomb's Law.

    1. The problem statement, all variables and given/known data
    Charges of -1 micro coulomb and -4 microcoulomb are placed at opposite ends of a meter ruler. Where on the ruler could...
    (a) a free electron be placed and be in electrostatic equilibrium?
    (b) a free proton be placed and be in electrostatic equilibrium?

    2. Relevant equations

    F= Force (Newtons)
    Q= Coulombs charges
    R= distance between the charges
    K= botlzmann constant

    3. The attempt at a solution

    I tried getting the answer by doing math without involving phyisics, like this:

    If 4 microcolombs is 100%
    then 1 microcoulomb might be 25%
    and I have a 1 meter long ruler,
    then the new coulomb would be placed 25% to the right from the force that has 1 microcoulomb. That would be 25 cms to the right from the -1 microcoulomb

    I reach this conclusion by math logic, but I believe it's wrong and my teacher told me it really is wrong, my method is ambiguous so..

    I hope someone helps me get me the correct answer.
  2. jcsd
  3. Feb 17, 2009 #2


    User Avatar
    Homework Helper

    Welcome to PF.

    You would be better served to set it up algebraically, where

    X = distance to one charge and 1 - X = distance to the other.

    Since you want the forces to be equal at that point ... then set them equal and solve for X.
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