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Homework Help: Electric Point Charge and Directional Force

  1. Feb 11, 2014 #1
    1. The problem statement, all variables and given/known data
    Three charged particles are placed at the corners of an equilateral triangle of side 1.20 m. The charges are Q1 = 7.4μC , Q2 = -8.0μC , and Q3 = -5.7μC .
    Calculate the magnitude of the net force on each due to the other two.
    (The answer is prompted to be in newtons).
    Calculate the direction of the net force on each due to the other two.
    (counterclockwise from the +x axis, which is shown as running along the bottom of the triangle, with the positive charge on top)
    2. Relevant equations
    sin(θ)=O/H or knowledge that equilateral triangle angles are all 60 degrees

    3. The attempt at a solution
    My first attempt was to use kqQ/r^2 to find the force between Q1 (+) and Q2(-), then add it to the force between Q1 and Q3(-). This would be my answer for the charge on Q1. I did this same addition for the Q2 and Q3 charges as well.
    I first changed μC to C and radius was already in meters, so I left it. I did not change "k".
    For Q1 force I got .63N, for Q2 I got .655N, for Q3 I got .549N.
    This wasn't right.
    Then, I thought, wait a minute, these charges are different signs, maybe some kind of subtraction is required. My second attempt was based on this and instead of adding all the forces I subtracted the charges with the same sign. (for example, Q2 was now F(Q1)-F(Q3), but Q1 was =F(Q2)+F(Q3)). This was also wrong. I think I am messing up the signs.
    As for the direction, I have a notion that it should be vector addition, but without the signs (or direction) of the force from step 1 I'm having trouble answering it.
  2. jcsd
  3. Feb 11, 2014 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You use the signs of the charges to determine the direction of the forces. Like charges repel; opposite charges attract. Once you have the proper force vectors, you add them.
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