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Stuck on electric field question

  1. Aug 29, 2005 #1
    I've been stuck on this relatively simple question for over an hour, can't get the right answer. I think my fundamental understanding of the units involved is flawed, I would really appreciate any kind of clarification as to what I'm doing... okay here goes.
    I'm given that three charges are at the vertices of an equilateral triangle, with side lengths of 0.8 meters. The charges are given in units of (mu)C, which I understand to be microCoulombs, is that correct? Two of the charges, mainly the left and uppermost ones, are positive; they are +2.4 microCoulombs and +6.8 microCoulombs, respectively. The third charge, on the right, is negative, a -4.2 microCoulombs. I am asked to calculate the electric field at the point of the leftmost (+2.4) charge, due to the other two charges. The answer is expected in vector format, and in kN/C (killoNewtons per Coulomb?).
    This shouldn't be hard, using the equation for electric field,
    Ke*(Q/r^2). What I do every time is:

    convert the microCoulombs to Coulombs by dividing by 1.6 * 10^6.
    So I have 6.8 * 10 ^ -6 Couloumbs, -4.9 * 10 ^ - 6 Couloumbs, and the third point doesn't matter, since that's where I'm calculating electric field at. Basically then [ Ke * (6.8 * 10 ^ -6 C) / (0.8 m) ^ 2 ], that's the magnitude, and I divide it into x and y components by multiplying by cos(60) and sin(60) respectively. x should be negative (it extends outward from positive charge to the left on the x-axis) and y is also negative (it extends outward from positive charge down on the y-axis).
    For the other one, it's [ Ke * (-4.9 * 10 ^ -6 C) / (0.8 m)^2]. It lays directly horizontal to the point in question, so no y-component, it's all x-component and it's positive since the negative charge produces a field directed toward itself.
    Add components, which should produce and answer in C/N, then divide by 1000 to get kN/C.
    Seems simple, what am I doing wrong? Sorry if the description is too in-depth or not enough so. Please help me someone!!!!!!!!!
  2. jcsd
  3. Aug 29, 2005 #2

    Doc Al

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    Staff: Mentor

    "micro" just means 1 millionth. So 1 microC = 10^-6 C.
  4. Aug 29, 2005 #3


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    I hope you're using "Ke" as a symbol for Coulomb's constant
    "Ke = 9E9 [Nm^2/C^2]" , NOT as "k*e" with e=1.6E-19 [C]
  5. Aug 30, 2005 #4


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    evangelic04, See attachments

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    Last edited: Nov 29, 2006
  6. Aug 31, 2005 #5
    thank you all

    Forgot to come back and thank you all! How rude of me!
    Of course the division by 10^-6 was a bonehead error on my part, once I got past that I almost got the problem...looking at andrevdh's generous response I could see that I was getting the y-component correct everytime, it was my x-component that was wrong...I was using sin and cos on the overall magnitude, but I don't think I was fully grasping the math behind what I was trying to figure out. Anyways thanks everyone!
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