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Homework Help: Coulomb's Law

  1. Sep 29, 2007 #1


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    1. The problem statement, all variables and given/known data
    Hi, I would appreciate it if someone could help me with this question, im a n00b here
    Please and thank you
    Okay heres the question.

    The particles have charges Q1 = -Q2 = 100nC and Q3 = -Q4 =200nC
    and distance a = 5.0cm.

    What are the x and y components of the net electostatic force on particle 3 ?

    So basically a diagram is given with the question showing the 4 particles equally seperated from each other at a distance of 5.0cm, making a square. And I have to find the force on particle 3.

    2. Relevant equations

    This is the equation I used.

    F1,3 = (8.854x10^-12) x (Q1 x Q3)/0.0707xR^2 (0.0707 comes from Phyagrous therom)
    H= (square root) 0.05^2 + 0.05^2
    F1,4 = (8.854x10^-12) x (Q1 x Q4)/0.0707xR^2

    Fnet = F1,3 + F1,4

    = F1,3 + (8.854x10^-12) x (Q1 x Q3)/R^2 Cos 45 (45 degrees middle of square)
    + (8.854x10^-12) x (Q1 x Q4)/0.0707xR^2 Sin 45

    3. The attempt at a solution

    I have a feeling this is seriously wrong and have made it more complicated than it is.

    I placed the values into the equation shown and came out with

    = -79.46 x10^3 + 79.46 x10^3

    Heres a site I found afterwards...

    I think it might go a little something like that instead^^

    If you could help that would be great thanks.
    Last edited: Sep 29, 2007
  2. jcsd
  3. Sep 29, 2007 #2

    This is a rather straight-forward problem essentially all you need to do is sum up the forces on particle 3 in the x and y directions.

    To get you started here is the general form for the superposition of forces (summing of force vectors).

    \sum{\vec{F}_{m}} = {\sum_{i=1}^{n}}{\vec{F}_{mn}}


    \sum{\vec{F}_{3_{}}} = {\vec{F}_{31_{}}} + {\vec{F}_{32_{}}}

    Breaking in to the x-components,

    \sum{\vec{F}_{3_{x}}} = {\vec{F}_{31_{x}}} + {\vec{F}_{32_{x}}}

    \sum{\vec{F}_{3_{x}}} = {\left(\frac{{k_{e}}{|q_{3}|}{|q_{1}|}}{{{r}_{31}}^{2}}\right)}{cos{\theta_{31}}}{\hat{i}} + {\left(\frac{{k_{e}}{|q_{3}|}{|q_{2}|}}{{{r}_{32}}^{2}}\right)}{cos{\theta_{32}}}{\hat{i}}

    From here you can use the symmetry of the problem to find your distances and evaluate.


    Last edited: Sep 29, 2007
  4. Sep 29, 2007 #3


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    Thanks, those are the forumulas I used.

    Made a silly few mistakes though
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