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Trouble with electric force question

  1. Dec 20, 2013 #1
    1. The problem statement, all variables and given/known data
    I'm having a little trouble with an electric force question I have been working on. It seems simple, but my answer keeps coming out incorrect. Using the graph, deduce the electric force vector on charge q3 by q1/2. In case it is not clear, q3 lies on point (2,2)
    Graph.jpg

    2. Relevant equations

    F=(kq1q2)/r^2*[itex]\hat{r}[/itex]

    3. The attempt at a solution
    Okay, well since electric force vectors simply add to give resultant force I can deduced the individual vectors and sum, as such.

    F(1/3)=(kq1q3)/r^2*[itex]\hat{r}[/itex](1/3)

    and the same applies to the second charge

    F(2/3)=(kq3q2)/r^2*[itex]\hat{r}[/itex](2/3)

    then I deduced r for both. Simply being 2 for q(2/3), and 5 for q(1/3) (from pythagoras)
    Then, the unit vectors, as (2[itex]\hat{i}[/itex]+[itex]\hat{j}[/itex])/5 for q(1/3)
    and [itex]\hat{j}[/itex] for q(2/3)

    Put it together I got
    F(1/3)=((kq1q3)/25)(2[itex]\hat{i}[/itex]+[itex]\hat{j}[/itex])

    F(2/3)=((kq2q3)/4)[itex]\hat{j}[/itex]

    Evaluate and sum, I got, 0.367[itex]\hat{i}[/itex]-0.412[itex]\hat{j}[/itex]

    which of course is not correct. I'm sure there is a mistake in there but can not find it. Would be greatful for some fresh eyes to tell me where.

    Thank you in advance. :)
     
  2. jcsd
  3. Dec 20, 2013 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Hello con31772 and welcome to PF!

    Oops. Check what you got for r for q(1/3).
     
  4. Dec 20, 2013 #3
    Oops. Check what you got for r for q(1/3)

    Haha, thanks very much. Could not find my mistake at all.
     
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