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Homework Help: Easy homework problem I know, how original.

  1. Sep 7, 2007 #1
    1. The problem statement, all variables and given/known data
    Four point charges are situated at the corners of a square with sides of length a.
    Find the resultant force on the positive charge q. HINT: write an expression for the resultant force, use Ke for Ke, q for q, and a for a! That makes sense, because not much information is given.

    HERE is a picture.

    2. Relevant equations

    F=Ke ( q1q2 / m^2 )

    Ke is the constant 8.99e9 Nm^2/C^2.

    Q1 and Q2 are any two charges of interest. That m^2 basically means the distance between two should be charges squared. So if distance between two charges is 5 cm, then it should be (0.05m)^2.

    3. The attempt at a solution

    F of top left negative charge on positive charge = Ke ( q^2 / a^2 ) ?

    F of top right negative charge on positive charge =Ke ( q^2 / ( (2sqrt(a))^2 ) ?

    F of bottom right negative charge on positive charge = Ke ( q^2 / a^2 ) ?

    I don't know what I'm doing.
    I figured that if you make a 45 45 90 triangle out of that square, the length between the positive charge and the top right charge should be 2sqrt(a). Other than that I do not know.
  2. jcsd
  3. Sep 7, 2007 #2


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    Homework Helper

    That distance should be [tex]\sqrt{a^2+a^2} = a\sqrt{2}[/tex] So the distance square is just 2a^2.

    Draw a picture of the 3 force vectors... You need to add the 3 vectors... but the problem can be simplified when you see the symmetry...
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