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Homework Help: Electric field at the center of a square homework

  1. Aug 22, 2006 #1

    Electric charge is distributed uniformly along each side of a square. Two adjacent sides have positive charge with total charge + Q on each. Each side of the square has length a.

    Image at bottom.

    Part A:

    Suppose the other two sides have negative charge with total charge - Q on each. What is the x-component of the net electric field at the center of the square? Give your answer in terms of Q, a, and epsilon_0.

    The electric field at the origin would point away from the positive charge and point towards the negative charge. The x-components would both be in the -x direction and equal in magnitude.

    I thought all I had to do was double the equation for an electric field with Q as the charge and (.5a) as the distance.

    My answer was - (2)*(Q) / (Pi* a^(2) * (epsilon_0).

    Because this is a Mastering Physics question and I was close, my feedback was "Your answer is off by a multiplicative factor."

    What did I do wrong?

    Attached Files:

  2. jcsd
  3. Aug 22, 2006 #2
    You can't just double it. Since each side is a charged rod, you will have to find the electric field a distance .5a away from a charged rod. What you did would work if it were a point charge.
  4. Aug 24, 2006 #3
    I exceeded my attempts (five) on the first part. It was the same as the second part, so I ended up getting 3/4s of the points.

    I don't understand why the answer is [tex]-\frac{\sqrt{2} \cdot Q}{\pi \cdot a^2 \cdot \epsilon_0}[/tex].
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