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Homework Help: Help with an electrostatic problem

  1. Sep 4, 2007 #1
    Hi I cannot get this problem for the life of me and I have no idea why

    Four charges q1 = q3 = -q and q2 = q4 = +q, where q = 5 µC, are fixed at the corners of a square with sides a = 1.3 m

    Calculate the x- and y-components of the net electric field at the midpoint M of the bottom side of the square.

    Ok so I know that the y component is zero. So I need the x component. First I did it from q2 to q3, where i have a vector going from q2 to the midpoint, and one going from the midpoint to q3. The magnitude of the electric field of the vector from q2 to the midpoint should be kq2/(a^2/4), which i get to be about 1.1 x 10^5. Then I need to find it from the upper midpoint to q1 (the x component of the vector from q1 to the lower midpoint. This should be equal to kq1/(a^2+a^2/4)*cos(theta). I measure the theta to be 63.4 degrees using tan(theta)=a/(a/2). so i get a magnitude of about -9.5 x 10^3 here. Then i multiply each magnitude by 2 and subtract (2*-9.5 x 10^3) from (2*1.1 x 10^5) to get a total magnitude of 2.4 x 10^4 in the x direction, but this is not correct. Please tell me what I'm doing wrong here. thanks

    Attached Files:

  2. jcsd
  3. Sep 5, 2007 #2

    Doc Al

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    Staff: Mentor

    Redo this final calculation.
  4. Sep 5, 2007 #3
    ok I typed in 2.4x10^4 instead of 2.4x10^5. 2.4x10^5 is not right though. is there anything in the rest of my work that you can see is wrong? thanks
  5. Sep 5, 2007 #4


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    Homework Helper

    I don't think you're carrying enough decimal places (the 1.1*10^5, I have 1.06*10^5)... do you already know the answer?
  6. Sep 5, 2007 #5
    no i don't know the answer, i have to type them in online, and then it tells me if i'm right or wrong. the thing is, that i have done the same calculations with the entire number, not in scientific form, i pretty much put them that way in this thread for the sake of saving time writing long numbers. i still got them wrong. i must be doing something wrong somewhere in my math, but i can't see where.
  7. Sep 5, 2007 #6


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    Homework Helper

    So what would your final answer be if you carried more decimal places in your calculations?

    Because 2.4*10^5 is wrong, even if you just round the final answer to 2 significant figures...
  8. Sep 5, 2007 #7
    ok i got it right. i wasn't taking the absolute value of the charges, which was causing me to get the wrong answer.
  9. Sep 6, 2007 #8
    Well what was the answer
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