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Finding magnitude of electric field strength!

  1. Mar 8, 2009 #1
    1. The problem statement, all variables and given/known data
    Consider three charges arranged as shown.

    What is the magnitude of the electric field
    strength at a point 2.7 cm to the left of the
    middle charge? The value of the Coulomb
    constant is 8.98755 × 109 N · m2/C2 . Answer
    in units of N/C.

    2. Relevant equations
    I'm truly lost. I've been looking over some other places, trying to put things together, but nothing is becoming clear to me... How do the positive and negative charges affect each other? For example if I have 2 equal positive charges, the electric field should be 0 right? How do I determine it if it is 1 positive and 1 negative charge? (Both the same value -1 and +1)

    3. The attempt at a solution
    I was looking at some similar problems.

    E1 = 8.98755e9 x 5.6e-6 / .064^2 = 1.228766602e7. I got that distance by finding the total distance between the point 2.7 cm to the left. So 3.7 + 2.7 = 6.4. That could be very wrong, I'm not sure.
    E2 = 8.98755e9 x 3.2e-6 / .047^2 = 1.301953825e7
    E3 = 8.98755e9 x -1.8e-6 / .017^2 = -5.597782007

    Then i just decided to add all three of those together... As I said I'm really lost, and I'm quite sure none of what I did is correct. Any help?

    Edit : Sorry... I inserted a picture to show the problem.

    Attached Files:

  2. jcsd
  3. Mar 8, 2009 #2


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    Homework Helper

    draw a picture of the field lines

    define a poistive direction along the axis the charges are arranged
    out from positive, in to negative

    work out the distance form each cahrge to the point you want to find

    then based on whether the arrow aligns with you positive dircetion (+ or -), add them all up...
  4. Mar 11, 2009 #3
    I think I may have done my distances correctly? Can anyone confirm that? I guess I was confusing the difference between electric force and electric field. I didn't know that I should be subtracting the negatively charged field, I thought it would be attracting so you add it.

    So I think the only thing possibly wrong are my distances. Assuming I did the distances correctly, and that I add the two positive and subtract the negative fields, I should find the correct answer right?
  5. Mar 11, 2009 #4


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    Homework Helper

    first one looks right 3.7+2.7
    shouldn't the 2nd be 2.7 as defined
    then the last will be 4.4-2.7

    best way to do these is draw the point on your diagram, then draw each line from each charg to the point, then you should be able to see what the distances are

  6. Mar 11, 2009 #5
    Yes this worked out. Thanks for helping me clear this up.
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