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Magnitude of electric field using point charges

  1. Mar 7, 2008 #1
    [SOLVED] Magnitude of electric field using point charges

    1. The problem statement, all variables and given/known data
    Two charges, Q1 = 3.15 μC and Q2 = 5.40 μC, are located at points (0, -2.80 cm) and (0, +2.80 cm), What is the magnitude of the electric field at point P, located at (5.45 cm, 0), due to Q1 alone?


    lQ2
    l
    l
    l
    l--------------P
    l
    l
    l
    l
    lQ1



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 7, 2008 #2

    Doc Al

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    Staff: Mentor

    In order to get help, post your attempted solution and point out where you got stuck.
     
  4. Mar 8, 2008 #3
    I used E=( k lq1l ) / r^2

    (8.99x10^9) (3.15x10^-6) / (0.0612m)^2
    =7.56x10^6 N/C
     
  5. Mar 8, 2008 #4

    Doc Al

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    Looks good to me.
     
  6. Mar 8, 2008 #5
    k got it, I think I was putting in 10^-6 by accident :S lol, thanks!
     
  7. Mar 9, 2008 #6
    Ok so there were two other parts to the question, I had to find the xcomponent and ycomponent of total electric field at P. I solved the one for x and got it correct which means I must be using the right angle and stuff to do my calculations. My answer for y was wrong, why is this?
     
  8. Mar 9, 2008 #7

    Doc Al

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    Beats me. Show what you did.
     
  9. Mar 9, 2008 #8
    Workings are on attached file...
     

    Attached Files:

  10. Mar 9, 2008 #9

    Doc Al

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    Check your signs. Realize that one field points up, while the other points down.
     
  11. Mar 9, 2008 #10
    opps so there should be subtraction to get my final answer...
    So the Q1 diagram needs to be a negative answer...

    (-3.44x10^6)+ (5.90x10^6)
    =2.46x10^6N/C
    Thats still not right tho, I dont know what Im going wrong
     
  12. Mar 9, 2008 #11

    Doc Al

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    Q1 is the one on the bottom, right? So where does its field point?
     
  13. Mar 9, 2008 #12
    Ok, so the Q1 line points in the positive direction while the Q2 points in the negative direction right? I got the right answer but I just want to make sure my reasoning for sign convention is understood.
     
  14. Mar 9, 2008 #13

    Doc Al

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    Right. The field from a positive charge always points away from the charge. So the y-component of the Q1 field is + and the Q2 field is -.
     
  15. Mar 9, 2008 #14
    Ok thanks once again!
     
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