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Homework Help: Adding electrical field vectors

  1. Sep 29, 2006 #1
    I have this standard homework problem to find the electrical field at a point.

    I figured out most of the math like the x and y components of the E Field from the two charges acting on the point. I also have the correct answer and i don't understand why the x components of the electrical charge cancel and all that's left is the sum of the two y components.

    Here's drawing of problem and me figuring out direction of e fields: http://focusedwolf.googlepages.com/work2.bmp [Broken]

    It just looks to me like the y components should cancel and it's the sum of the x components.
     
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Sep 29, 2006 #2
    If the charges are equal in magnitude, the y-components should cancel, and the resulting field would be twice the x-component (in magnitude) of the field of any one charge.
     
  4. Sep 29, 2006 #3
    So your agreeing with my intrepration?

    Here is the problem... with "work". they skpped alot of steps, but notices at end says the x components cancel due to symmetry, and not the y. zzz

    http://focusedwolf.googlepages.com/problem.jpg
     
  5. Sep 29, 2006 #4
    Although not specified, I think they may have defined [tex]\theta[/tex] with respect to the y-axis. In that case, what they've stated is correct. [tex]cos\theta[/tex] components, and not the "x-components", do cancel out due to symmetry.
     
  6. Sep 29, 2006 #5
    Hmm i think i get it...so its its not x = Ecos(theta) and y = Esin(theta) but instead, cause theta is "looking down", x = Esin(theta) and y = Ecos(theta)... so the x's do sum and the y's do cancel... just backwards :rofl:

    So if theta was defined from between x axis and P, the "normal" way could be used with x = Ecos(theta)... and get same answer?
     
  7. Sep 29, 2006 #6
    The answer would be the same, but your new theta would be pi/2 - old theta.

    Remember, the direction of the field does NOT depend upon what coordiante system or the angle convention you use. If you're in doubt, think of it in physical terms.
     
  8. Sep 30, 2006 #7
    Yep it worked :biggrin:

    http://focusedwolf.googlepages.com/answer.jpg [Broken]
     
    Last edited by a moderator: May 2, 2017
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