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Cartesian coordinates vectors

  1. Feb 10, 2017 #1
    1. The problem statement, all variables and given/known data
    upload_2017-2-10_15-9-0.png

    2. Relevant equations


    3. The attempt at a solution
    upload_2017-2-10_15-10-26.png

    the answer given is the same but without the negative sign, I don't understand because the crossproduct of unit vectors upload_2017-2-10_15-13-41.png
    when using a Cartesian coordinates of the directions given by the right-hand rule? Is the positive z direction pointing out of the page if X and Y are as follows
    upload_2017-2-10_15-18-3.png
    apologies if this is in the wrong section, thanks for any help in advance
     
  2. jcsd
  3. Feb 10, 2017 #2

    BvU

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    Yes. ##\hat {k} = \hat {\bf \imath } \times\hat {\bf \jmath}## , so z points towards you, out of the screen.

    Corkscrew rule I call it. Turn ##\hat{\bf \imath }## over the smallest angle towards ##\hat {k}##. Corkscrew will go in the minus y direction : $$\hat \imath \times\hat k = -\hat \jmath $$
     
  4. Feb 10, 2017 #3

    TSny

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    The problem doesn't specify which of the two wires the charge -q is a distance b from. Does the answer change if you pick the other wire to be a distance b from -q?
     
  5. Feb 10, 2017 #4
    Thank you both your answers, TSny I thought the same, closer to the top wire the combined magnetic field would be in the opposite direction . Maybe the question is just not very good
     
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