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Direction of magnetic force

  1. Oct 31, 2013 #1

    Zondrina

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    1. The problem statement, all variables and given/known data

    Just a quick question I had about finding the direction of a magnetic force.

    I have abandoned my 2D way of thinking when it comes to these because perceptually it's completely wrong to think about it that way.

    Lets say I have an electron moving with a constant velocity through a magnetic field.

    Some relevant info will be provided below.

    2. Relevant equations

    ##\vec{B} = 1.4T [Up]##
    ##\vec{v} = 5.00x10^6 m/s [North]##
    ##q_{electron} = 1.60x10^{-19}C##

    3. The attempt at a solution

    The magnetic force can be calculated:

    ##F_M = qvB = 1.60x10^{-14}N##

    To find the direction of the magnetic force, the right hand rule can be applied. The electron is initially moving [North], so to be able to use my right hand to find the direction, I should initially point my thumb [South].

    There is a magnetic field in the region in the [Up] direction, perpendicular to the motion of the atom. Using the right hand rule, my remaining fingers point in the [Up] direction.

    I believe this will result in my palm pushing to the west, in the direction of the magnetic force.

    Here's a drawing of how I'm visualizing the region:

    http://gyazo.com/3846aa2d5e7826878ea69965d528da27

    Would all of this be reasonable?
     
    Last edited: Oct 31, 2013
  2. jcsd
  3. Oct 31, 2013 #2

    UltrafastPED

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    I get East for the direction; F = v x B gives a right handed coordinate system based on v, B, and F corresponding to i, j, k.

    So if you align i with North, j with Up, k must be pointing East.

    And when I use my right hand, thumb pointing north, index finger pointing up, my palm points east.

    See http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html
     
  4. Oct 31, 2013 #3

    Zondrina

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    [East] is my original answer for the problem. My question is, why is the diagram I've drawn been so misleading then?
     
  5. Oct 31, 2013 #4

    UltrafastPED

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    The diagram was too complicated and compacted for me to follow.

    I simply sketched the field, the velocity, and the force as three perpendicular axes which obeyed the right hand rule ... simple is better?
     
  6. Oct 31, 2013 #5

    Zondrina

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    Simple is better.

    You said this though:

    Why point your thumb north? When I got my answer as [East] I used a slightly different method:

     
  7. Oct 31, 2013 #6

    UltrafastPED

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    I forget that it was an electron; this just changes the sign on the force when you are done.

    In that case the force is pointing West. This is same as you got in the OP.
     
  8. Oct 31, 2013 #7

    Zondrina

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    So the answer is actually west and not east?
     
  9. Oct 31, 2013 #8

    UltrafastPED

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    I agree with your original answer: you got west!
     
  10. Oct 31, 2013 #9

    Zondrina

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    Okay. So the force is actually pointing west.

    There wasn't anything misleading about my interpretation then (the drawing)?

    The NESW arrangement represents the "sheet of paper" if you will in the x-y plane and the "up" and "down" on the z axis represent "out of" and "into" the page respectively.
     
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