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Homework Help: Direction of force on circular loop.

  1. Jan 3, 2014 #1
    1. The problem statement, all variables and given/known data

    Find direction of force on circular loop

    http://pokit.org/get/img/b886e52efb703d47d09fa6e781c74461.jpg [Broken]

    What would hapend if direction of the current in a straight conductor were different ?

    2. Relevant equations


    3. The attempt at a solution

    http://pokit.org/get/img/7b5d76e176716f45edb5dbf331ff9e3e.jpg [Broken]

    I try to do this by using right hand rule, but this is just assumptions, im not sure is this correct or can it be done like this. When direction of current change i dont know what would happend, or how straith conductor influence? I hope you would help me ;)
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jan 3, 2014 #2


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    welcome to pf!

    hi diinnoo! welcome to pf! :smile:

    you're only asked for the direction, so can't you just use general principles, and symmetry?

    the force between two straight currents in the same direction is … ?

    in the opposite direction is … ?

    now apply that to tiny bits of the circle, in pairs at opposite ends of the same diameter :wink:
  4. Jan 3, 2014 #3
    thanks for welcome tiny-tim.

    Yes only direction was asked, i think two straight current same direction force would attract each other, when they are opposit they wil reject each other, two forces.

    I tried this using unit vectors i tried something like this

    http://pokit.org/get/img/96569110fd7b5c1228052db376c5fecd.jpg [Broken]

    but i can do this onle wher direction of current is same as unit vector i or j, i dont know how to do it on whole circut. but in this examle above i again don see influence of straight current.
    Last edited by a moderator: May 6, 2017
  5. Jan 3, 2014 #4
    if the loop is is placed parallel to the field there will be a couple acting on it, when it's perpendicular the two forces will be equal opposite and have the same line of action
  6. Jan 3, 2014 #5
    I realy didnt understand you, sorry, or you just can simply draw what you thought. In school we did task like this, with straight conuctor and metal frame, in same plane. Direction we find using units vector, as my example up.

    http://pokit.org/get/img/23dd764ee7a54aef8c120e0932b3eb97.jpg [Broken]

    And this task should be on same principes but much harder :(
    Last edited by a moderator: May 6, 2017
  7. Jan 4, 2014 #6


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    hi diinnoo! :smile:
    yes … so first consider two very small parts of the circle, at 12 o'clock and 6 o'clock …

    which one will have the larger force, and so what will be the direction of the resultant force on the pair?

    (this is similar to the question in your diagram, but your "hollow" arrows are wrong …)
    then consider the resultant force on four very small parts, at 1 5 7 and 11 o'clock :wink:
    Last edited by a moderator: May 6, 2017
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