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Finding the direction of current given a varying magnetic...

  1. Apr 18, 2017 #1
    1. The problem statement, all variables and given/known data
    YZgf1wB.jpg

    2. Relevant equations
    μ0 In,e = ∫B⋅ds

    3. The attempt at a solution
    I really don't how to approach this question at all.

    Do you have to integrate counterclockwise around the loop every time? If the field was decreasing as y decreased, would you integrate CW or is it still CCW?

    Why does μ0 In,e = ∫B⋅ds being >0 mean that it must be out of the page.
     
  2. jcsd
  3. Apr 19, 2017 #2

    BvU

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    The direction of ##\vec {ds}## has a meaning. You go around in a mathematically positive direction (CCW).

    Ampere's law is an integral form of one of the maxwelll equations.

    If ##d\vec l## in the link (your ##\vec {ds}## ) goes around CCW then ##\vec {d\bf {S}}## is pointing towards you (the positive z direction).

    If the field is decreasing you still go around CCW and you get a negative ##\vec I## -- i.e. pointing in the negative z direction.
     
  4. Apr 19, 2017 #3
    Okay, I understand. Thank you.
     
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