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Homework Help: Magnetic Field Question

  1. Jan 19, 2015 #1
    1. The problem statement, all variables and given/known data

    A magnetic field of 1.4 T [N] is 4.0m wide. A very long wire crosses the field. The current in the wire is 2.0 A [W].

    What would happen if the wire is rotated slightly in the horizontal plane?
    2. Relevant equations

    F = ILBSinx

    3. The attempt at a solution
    I dont understand what it means by "rotated in the horizontal plane". If it was moved left or right, L would increase because the wire isnt moving straight across the electric field. However, it says the current in the wire is moving West which confuses me.

    Is it saying that the wire is lifted to create an angle with the surface it is on?
  2. jcsd
  3. Jan 19, 2015 #2


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    The wire is sitting on a horizontal table, current travels leftward (West), from right (East).
    The Magnetic Field points forward along the table (Northward).
    In this situation, only 4 meters length of the wire (current) is in the B-field,
    so the magnetic Force applied to the current is F = ILxB = ILB sin 90 (downward).

    The wire stays horizontal as it rotates, and the current rotates with it
    ... suppose, to 10deg North of West.

    the wire is "very long" ... much longer than 4 m ; is more wire in the B-field now, than was in it before? Is the extra length made up for by sin 10?
    Last edited: Jul 24, 2015
  4. Jan 19, 2015 #3
    Thats all it says. I actually wrote the question wrong, it says:

    What will happen to the force on the wire if the wire is rotated slightly in the horizontal plane?
  5. Jan 19, 2015 #4


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    Staff: Mentor

    Is this the total information you have re this problem? Is it from a textbook?
  6. Jan 19, 2015 #5
    yes to both
  7. Jan 19, 2015 #6


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    So, if the wire is rotated by 10o , so it doesn't cross "straight West" across the B-field's width,
    does the Force increase, decrease, stay the same ... and why? I would stick to Force strength (ie, magnitude)
  8. Jan 19, 2015 #7
    L increases so the force should increase but it is also not perpendicular to the magnetic field so I dont get it.
  9. Jan 19, 2015 #8


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    IL x B means IL B sin θ ... θ is now 80°, rather than the 90° that it used to be ;
    but the length is a *bit* longer (along the trig triangle hypotenuse; 4m is the long leg).
    Last edited: Jul 24, 2015
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