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Homework Help: Magnitude and direction of the total force on the wire

  1. Mar 16, 2009 #1
    1. The problem statement, all variables and given/known data
    A stiff wire 50 cm (0.5 m) long is bent at a right angle in the middle. One section loes along the z axis and the other is along the line y=2x in the xy plane. A current of 20A flows in the wire -down the z-axis and out the line in the xy plane. The wire passes through a uniform magnetic field given by B=(0.318i)T determine the magnitude and direction of the total force on the wire.

    2. Relevant equations
    F = I l B sin(theta)
    F = I lxB

    3. The attempt at a solution

    I tried solving the problem by finding the force on each section and adding them.

    Along the z-axis the angle with the mag field would be 90, so I get the force to be 1.59N

    In the xy-plane, the section lies on the line y=2x. So it makes a 27 degree angle with the mag field. So I get the force to be 0.72N.

    After adding them I get the force F = 1.59 + 0.72 = 2.31N

    Now I have an answer key that says the force F=2.13N and lies 41.8 degrees below negative y-axis.

    How do I get these results? What am I doing wrong? Thanks for your time!
  2. jcsd
  3. Mar 16, 2009 #2
    Re: Electromagnetism

    Well, you're dealing with forces - i.e. vectors. Vector addition isn't quite as simple as you've made it..

    Ah, and one other thing - I don't quite agree with you on your angle!
  4. Mar 16, 2009 #3
    Re: Electromagnetism

    Sorry, it was a 63 degree angle I had.
  5. Mar 16, 2009 #4
    Re: Electromagnetism

    sorry, can anyone walk me through this problem or hint at some steps i need to do this. why cant i simply use the equation f= I l b sin(theta) using the angles i found the wire to be.

    Is when the mag field is given as B=(.318i)T mean that it is only in the x direction? Thats what i am thinking but im confusing myself now. Thanks guys!
  6. Mar 16, 2009 #5
    Re: Electromagnetism

    ok, i solved this, thanks!
  7. Sep 29, 2009 #6
    Re: Electromagnetism

    I have a similar method. I tried it the exact way that you tried. Did you use any other equations?
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