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Homework Help: Question about magnetic fields

  1. Feb 9, 2005 #1
    A conductor suspended by two flexible wires as in the figure below has a mass per unit length of 0.0800 kg/m.

    What current must exist in the conductor in order for the tension in the supporting wires to be zero when the magnetic field is 2.60 T into the page?

    Figure: http://east.ilrn.com/books/sepsp06t/pse6e.29.14p.e.jpg

    I know you have to use the formula F=IL x B to get the current but I don't know how to put it all together...
    I think the force is the tension so you can make that zero. I don't know how to get the length because it only gives you the mass per unit length.
    Maybe I'm doing this wrong...I don't know...
    Please help me! Thanks in advance!!!! :)
  2. jcsd
  3. Feb 9, 2005 #2


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    The magnetic force (F) per unit length (L) is:
    F/L = BI

    It's weight (W) per unit length is:
    W/L = mg/L
    = (m/L)*g
    = 0.08*9.8 N/m
    = 0.784 N/m

    To have zero tension in the supporting wires, the conductor's magnetic force must cancel out it's weight (i.e. W=F)

    BI = 0.784 N
    -> I = 0.784/2.6 A
    = 302 mA (current must go from left to right, using the right-hand rule)
  4. Feb 9, 2005 #3
    thx for the help and the quick response! :)
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