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Homework Help: Give the magnitude of the magnetic field that will.

  1. Mar 7, 2010 #1
    Give the magnitude of the magnetic field that will.......

    1. The problem statement, all variables and given/known data

    GIANCOLI.ch27.p59.60.jpg

    Suppose the rod in the figure has mass m = 0.38 kg and length 26 cm and the current through it is I = 40 A.

    If the coefficient of static friction is us = 0.49, determine the minimum magnetic field B (not necessarily vertical) that will just cause the rod to slide. Give the magnitude of B.

    Express your answer using two significant digits.

    2. Relevant equations

    F = IBd
    F = usFN

    3. The attempt at a solution

    Since its just "just beginning to move", I know that the two forces are equal to eachother.

    So:

    IBd = usFN
    IBd = usmg

    which means that:

    B = (usmg) / Id

    I plug in the values (after converting them to standard SI units), and I get .175 T. To two significant figures would be .18. But thats not right, I also tried .17, not that either, .2 was also wrong. I don't know what I'm doing wrong.
     
  2. jcsd
  3. Mar 7, 2010 #2
    Re: Give the magnitude of the magnetic field that will.......

    never mind, i figured it out. Our teacher gave us a message saying that if you tilt the mag field west, it will reduce the normal force, and then the overall magnetic field strength. I modified the equations to make them functions of theta. Then from there, I found the value of theta that gives a minimum for B.
     
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