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Homework Help: AP problem

  1. Oct 19, 2004 #1
    I been having trouble lately with my phyiscs HW probs. Here is only that ti'm confused on:

    Three wires are connected to form a vertical H. The cross piece, mass 0.050 kg and length 0.10m , can slide up and down the vertical wires without friction. The device is placed in a horizontal field that varies B=0.10y where y is the vertical distance from the bottom of the H. What current in the H is required to lift the cross piece .05m

    MY q's:

    Is this a magnetism prob? (thats what we are learning in class) also can i make the cross piece effectively start from the bottom? Must I incorporate gravity?
  2. jcsd
  3. Oct 21, 2004 #2


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    Yeah, it's magnetism allright. A magnetic field will exert a force on a current carrying wire. If you look at the H from the front and the current is going from left to right and the magnetic field points 'into' the H (away from you) then the magnetic force on the wire will point upward.
    If you don't incorporate gravity, the wire wil simply accelerate according to Newton's second law. You'll have to find the right current for equilibrium.

    Since the B-field increases as y increases, I don't think you have to start from the bottom or else the piece will just keep accelerating.
  4. Oct 21, 2004 #3
    I see the same problem that Galileo sees:

    The question ought to be "what current will hold the cross-piece at .05 m." For this you just need to balance two equations, one for the force of gravity, and one for the Lorenz force (that's the "BIL" formula). Solve for I.

    Accoring to "B = 0.10y" the magnetic field will always be zero at the bottom, so NO current will ever get it to move from the bottom since B=0. If you did give the cross-piece a little "bump," then any current that lifts it at all will cause a greater and greater force and therefore a greater acceleration as it rises. Double check the way the question is phrased.
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