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Homework Help: Experiment, magnetic forces between current-carrying wires

  1. Sep 25, 2011 #1
    1. The problem statement, all variables and given/known data

    There is a coil placed on a very sensitive weighing scale that has a variable resistor attached it to control its current, and is also connected to a power supply. The wires to it have tape to hold them in place. There is another coil held a few millimeters above this coil and fixed in place to the wall, it has current flowing through it, but I don't know which direction it flows in. I also don't know which direction the current flows in the coil that is on the weighing scale.

    1) Does decreasing the variable resistor's setting increase or decrease the weight read on the weighing scale? Is the magnetic force attractive or repulsive?

    2) Why are the wires with current through them, with which the coil is connected to the resistor and power supply, taped?

    3) If the weight decreases as current increases, is the magnetic force attractive or repulsive?

    2. Relevant equations

    Decreasing the resistance increases the current.
    Magnetic force on a current carrying wire F = ILB
    Magnetic field produced by a straight wire (but I have a coil) B = u0*I/(2*pi*r)

    3. The attempt at a solution

    If the magnetic force exerted by current-carrying wire coil is attractive towards the fixed coil above it, it will counteract gravity slightly and decrease the weight reading on the weighing scale. But I don't know whether force exerted by the current-carrying coil on the weighing scale is attractive or repulsive to the fixed coil above it.

    The wires are taped to stop the magnetic field exerted by the current-carrying coil from moving them.

    If the weight decreases, the magnetic force is attractive (i.e. pulls upwards).
    Last edited: Sep 25, 2011
  2. jcsd
  3. Sep 25, 2011 #2
  4. Sep 25, 2011 #3
    In bar magnets, the attractive force is between opposite poles, so I suppose that the force is attractive, but the issue is that I don't know which direction the current flows in the top coil.
  5. Sep 25, 2011 #4
    Two bar magnet ends attract when their magnetization currents flow the same way, likewise coil ends that carry current in the same direction will attract.
  6. Sep 25, 2011 #5
    Yes, but to solve my problem, I first need to know if the current in both coils is in the same direction, or if one is going the opposite way. :smile:

    Edit: The question in which it results that they are attractive (3) is a hypothetical question that is separate from (1).
  7. Sep 26, 2011 #6
    If the weight of the one coil is reduced then the currents go in the same direction.
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