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Homework Help: Motional emf (conceptual)

  1. Oct 12, 2011 #1
    1. The problem statement, all variables and given/known data
    Hello all,
    If we have a bar that is perpendicular to a magnetic field, and that bar is being pushed to the left via an applied force, all the while a current (due to a voltage source and resistor) runs down through the wire. Therefore, there would be a total force comprised of a magnetic force due to the current and an applied for, both directed to the left.

    My question is: since another magnetic force develops to oppose this motion and allow dv/dt=0, it will have to have current go UP through the metal rod, and therefore oppose the initial current that was supplied by the voltage source, correct?

    Hopefully that makes sense without the need of a picture. If not, please let me know and I will attempt to draw a picture. Regards, Sandy.
  2. jcsd
  3. Oct 12, 2011 #2

    Here, I added a picture. F is the applied force acting on the bar. The B field is directed out of the picture, and there is a voltage source and resistor in series with the conducting bars. Friction is neglected. Obviously there will be current that enters the system and thus, there will be a magnetic force acting on the wire directed in the same direction as the force F. Now, another force, say F2, will have to induce a current in the opposite direction in order to obtain constant velocity. F2 will act opposite to F and the magnetic force developed due to the initial current flowing.

    The current, once steady state is attained, will be clockwise in this picture.

    Does this seem right?
  4. Oct 12, 2011 #3
    No, if the system is in steady state, the magnetic force will oppose the velocity, and therefore the current will flow clockwise as you said. However, you introduced an unnecessary force.

    All you would need to do is set F=Fmag for steady state conditions.

    Feel free to correct me if I am wrong, considering I am new to this type of theory.
  5. Oct 12, 2011 #4
    Also note that the only time that you would have had the conditions as you had it, is if the question explicitly stated that they wanted to prevent the system from moving, ie equilibirium with the sum of forces equal to zero.

    In that case, it would be F + F(magnetic) = F(external)
  6. Oct 12, 2011 #5
    I can't see it. Shouldn't there be a magnetic force along with the applied force F in the same direction, that would have to be overcome by another, but stronger magneticforce in the opposite direction?
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