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Homework Help: Find the power required to pull loop through constant magnetic field

  1. Nov 14, 2011 #1
    1. The problem statement, all variables and given/known data

    The wire shown is being pulled through a constant magnetic field of 0.25 Tesla at 3 m/s.

    A) Find the power required to pull the loop at this speed.
    B) If the wire has a resistance of 100 ohms, what is the current in the wire?

    2. Relevant equations

    3. The attempt at a solution

    I am trying to teach myself this problem, as I missed a week of school because of a back surgery and the slides I was given are wholly inadequate. I am not looking for a simple solution, but an explanation and a little bit of guidance to point me in the right direction would be stupendous.

    This isn't a homework problem (as in it has a due date) but it is in the back of the book and is listed as difficult.

    The picture (I have drawn) is here: http://twitpic.com/7e6nad [Broken]

    Thank you all.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Nov 14, 2011 #2
    Consider the loop consisting of atoms.Each atom has electron's and in metals these electrons are free to move.Due to action of magnetic field we have force on electron as e*(v x B). Here 'x' is cross product of vectors.So electrons move to one end and all protons remain where there are.Thus creating a potential difference and causing a current.And yes power is force is= Force*Velocity=e*(v x B) * v=e*(v^2)*B.
    Value of potential Difference=B*l*v
    Where l is length of rod.It must be given.
    Otherwise use power I^2*R(resistance)=F*v.
    Get current.
    B is magnetic field and v is velocity of electron and e is charge on electron.
    Last edited: Nov 14, 2011
  4. Nov 14, 2011 #3
    Power = force x velocity.
    Do you know how to calculate the force on the wire in the magnetic field?
  5. Nov 14, 2011 #4
    what is the force acting on the wire due to the magnetic field?

    q(vxB)=F since there is no electric field.

    and current is dq/dt which you could say is qv, but i think power is I2R = P

    Power is F*v=P
  6. Nov 14, 2011 #5
    Isn't it F=BILsinĪø? But surely that doesn't apply here? There is no length given, after all.

    For B) If the wire has a resistance of 100 ohms, what is the current in the wire? I=V/R, but I do not know the voltage, nor am I certain how I can ascertain it with the given information.

    I feel that I may have jumped ahead of the class with this problem, but I really want to grasp the material.
  7. Nov 14, 2011 #6
    Length of rod is not given. Truly.

    e*(v^2)*B would be 3.6E-19 N?

    I^2*r=F*v => I = 1.04E-10 amps? Does that seem correct?
  8. Nov 14, 2011 #7
    If you have(Covenant32) calculated correctly then it might be.Method remains as stated.All rest is calculation.
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