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Motor Principle - Direction of Rotation

  1. Jan 27, 2012 #1

    I'm trying to work out the direction a loop of wire will rotate about an axis in an increasing magnetic field.

    From what I understand by Lenz's law the induced current in the loop will create it's own magnetic field as to counteract the change in external magnetic field. I'm struggling by Fleming's left hand rule however to determine a direction of rotation.

    I have attached an image, if somebody could clear this up this will definitely put me at rest.


  2. jcsd
  3. Jan 27, 2012 #2
    You are mixing up a lot of physics ideas.
    This is about the FORCE on a current carying conductor in a magnetic field.
    Fleming's Left hand rule gives the direction of the force.
    Your diagram does not show the direction of the current in the coil of wire!!
    You need to know that
  4. Jan 27, 2012 #3


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    The direction of the current just affects the direction of the turning moment,not the magnitude.

    Lenz's law will only come into play once the coil starts to move and will limit the acceleration.
  5. Jan 27, 2012 #4
    Sorry perhaps I should elaborate,

    In this example what I am saying is the magnetic field shown in blue is increasing in the direction shown. Therefore an emf will be induced in the wire equal to the rate of change of this magnetic flux (Faraday's law). This emf will produce a current in the direction according to Lenz's law.

    The induced current will obviously create it's own magnetic field as such to oppose the changing magnetic field (Lenz's law). It is the direction of which I am struggling to ascertain. So this loop is now carrying current and so should experience a force equal to the Lorentz Force (except the current is changing).

    I am thinking about the AC induction motor where in this example the wire loop is the stator and the N-S pole is coil wound in the stator.

  6. Jan 28, 2012 #5
    My apologies fonz! I mis-read. Your first sentence. I thought you were looking at a coil with current flowing in it.
  7. Jan 28, 2012 #6
    The way your diagram is drawn there is nof flux linking (going through) the loop and therefore the increasing magnetic field will not induce an emf.
    If the loop is at 90 degrees to the field then the maximum induced emf (and therefore current) will be induced.
    The emf is given by Faraday's law = rate of change of magnetic flux linkage and the direction of the emf and resulting current will try to prevent the change (increasing magnetic flux) occurring.
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