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Homework Help: Induced motional emf conceptual question

  1. Nov 10, 2011 #1
    A magnetic field points into the page. A conductive bar is moved to the right (perpendicular to the magnetic field). Why do the electrons move downwards toward the bottom of the bar? The book doesn't offer a good explanation of this... using right hand rule I would say that the electrons are moving to the right and therefore there is a force pointing upward?
    Last edited: Nov 10, 2011
  2. jcsd
  3. Nov 10, 2011 #2

    Doc Al

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    Staff: Mentor

    The right hand rule will give you the force on a positive charge. You need to reverse it, since the electrons are negative.
  4. Nov 10, 2011 #3
    When charges move through a magnetic field they experience a force.
    The force is at right angles to the magnetic field and at right angles to the velocity of the charge.
    This relationship is given in Fleming's LEFT hand rule. I hope you recognise this and realise in this rule the fingers of the LEFT hand have the following meaning:
    1) The First finger points in the direction of the magnetic Field ( The 'F' in 'F'ield)
    2) the seCond finger is the direction of the Current (this is the direction + charges would move in). This is a major issue !!! conventional 'current' is the direction + charges would flow. When you are dealing with a flow of electrons (- charge) the second finger must point in the opposite direction of electron flow. This is the main aspect of this work to sort out !!!!
    3) The thuMb points in the direction of the force on the charges. ( M = Motion !!!)
    In your example the magnetic field is INTO the plane of the paper....point the first finger of your left hand into the paper. The electrons in the wire are moving to the right so you must point your second finger to the LEFT (it takes some getting used to !!!)
    Which way is your thumb pointing? ( I hope that it is DOWN (towards you if you are sitting like I am !!)).
    Electrons feel a force towards the bottom of the wire, they will move in this direction until a voltage is built up, with the bottom end of the wire -ve and the top end +ve opposing the further movement of electrons. A voltage is developed between the top end and the bottom end of the moving wire.
    This effect is called the Hall effect and the voltage is called the Hall voltage.
    If you need to know more look up Hall effect.
  5. Nov 10, 2011 #4
    I should add and qualify that the Hall effect is usually associated with a current flowing from one end of a PLATE (rather than a wire) to the other. The electrons are deflected down as in your example and a voltage appears between the top edge of the plate and the bottom edge.
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