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Homework Help: Direction of emf, Flemings left & right hand rules

  1. Oct 4, 2012 #1
    1. The problem statement, all variables and given/known data
    A metal strip moving to the right in a B field which is going into the plane of the page.

    Can this problem be considered using both Fleming's right and left hand rules?

    2. Relevant equations

    3. The attempt at a solution
    If I consider Fleming's left hand rule: the negative charges in the strip are being moved to the right and so experience a force up (not what it seems to say in the books). Then the strip will have negative charges at the top and positive charges at the bottom.

    Or should I think of it as that they are feeling a force to the right (because the strip is being moved) and the current is down? Or maybe Fleming's left hand rule is not for this and is only for a negative charge moving in to a B field and it tells the direction of the force on it.

    But is the current I in Fleming's left hand rule the direction of the actual negative charges or the conventional current of positive charges?

    If I consider Fleming's right hand rule: The force is to the right and the emf is up. But is the emf direction from + to - or from - to +?

  2. jcsd
  3. Oct 4, 2012 #2
    I think I understand it now.

    Firstly the I for current is conventional current and so it is the direction that positive charges would flow.

    In Fleming's left hand rule the force is as a consequence of the B-field and the velocity of positive charges (or conventional current).

    In Fleming's right hand rule then current, I, is as a consequence of the force and the B-field. Then this is what should be used in the example above. The B-field is in to the page, the force to the right and these two create a current, 'I' going up. I is the conventional current and therefore the direction positive charges flow in. So positive charges are going to the top of the strip. Then the top of the strip becomes positive relative to the bottom of the strip.

    I know the emf also points up and so the direction of emf must be from - to +. That was my original question.

    Is this all correct?

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