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What causes the induced EMF?

  1. Feb 14, 2016 #1
    I read many different explanation but none of them is satisfying.
    There is two cases:

    1. I move an iron rod in a magnetic field, between two bar magnet for example.

    To this case I read that when I move the rod the electrons on it also move and moving charges produce magnetic field which interact with the external B field. I think this is impossible because those electrons don't move locally just relatively. This cannot produce net magnetic field arround the rod just allign the spin of the electrons. Maybe this B field interact with the external one?

    2. Iron core of a transformer transfer the energy.
    In this case electrons in the secondary winding don't move at all. Allign to the external B field but just due to its spin magnetic moment. Just the spin allign and e- doesn't move at all. What causes the charge separation?
    Last edited: Feb 14, 2016
  2. jcsd
  3. Feb 14, 2016 #2


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    Lorentz's force acts on the charges when they are moving relative to a magnetic field with some velocity V.
    What's "trafo"? Do you mean a transformer? If yes, I still don't understand your question.
  4. Feb 14, 2016 #3
    B field can interact only with B field. Where is the B field of those "moving" electrons? The spin magnetic moment interact here? (Because at the Lorentz force two B field interact each other.)

    Yes, transformer. What cause the EMF in the secondary coil? The changing magnetic field just allign the spin of the electrons. Why they move?
    Last edited: Feb 14, 2016
  5. Feb 14, 2016 #4
    "B-fields" do not interact with each other.
    Magnetic fields interact with moving charges.
  6. Feb 14, 2016 #5
    Read this:

    "Cause of Lorentz Force

    A magnetic field is created by the motion of an electrically charged particle—such as a proton or electron. If that electrical charge is moving through an external magnetic field, there will be a magnetic attraction or repulsion force, depending on how the two magnetic fields interact."
  7. Feb 15, 2016 #6


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    But when you move a rod, electrons and protons both move in the same direction with same velocity. Therefore, the rod can't generate a magnetic field of its own.
    When only the electrons are moving, i.e. when a current is flowing, a magnetic field is set up.
    According to Faraday's law of EM induction, emf is induced in the secondary because there is a changing magnetic flux through the secondary winding.
  8. Feb 15, 2016 #7
    You are introducing an artificial (or maybe meaningless) distinction here. A charge moving in a magnetic field experiences the Lorentz force. It does not matter how did it get to move. In a metal even the thermal motion of the free electrons results in a magnetic force that curves their trajectories. See "cyclotron resonance in metals" (can be used to measure the effective mass). When the bar has an overall translation, there is an extra effect on top of this, which we measure as an overall emf.
    An by the way, is there any other way to move than relatively (to something)?

    The spins of the electrons have nothing to do with this.
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