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EMF induced in conducting wire

  1. Sep 13, 2012 #1
    We just went over Faraday's law in class and our teacher stressed how a changing flux is needed to induce an emf in some coil/loop/wire.
    I was just wondering then, how is an emf induced in a straight conducting wire moving at a constant velocity in a constant magnetic field? It seems as if Δϕ=0...
    I think ε=vBl is the equation which is used to describe this phenomena.
  2. jcsd
  3. Sep 14, 2012 #2


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    yes. Your teacher meant that for a stationary wire, a changing flux is required to create an emf. But if the wire is moving through a magnetic field, then an emf can also be created, as you said.
  4. Sep 14, 2012 #3
    But if you have a loop of wire moving through a constant magnetic field then there is no change in flux so no induced emf.. What is the difference between a moving straight wire and a loop/coil?
  5. Sep 14, 2012 #4
    I suggest you draw a picture & you can see things better. A pic always helps. If the loop is normal to the flux continuously & flux is uniform, there is no induction. But if the loop spins so that the angle w/ the flux changes, then induction takes place.

    A good book on motors/generators will illustrate this better than I can verbally describe things. My advice to all pondering such questions is to - draw a picture. Drawing a picture really makes things easier to understand. That is why I always like to draw a picture. It beats words by a big margin.

    Draw a picture. BR.

  6. Sep 14, 2012 #5
    prior posts all look ok....

    Any way you can produce a change in electric potential will push charges around a closed circuit.
    Some explanatory viewpoints here:

  7. Sep 15, 2012 #6


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    There is no difference between the loop and straight wire (except that a current can flow in the coil, causing secondary effects, but we don't need to make this more complicated by thinking about them).

    As I said before, if the wire is stationary, then there must be a change in flux to create an emf, but this is not true if the wire is moving.

    Think about the charge carriers moving through the wire. If the wire is moving (in addition to them moving along the wire), then there is an extra velocity which the charge carriers must have. And so the magnetic field will have an extra effect on the charge carriers due to the fact that the wire is moving.

    Or another way to think about it is that although the flux is not changing, the magnetic field is being 'swept through', as the wire moves through it. P.S. this kind of emf is called motional emf (which makes sense really).
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