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How do you induce a current?

  1. Oct 16, 2004 #1

    I've just realised, when trying to do a write up for a Physics experiment (about a coil and a magnet) that I don't know how you get an induced current!

    What's actually going on when the magnet moves through the coil? What are the electrons doing to induce a current?

    I'm totally confused and my Physics textbook is not helping at all!

    Please help!
  2. jcsd
  3. Oct 16, 2004 #2


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    Take a look at the Faraday-Maxwell's law:

    [tex] \epsilon=\oint\overline{E}\cdot\overline{dl}=-\int\int\frac{\partial \overline{B}}{\partial t}\cdot \overline{dS}+\oint(\overline{v}\times\overline{B})\cdot \overline{dl}[/tex]

    A FEM can be induced by two effects:

    -An unsteady magnetic field inside a steady electric closed circuit.
    -The movement of a conductor inside a magnetic field.

    In your example, you are causing a [tex] \frac{\partial \overline{B}}{\partial t}[/tex] moving the magnet inside a cylindrical-shaped coil of section S. The FEM is induced internally in the conductor displacing the electrons towards one of the extremes, and positive charges to the other. The vehicle of transmision is a bit heuristic for me, so that maybe a physicist could help you very much.
    Last edited: Oct 16, 2004
  4. Oct 16, 2004 #3


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    It might help to think about how things look in a frame of reference moving with the magnet. There the magnetic field is static and the charges in the coil are moving through that static field. They will be subjected to the [itex]\vec v \times \vec B[/itex] force and respond to it. In the coil frame, that response is said to be due to the induced EMF.
  5. Oct 16, 2004 #4


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    A nice change of reference frame, Tide. Sorry for writting upside down the letters FEM and EMF.

    FEM=Fuerza ElectroMagnética (spanish)
    EMF=ElectroMagnetic Force (english)

    It's an usual error in my spelling to think of my native language... :frown:
  6. Oct 16, 2004 #5


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    I knew what you meant by FEM and I was by no means attempting to correct you! Everyone working in the sciences is aware of the linguistic differences used in writing abbreviations.
  7. Oct 17, 2004 #6
    Thanks everyone! It all makes sense now!

    I have another question though! The graph of voltage against time: what physical significance is the gradient and the area under the curve?
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