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EMF, Current and Electric/Magnetic fields

  1. Oct 31, 2012 #1
    Hi, I've been trying to understand electricity... and series of questions came into my mind..

    1. I know if i move a conductor in an external magnetic field, it induces EMF, but
    I don't really understand INDEPTH how CHANGE in MFlux induces EMF?
    I think something to do with EF and MF...

    2. Also with the conductor in an external magnetic field, since current is induced by voltage,
    do electrons flow from one side of the rod to another? So is there current even if there's no 'circuit', furthermore does the rod become positively charged in one side and negative in the other?

    3. As a motor spins, back emf is induced, to eventually result in NET VOLTAGE of zero
    but how is there flow in current if there is no potential difference?
    Also is there back EMF in AC motors?

    Thanks =]
  2. jcsd
  3. Oct 31, 2012 #2
    1/ When a conductor and a magnetic field move relative to each other, the charges in the conductor experience a voltage.
    It doesn't matter if you move the conductor and keep the magnet still, or if you move the magnet and keep the conductor still. It's the same thing, relative movement produces the voltage.
    (You might be interested to know that Einstein was puzzled by this too and it was what made him start thinking about relativity - he was 16. The first part of his famous paper on relativity is about it)

    2/ Yes

    3/ The same way that if you fill a pipe with water then and a pressure to get it going, then connect the two ends of the pipe together. For a little while at least, the water keeps going even though there's no pressure difference.
    And again -yes.
  4. Oct 31, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    Look at it this way. If you couldn't see the magnet moving towards you, or you moving towards the magnet, would you be able to tell if you were actually moving towards a magnet or if the flux was just increasing?
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