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Faraday's Law

  1. Oct 25, 2007 #1
    Ok so we are studying Faradays law right now and am a little confused. I will try and state my understanding of it, please correct of wrong.

    If we have a change in magnetic flux in a certain area in space there is an induced current. This current must obvoiusly come from an electric field whos direction comes from the right hand rule , the thumb point opposite to the direction of change in the magnetic field and curls.

    But there are a number of things that elude me:
    1) Faradays law always hold-->>even for non conservative system, how and why?
    2) How exactly do you determin change in B?
    For example if I have a magnet pointing north and move it towards a ring at a nonuniform velocity, the ring is facing the magnet through its opening, is there a change in B?
    How about at different orientations of the magnet? This is where am just beyond confused, especially with different oreintations.

    Are there any books/supplements you guys can recommend? In desperate need.
  2. jcsd
  3. Oct 28, 2007 #2
    Holy Cow nothing yet!?
  4. Oct 28, 2007 #3


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    Staff: Mentor

    Well, apparently it was a big party weekend at many universities this weekend... I wasn't invited, so that's not my excuse, however :rolleyes:

    I'll try to answer a part of your question -- The B field diverges as it traces away from a permanent magnet, so Yes, as you move a permanent magnet closer to a ring, the B field cutting through the area of the ring increases:


    So moving a permanent magnet closer to a metallic ring will induce a current in that ring.
  5. Oct 29, 2007 #4


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    Homework Helper

    Faraday's law is one of the four Maxwell's equations. Its place in the theory of electromagnetism is thus comparable to one of Newton's laws in classical mechanics. So it always holds.

    You have to consider why and if the flux changes in a particular situation. It could be because the field strength changes in time, or because you move the ring through a nonuniformmagnetic field, or because the loop bounding the surface is changing shape.
  6. Nov 1, 2007 #5
    Well the answer of the question "How the magnetic field changes?" is this that the magnetic field around any magnet or around any charge particle depends upon the strength of the charge or magnet. As we move away from the magnet the field becomes weaker so when a magnet brings close to coil then obiviously there will be change in magnetic field
  7. Nov 2, 2007 #6
    Not sure what you mean by that question, sorry.

    Think of the flux density. Picture the old highschool iron magnet with the filings sprinkled around it. The more lines the higher the density. As you move the magent towards the ring, whether uniform or not, the number of flux lines intersecting the ring changes. Increasing or decreasing depending on direction of travel and orientation.

    Specifically about orientation: The magnet doesn't even have to move towards the ring. Just imagine a standard bar magnet spinning in place with first one pole facing the ring, then a side, then the other pole, then the other side and back to the first pole. As the flux density is higher at the poles than the long sides (same flux, spread over a larger area) then B will float up and down across the ring as the magnet spins. It's the basis for how generators function.
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