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Coil loop with a magnet through it produces AC or DC?

  1. Jul 23, 2015 #1
    Here is my misunderstanding , for the example let's ignore resistance losses , say i have a infinite length of coil , and an infinite strip of magnets passing through that coil, each magnet seperated by some distance from the next one , the strip of magnets move through the middle of the coil , what kind of current I would get out of the coil , AC or DC?
    I understand that when a magnet approaches a coil and goes past it it creates AC since the field gets stringer and then gradually decreases , but here the flux changes along the length of the coil but overall the magnet never approaches the coil itself nor leaves it.

    a similar situation could be modelled by using a toroid coil and a strip of magnets rotating inside the coil.
  2. jcsd
  3. Jul 23, 2015 #2
    you would have no net current, you would only have local eddy currents. just think about an infinitely long magnetic rod inside the coil, there can never be any flux change because the set up is invariant under translations of an infinite magnet. it's clear if you cut a chunk out of the middle, you still would have no net current. if you consider the long rod with the gap to be moving to the left, then the net flux through a surface at the midpoint between the gap would be 0 because one end of the magnet is contributing to a decreasing magnetic field to the left and the other end is contributing to an increasing magnetic field to the left. it's clear that any 2 surfaces symmetrically placed on either side of this initial surface will create equal current in opposite direction so that the net current would be 0, but you would have local eddy currents. the same thing applies when you have these gaps equally spaced along the entire infinitely long magnet
  4. Jul 24, 2015 #3
    ok I understand that's about the infinite coil matter, but what happens in a toridally shaped coil? where the end of the coil meets the start , and a magnet rotates inside the coil , there should be induction?
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