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Rotating magnet inside coil = flux change?

  1. Jan 17, 2012 #1
    Apparently, that's how a lot of car speedometers work... but I don't understand how a rotating magnet inside a coil (or any conductive material, really) can induce a current in the coil.

    I can understand how pushing a magnet in and out through a coil can induce current, because
    Flux = Area * Magnetic field strength * sin θ, and the magnetic field acting upon the coil is becoming stronger.

    But I don't see how spinning a magnet inside a coil is going to do anything, much less create eddy currents to oppose the spinning and somehow create a torque.

    Could someone refer me to a video or an image of some kind? I've tried to figure this out but nothing really comes to mind.
  2. jcsd
  3. Jan 17, 2012 #2


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    Staff: Mentor

    Do it like this -- put half the coil on one side of the magnet and the other half on the other side. You can space the two half-coils arbitrarily close together, if you use a short magnet. As the magnet turns, its flux goes one way through the coil, and then the opposite way as it turns to 180 degrees... Make sense?


    . 02002.png
  4. Jan 18, 2012 #3
    If you mean a car speedo of the 'old fashioned' type where a needle moves across a scale then this works by causing eddy currents in an aluminium disc by means of a spinning magnet. The disc has a spring to counteract the turning effect and the pointer is connected to the disc. The faster the magnet spins the greater the eddy currents and therefore the greater the drag on the disc caused by the magnet.... the greater the pointer deflection.
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