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Electricity generation - simple bike dynamo - can anyone explain this design?

  1. Aug 7, 2012 #1
    http://img404.imageshack.us/img404/9523/dynamos.jpg [Broken]

    I understand that when the rotating magnet spins, current should be induced in the coils which powers the lights.

    So what is the point of the soft iron core? The rotating magnet should magnetise it because it is near it, and it should become an electromagnet when current flows through the wire, but I can't see a point to any of this. I would think the design should work even if you replaced the soft iron core with a piece of wood because I think all you really need is for the magnet to spin near the wires for the electricity you need.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Aug 7, 2012 #2


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    The soft iron has a very high magnetic permeability and essentially the magnetic field lines are concentrated (or focused) within it. So the magnetic flux through the coil is much larger with the soft iron present than if you were to replace the soft iron with a non-magnetic material like wood. You might start reading here.
  4. Aug 7, 2012 #3

    Simon Bridge

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    The point of the core is to maximize the magnetic flux through the center of the solenoid.
    Without it, there would still be some flux through the coil - just not as much, so you won't get as much electricity for the speed.
  5. Aug 7, 2012 #4
    I see, so the rotating magnet is not strong enough to power the lights on its own which is why the iron core is needed.

    However the iron core and the wires are not moving moving relative to each other (From the picture it looks like the rotating magnet spins but everything else remains in place) which I believe is a requirement for current generation, so how does this electromagnet supply the wires wrapped around it with voltage?
    Last edited: Aug 7, 2012
  6. Aug 7, 2012 #5


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    Hi Steve

    its not an electromagnet...
    An electromagnet is a coil with an iron core that YOU supply voltage to to generate a magnetic field
    The bike dynamo aka generator uses a spinning magnet in a iron cored coil to induce a voltage into the coil

    have a look at Lenz's Law .... http://en.wikipedia.org/wiki/Lenz's_law[/PLAIN] [Broken]

    Last edited by a moderator: May 6, 2017
  7. Aug 7, 2012 #6

    Simon Bridge

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    It may or may not be needed - but the iron core makes the generator more efficient - so you don't have to pedal as hard to get the same speed.

    Note: you only need a changing magnetic flux through the coils to induce electric current - the flux changes as the position of the rotating magnet changes. The effect of the core is to concentrate that flux.

    Try sketching the flux lines from an overhead diagram of the setup.
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