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Why an electromagnet doesn't makes a short circuit?

  1. Feb 15, 2013 #1
    How can you have a simple coil connected to a battery without having a short circuit in it? How do they make electromagnets that don't heat the battery?

    Thanks :)
     
  2. jcsd
  3. Feb 15, 2013 #2

    cjl

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    Wire has a nonzero resistance, and electromagnets often use a fairly long length of fairly skinny wire.
     
  4. Feb 15, 2013 #3

    Philip Wood

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    Gold Member

    The wire is coated with insulation (except, of course, at the ends, where the connections are made). So it doesn't matter if the turns touch each other; the current can't cut corners.
     
    Last edited: Feb 15, 2013
  5. Feb 15, 2013 #4
    Yep but I'm talking about the fact that the coil is a simple wire. And if we connect a simple wire to the terminals of the battery it should be a short circuit and heat everything :(
     
  6. Feb 15, 2013 #5

    Philip Wood

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    A short-circuit means a circuit which is too short - usually unintentionally. Its resistance is so low that it takes too large a current, overloading the power source and/or overheating the wires.

    As cjl pointed out, the wire of the electromagnet is chosen to be long enough and thin enough for its resistance to be large enough for only a modest current to flow.
     
  7. Feb 15, 2013 #6

    Drakkith

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    And if all else fails you can just add a resistor to the circuit.
     
  8. Feb 16, 2013 #7
    I do believe that if you stall most electric motors, they will overheat precisely because they have essentially become "short circuits" (albeit not so short).

    So...yeah, I believe that a simple electromagnet will actually overheat unless you do something to stop it from doing so.
     
  9. Feb 16, 2013 #8

    Philip Wood

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    It's certainly true, as Lsos implies, that if there is a back-emf present, as with an electric motor when it's running, or in an electromagnet when fed with a.c., then the current will be lower than for the same system with no back-emf.

    Yet it is possible to design an electromagnet so that it takes a safe current when a suitable p.d. is placed across it, even when there is no back-emf. Like many of us, I would imagine, I played with home-made electromagnets as a child. There were no nasty incidents of overheating, and the batteries lasted a reasonable time. I'm sure someone had advised me to put plenty of thin wire on my iron core!

    It's all a matter of design. In general, motors use thicker wire and less of it than you'd use for an electromagnet designed to run for more than a few seconds on a d.c. supply.

    [Can't resist saying to Drakkith that he's right, of course, but making the resistor out of extra wire added to the electromagnet coil will have the advantage of making the electromagnet stronger! Just teasing.]
     
    Last edited: Feb 16, 2013
  10. Feb 16, 2013 #9

    Drakkith

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    Interesting. I guess the shorter, thicker wire allows for a higher current to run the motor, especially at startup, and the back EMF generated when it is running keeps the current from getting out of hand?

    Yep!
     
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