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Why an electro magnet use energy?

  1. Apr 10, 2008 #1
    Why an electro magnet use energy?
    the resistance is reason ?
     
  2. jcsd
  3. Apr 10, 2008 #2

    pam

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    Yes. The power dissipated is I^2 R.
     
  4. Apr 10, 2008 #3

    f95toli

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    Superconducting electromagnets (used in e.g. MRI) do not use energy. Once you have a current circulating in the magnet you can "disconnect the cables" and the field will (ideally) never decay*

    *in reality the field DOES decay, but for a good magnet this happens very slowly.
     
  5. Apr 11, 2008 #4
    SO we can build electromotors with Superconducting that don't use energy !

    YES?!
     
  6. Apr 11, 2008 #5

    pam

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    No. A superconducting motor would not lose energy through I^2R, but energy input is required for the motor to do work. Also, energy is required to refrigerate and keep the wires at a low enough temperature.
     
  7. Apr 11, 2008 #6

    dst

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    In general, that's why we have the definition of WORK. As they say, no such thing as a free lunch - even God agrees. Regardless of how efficient your motor is, and regardless of the fact that if unperturbed it would go on for eternity, the second you attempt to make it do something useful for you is the second it stops being a "perpetual motion machine".

    On the other hand, such motors do have their applications, being ultra-high efficiency and all.
     
  8. Apr 11, 2008 #7
    I don't understand
    If the superconducting have not resistance;
    So where the input energy used?


    Whe have only one superconductor that motor builded with that , if it have not resistance ((don't use energy)) , So which one use the Energy ?????!!!!!
     
  9. Apr 11, 2008 #8
    I don't get how this could possibly happen. If i disconnect the wires, there are two ways:

    i] Disrupt the circuit: In which case the current will be zero and there is no magnetic field.

    OR

    ii] Connect the ends of the superconductor in a circuit. Assuming that this is a superconductor, there is a completely non-resistive circuit. As such, there will be no potential difference across any two points on the circuit. In this case, the current won't flow at all.

    In either case, you need an extra driving potential to maintain the field [as i see it]. But, if i just disconnect the wires, and the field vanishes.. then the potential energy of the inductor will drop. But doing this will violate the law of conservation of energy. What am i doing wrong?
     
  10. Apr 11, 2008 #9
    If we built an electric motor that used superconducting solenoids with no resistance, and we ignore the need for a refrigeration system and whatnot, what will happen when we turn it on (connect the electricity)? Unfortunately, we still haven't got rid of friction produced by the motor as it spins, so in this case it is friction that "uses" the energy. If we somehow got rid of friction, the energy still must go somewhere, like you said. Therefore, I think in this case the energy would go to increasing the speed of the motor

    Rohan: To answer your question about disrupting the circuit, I believe in this case disconnecting the solenoid means disconnecting the power source and connecting the solenoid to itself. If the resistance is zero, the current will continue to flow, and the magnetic field will continue to exist.
     
  11. Apr 11, 2008 #10

    dst

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    Doesn't Newton's first law work in this scenario? If there is 0 resistance but an initial amount of energy it should just keep moving and moving and moving.
     
  12. Apr 11, 2008 #11

    f95toli

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    Start with a closed circuit, i.e. weld (or crimp, it doesn't matter as long as the joint is superconducting) the two ends of the superconducting wire together so that the wire forms a closed circuit.
    Now, in order to be able to "inject" some current into the circuit we use a heat switch. These are made by simply winding a few turns of the superconducting wire around a resistor. The resistor is used as a heater and drives a small part of the superconducting wire normal (i.e. a few tens of cm of wire is heated above Tc) meaning that part of the circuit is now resistive.
    The circuit is of course still closed, but since a part of it is now resistive we can inject current into it by connecting cables to each side of the resistive part, this works as long as the resistance of our source+cables; is much lower than the resistance of the heated part of the wire (just about any current source will do since the resistance of the heated part of the wire is of the order of one ohm).
    Once the field have ramped up the field to whatever strength we need the heater can be turned off, the resistive part of the wire cools down, goes through Tc and becomes superconducting again and the current in the circuit will just circulate forever (well, for a long time).
    Once all the the wire is superconducting the current source can be turned off.

    This is the basic operating principle for all superconducting magnets. I have actually made a few magnets myself (small ones, about 1T) and it is quite easy. All you need is some superconducting wire (e.g. Nb-Ti), a resistor, some copper wire and two current sources (one for the heater and one for the field).
    The main problem is the superconducting joint, welding Nb is difficult but for small magnets you can actually get away with just crimping the two ends together; this will give you an Ic of a few amps which is good enough for small solenoids.
     
  13. Apr 11, 2008 #12

    LURCH

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    If you put the energy into the motor, it will remain there virually forever. If you try to use the motor to do anything, energy is removed. There is no enrgy loss caused by the superconducting magnet itself, but whatevr you attach it to will extract energy
     
  14. Apr 11, 2008 #13
    motors need energy regardless. They operate via an armature that rapidly reverses its magnetic field. I've yet to see a magnet that does this unaided.
     
  15. Apr 11, 2008 #14
    1-The superconducting is very very very stronger than friction in one spot
    2-The Power from superconductor is continuous ((if the power from superconductor arrived wee , so the friction destroyed the power after some circuits and the swing ended , But the power from magnet is strong and perpetual !))

    >> So we enter a little energy for superconductor and it be a perpetual magnet with a high power , So this vigorous magnet can move and move ... until the magnet weaken after some time ... ((i think that it don't weaken ! ))



    I'm right ?!?
     
  16. Apr 11, 2008 #15

    stewartcs

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    There are also friction and windage losses that exist as soon as the motor begins to rotate. So even if you put energy into the motor and run it unloaded without any copper losses, it will still incur a power loss.

    CS
     
  17. Apr 11, 2008 #16

    russ_watters

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    No. Electromagnets can produce a static force and use no energy, but once you start to use that force to move something, energy will need to be extracted from the process. This has nothing to do with friction: a 1 kW (mechanical output) electric motor requires 1 kW of electrical input.

    A moving magnet induces a current in an inductor. A current in an inductor induces a force to move a magnet. These two processes are mirror images of each other - there is a force involved in both of them. Force * motion (velocity) = power
     
    Last edited: Apr 11, 2008
  18. Apr 11, 2008 #17

    russ_watters

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    I prefer Lurch's approach of ignoring friction because - in the case of the op and others - people will get the wrong idea that it is friction that uses up most of the energy in these things, implying that if you could reduce the friction you could end up with a perpetual motion machine. But the fact is that friction accounts for a trivially small fraction of the energy lost by a high quality electric motor. A fraction of a percent. And electrical losses account for only a few percent. The other ~95% of the energy used to power an electric motor is accounted for in the load itself.
     
  19. Apr 11, 2008 #18

    Dale

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    Hi Everyone,

    f95toli has given some excellent descriptions of superconducting magnets, how they work, and how to energize them. One thing that is important to realize is that there is energy stored in the magnetic field itself. In other words, when you are charging up the superconductor a small portion of the energy goes into heating resistive area, but the rest of the energy goes into the magnetic field itself.
     
  20. Apr 11, 2008 #19

    stewartcs

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    I hope they don't get that idea!

    Plus there are still core losses and stray losses that occur in addition to the friction and windage. In total, as a percentage of total power losses, they make up about 40% typically for an induction motor.

    Typical breakdown of the power losses:
    Stator conductor losses, 35-40%
    Rotor conductor losses, 15-25%
    Core losses, 15-25%
    Stray losses, 10-15%
    Friction and Windage losses, 5-10%

    CS
     
  21. Apr 12, 2008 #20
    for an electromagnet is energy required to align the domain is such a way that a piece of steel become a magnet and start attracting other metallic ojects??
     
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