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Is it true that superconducting electromagnets don't need any power? So... energy to created a magnetic field via a superconductor would be zero?

Since resistance is zero, does that also mean that the time it takes for current to flow & for the magnetic field to induce is instantaneous? The issue of self-inductance is irrelevant?

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    $\begingroup$ The self-inductance is quite relevant, as you guessed. Furthermore, the fact that the superconductor doesn't lose energy as time goes on does not mean it doesn't take energy to build up the magnetic field in the first place. $\endgroup$
    – DanielSank
    Commented Oct 8, 2014 at 3:16
  • $\begingroup$ The superconducting part of the circuit doesn't suffer resistive losses, which is a big win, but for practical current tech magnets you have to keep them cool which is expensive. That's why they are only used in places where you need very high fields for sustained periods. $\endgroup$ Commented Oct 8, 2014 at 3:21
  • $\begingroup$ @dmckee, how are they usually cooled, there are the cryogen free ones? Also, could these electromagnet be one of the fastest? Could they be light as well in mass? relative to copper base ones. $\endgroup$
    – Pupil
    Commented Oct 8, 2014 at 3:55
  • $\begingroup$ All the ones I'm familiar with are liquid helium cooled. I don't know if LN2 temperature magnets have made it to commercial production yet, but then it is not a field I pay close attention to. $\endgroup$ Commented Oct 8, 2014 at 3:59
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    $\begingroup$ The current in a superconducting magnet can only be changed slowly, otherwise the magnet will "quench": en.wikipedia.org/wiki/Superconducting_magnet#Magnet_quench. So quite the contrary, superconducting magnets have to be treated like raw eggs, otherwise they do nasty things, and that is, for one thing, not fun, for another it may be very costly. $\endgroup$
    – CuriousOne
    Commented Oct 8, 2014 at 6:28

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Is it true that superconducting electromagnets don't need any power? So... energy to created a magnetic field via a superconductor would be zero?

They have to be at very low temperatures, that takes a lot of energy.

The ATLAS Barrel Toroid was first cooled down over a six-week period in July-August to reach –269°C . It was then powered up step-by-step to higher and higher currents, reaching 21 thousand amps for the first time during the night of 9 November. This is 500 amps above the current needed to produce the nominal magnetic field. Afterwards, the current was switched off and the stored magnetic energy of 1.1 GigaJoules, the equivalent of about 10 000 cars travelling at 70km/h, has now been safely dissipated, raising the cold mass of the magnet to –218°C.

So it also takes a lot of energy to build up the magnetic field and the energy is stored ; one has to be careful in switching off that disaster does not happen.

Since resistance is zero, does that also mean that the time it takes for current to flow & for the magnetic field to induce is instantaneous?

There is the bound of the velocity of light, for everything. Nothing is instantaneous in electromagnetism of anything else.

The issue of self-inductance is irrelevant?

In charging the magnet and in a controlled quench it is relevant.

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Once produced, any magnet does not give nor take energy from the environment. The magnet field lines are closed and could only be bend (to work with some physical model).

Any magnet, a permanent as a electric as a superconducted, works like a spring in mechanical systems. This you can see by the way that an electric or a permanent magnet from the same strength induce the same Lorentz force and "bend" the trajectory of electron by the same way.

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    $\begingroup$ The question is about electromagnets, in particular about the energy cost of running them. $\endgroup$ Commented Oct 8, 2014 at 5:50
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Any work done by a supercooled magnet would dissipate energy. An electric induction motor using supercooled state and field would simply have nearly zero internal resistive loses, resulting a motor nearly 100% efficient. Yes every Joule of work output would require at least a joule of input energy.

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