questions on superconductor electromagnet and soft iron core

Discussion in 'General Electronics Chat' started by henry newton, Jul 23, 2008.

  1. henry newton

    Thread Starter Member

    May 5, 2008

    The Meissner Effect is an effect whereby the magnetic field created in a superconductor will repel all other magnetic fields, regardless of whether they are changing or not. This means that if a magnet is placed over a superconductor it will levitate there inside the magnetic field.

    The amazing fact about the magnetic levitation observed in superconductors is that even
    though the two objects repel each other, they are not pushed away entirely, but remain ‘stuck’ a certain distance apart. If two normal magnets’ North poles where placed facing each other, the magnets would be pushed apart by a force that exists (even though minutely) to a distance of infinity.

    However, a superconductor will repel a magnet a certain distance but then keep it at that
    distance. This is seen effectively in the video where, initially, the objects are kept apart, but when the magnet is lifted, the superconductor comes with it.

    This is due to the way in which a superconductor sets up its magnetic field. When a magnetic field is created in a superconductor, poles are created to repel all fields.

    The Meissner Effect is different from regular diamagnetism in that it repels all magnetic fields, not just changing ones. Unlike a regular magnet, which has just a North Pole and a South Pole, a superconductor can create many poles to ensure that all poles are repelled depending on what it is trying to repel. This same effect, however, is responsible for holding the magnet at a certain distance away. This is because when a magnet is pulled away from it, the poles are reversed to hold the magnet in place.

    The typical way of making an electromagnet is to place, say a soft iron inside a copper wire solenoid carrying direct current. Thus, a NORTH POLE is form at one end of the solenoid and a SOUTH POLE at the other end. Therefore, if the north pole of a permanent (say bar) magnet is brought to the north pole of the solenoid (electromagnet) it will be repel away. If the south pole of the permanent magnet is brought close to the north pole of the solenoid it will be attracted to it.

    Now what if instead of a copper wire, a superconductive wire is used to form the solenoid and a soft iron is place inside it, then everything is dip into a liquid helium or nitrogen in other to cool it below its critical temperature, thus making it to be superconductive (with zero resistance).Once the direct current source is turn on, electric current flow through the solenoid of superconductive wire, therefore an electromagnet is created. In the context of those facts about superconductor with respect to the phenomenon called MEISSNER EFFECT stated above, would the superconductive electromagnet behaves like the copper electromagnet when a permanent magnet is brought close to it( i.e. their like poles of the superconductive electromagnet and the permanent magnet repels each other while their unlike poles attract each other.)



    1. Soft iron is attracted to a permanent magnet when it is brought close to it.

    2. When a soft iron is place inside a solenoid carrying an electric current it becomes magnetized, thus an electromagnet is form.

    Now, my question is this: Is there a particular material that lacks the first property (1) stated above (i.e it does not get attracted to a permanent magnet when brought close it) but still possess the second property(2) (i.e it becomes magnetized when place in a solenoid to form an electromagnet.

    Thanks .

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  2. theamber

    Active Member

    Jun 13, 2008
    First Superconductors act very weird indeed because of the lack of resistivity.
    Superconductivity does not occur in noble metals, nor in most ferromagnetic metals. All superconductors have zero resistivity to low applied currents when there is no magnetic field present. The physical properties of superconductors vary from material to material, such as the heat capacity and the critical temperature,critical current density, Because of these properties it implies that superconductivity is "thermodynamic phase" dependand (which is similar to the states of matter and diferent properties they have depending on the state they are, e.g. solid, liquid...). This make superconductivity possess certain properties which largely are independent of microscopic details.
    Answer 1. I don't think you can create an permanent electromagnet with superconductivity. First an electromagnet is created through "remanent magnetism" in order to be a permanent magnet. With a superconductor you can create what is called "superdiamagnetism" phenomenon.
    Why is that, I asume it has to do with the penetration depth(absortion of electromagnetic waves), the magnetics lines of force differences and lack of frictions between electrons. You can read volumes about it on Wikipedia.
    Answer 2. If it is magnetized then it has the property of attraction, one cannot exist without the other. Many materials remain temporary magnetic depending on current flow but do not became permanet magnets.