Meissnereffekt
Meissnereffekten er eksklusionen af magnetfelter i en superleder. Denne effekt er en definerende egenskab ved superledere. En almindelig misforståelse er at Meissnereffekten følger af perfekt ledningsevne. Men perfekt ledningsevne fører kun til et konstant magnetfelt i lederen, gennem Faradays lov eller Lenz' lov. En superleder vil ikke blot forhindre ændringer i det interne magnetfelt, men ekskludere magnetfeltet.
Dette kan ses ved at observere en magnet oven på en superleder. Den kritiske temperatur er den temperatur hvorunder et materiale bliver superledende. En magnet stilles på en superleder ved en temperatur over superlederens kritiske temperatur, og når temperaturen sænkes til under den kritiske vil magneten levitere, grundet eksklusionen af det magnetiske felt.
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Forfatter/Opretter: Geek3, Licens: CC BY-SA 3.0
Diagram of a solenoid and its magnetic field lines. The shape of all lines was computed according to the laws of electrodynamics.
Nuclear physics
Diagram of the Meissner effect. Magnetic field lines, represented as arrows, are excluded from a superconductor when it is below its critical temperature.
Forfatter/Opretter: Maxim Bilovitskiy, Licens: CC BY-SA 4.0
A superconductor or a high-temperature superconducting ceramics is an alloy of oxides of yttrium, barium and copper in proportions (which you see on the screen) YBa2Cu3O7-x and abbreviated as YBCO. In order to demonstrate the properties of this object let’s conduct an experiment. First, let’s take a small piece of ceramic and then put a piece of styrofoam and a powerful neodymium magnet on top of it. Next, the superconductor is cooled with liquid nitrogen down to -196 degrees Celsius. After the ceramic is cooled, take a piece of styrofoam out from underneath of the magnet. The effect of levitation is caused by the ceramics cooling down to the critical temperature, it then becomes a superconductor, as well as a perfect diamagnetic.
This means that it can repel any magnetic field, as well as create its own when being near a strong magnetic field. The phenomenon is called the Meissner effect.
If the magnet is lifted the conductor next to it begins to lift with it as well. The magnet and the superconductor are almost like "frozen" together in space.