Why does the magnet continue to levitate above the superconductor for a while af

Question

Why does the magnet continue to levitate above the superconductor for a while after you remove the sample from the liquid nitrogen? The sample and brass structure develops a layer of frost only after the liquid has all boiled away. Why is this? The transition to zero resistance is gradual with temperature rather than abrupt. Why? A simple two-probe measurement of resistance as a function of temperature yields a non-zero value for the resistance even below the superconducting transition. Why? Why is it necessary to use a small, but very strong, permanent magnet in the demonstration of the Meissner effect? Draw a free-body diagram for the levitated magnet.

Explanation / Answer

(a) The superconductors expel magnetic field and hence they repel each other. Now the magnet stays as a superconductor as long as the temperature is below a specific range. Now once the temperature has incrased beyond that limit by bringing it away from liquid nitrogen it will stop repelling.

(b) the frost is due to moisture outside. As long as the liquid nitrogen was there the sample is in cold surroundings but once that is gone, the sample is at very low temperature compared to the surroundings. So the moisture will be frozen on the low temperature sample.

(c) Normal conductors have the property that with the decrease in temperature the resistance gradually decreases. But only with the superconductors, the resistance falls to absolute zero from finite value. This transition to superconducting state happens below the critical temperature.

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