Meissner Effect: Levitating a Magnet Before Cooling

[niehls]

I've just successfully built and tested a superconductor. I've seen the meissner effect in action but there is one thing I'm wondering. If the superconductor is cooled below its critical temperature, and the magnet is then put on top it floats, as it should. It also levitates if the magnet is first put on the superconductor and it is then cooled. This behaviour seemes strange to me. If the magnetic field is present when it makes the transition to superconducting state shouldn't it "just be happy with that", and not try to prevent the magnetic field by inducing a current. I want the current to be induced when the magnet is removed, to counteract the diminishing magnetic field. I need some explanation why the magnet levitates if it is placed on the supraconductor before it is cooled.
Thanks

 

[ZapperZ]

I've just successfully built and tested a superconductor. I've seen the meissner effect in action but there is one thing I'm wondering. If the superconductor is cooled below its critical temperature, and the magnet is then put on top it floats, as it should. It also levitates if the magnet is first put on the superconductor and it is then cooled. This behaviour seemes strange to me. If the magnetic field is present when it makes the transition to superconducting state shouldn't it "just be happy with that", and not try to prevent the magnetic field by inducing a current. I want the current to be induced when the magnet is removed, to counteract the diminishing magnetic field. I need some explanation why the magnet levitates if it is placed on the supraconductor before it is cooled.
Thanks

What you are describing is exactly the difference between a diamagnet property of a "perfect conductor" and an ideal diamagnet property of a superconductor. A perfect conductor will be "happy" with the trapped field when it becomes a perfect conductor. A superconductor is a more special case and requires the expulsion of all magnetic field from the superconducting region.

If you want an "explanation" for why this is, you will need to apply Maxwell equation into the London equation to obtain the special case for the Meisner effect. I'm not sure this is what you want.

Zz.