The Meissner Effect: What Causes Magnet Repellency?

[leroyjenkens]

My question is why does the material having no electrical resistance cause it to repell a magnet?

And how can it repell and attract at the same time? I don't understand how the "flux trapping effect" works.
I watched this video and it got me really interested in it.
Thanks.

 

[Gokul43201]

Here's a simplistic, hand-waving picture.

Do you know what happens if you try to subject a regular metal to a magnetic field? Have you heard of eddy currents? There's a similar reaction when you try to subject a zero-field cooled SC to a magnetic field. Eddy currents are set up along the surface of the SC which oppose the change in the applied field.

However, there's a limit to how much magnetic field a SC can push out against. If you exceed this critical field, then it becomes energetically easier for the SC to just turn into a normal material and let the field through. In certain kinds of SC materials (Type II SCs), however, before completely giving up all hope, the SC chooses (for a certain range of fields) an intermediate phase where it let's small portions of itself turn into a normal material and let the field pass through only those regions. This is what leads to flux trapping. The rest of the material is still a SC an still repels the applied field.