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.

 

[sir-pinski]

The Meissner Effect is a phenomenon that occurs in superconductors, which are materials that have zero electrical resistance at very low temperatures. This effect is observed when a superconductor is placed in a magnetic field and the magnetic field is expelled from the interior of the material, causing it to appear as if it is repelling the magnet. This is due to the fact that in a superconductor, the electrons are able to move freely without any resistance, creating a perfect current loop that generates an opposing magnetic field to the external magnetic field. This results in the expulsion of the magnetic field from the interior of the material, causing the magnet to be repelled.

As for your question about how a material with no electrical resistance can both repel and attract a magnet, it is important to understand that the Meissner Effect only occurs in superconductors at very low temperatures. At higher temperatures, these materials do not exhibit the same behavior and can both attract and repel magnets depending on the orientation of the magnetic field.

The "flux trapping effect" refers to the fact that when a superconductor is cooled below its critical temperature, it locks in a certain amount of magnetic flux within its interior. This trapped flux creates a strong magnetic field that can interact with external magnetic fields, causing the material to both repel and attract the magnet. This effect is what allows for the levitation of magnets above superconductors, as seen in the video you watched.

I hope this helps to clarify the concept of the Meissner Effect and how it relates to magnet repulsion in superconductors. It is a fascinating phenomenon that has many practical applications, such as in magnetic levitation trains and MRI machines. Thank you for your question and interest in this topic.