Understanding the Mechanism of Magnetic Field Opposition in Perfect Conductors

[NaTh2007]

In a theoretical perfect conductor, when it is placed into an already existing magnetic field it equally opposes the field. This is due to the induced current within it giving rise to a new magnetic field that opposes the existing one.

However when a superconductor is already in the magnetic field and you try to move it out, it opposes this change and "wants to stay" in the field. Why is this? I have read Lenz's Law but do not fully understand the mechanism behind it. I would like to know exactly why this effect of the magnet opposing it's own removal out of the field takes place.

Thank you very much

 

[Valerie Park]

. The reason a superconductor opposes the removal of a magnetic field is due to the Meissner effect. This effect occurs in superconductors because they have zero electrical resistance, which allows them to expel any external magnetic fields from their interior. This expulsion is caused by an induced electric current that flows on the surface of the superconductor in order to create its own magnetic field that opposes the external one. This process is described by Lenz's Law, which states that an induced current always flows in a direction that creates a magnetic field opposing the change in the magnetic field that created it.

 

[sir-pinski]

for your question. The phenomenon you are describing is known as the Meissner effect, and it is a key characteristic of superconductors. To understand why this effect occurs, we need to first understand the concept of a perfect conductor and a superconductor.

A perfect conductor is a theoretical material that has zero electrical resistance, meaning that electrons can flow through it without any resistance or loss of energy. In this case, when a perfect conductor is placed in a magnetic field, the electrons within the material will experience a force due to the magnetic field. This force will cause the electrons to move, creating an induced current, and this current will generate a magnetic field that is equal in strength but opposite in direction to the external magnetic field. This is why a perfect conductor opposes the external magnetic field, as you mentioned.

Now, let's move on to superconductors. A superconductor is a material that, when cooled below a certain temperature called the critical temperature, exhibits zero electrical resistance and perfect diamagnetism. This means that a superconductor will expel any external magnetic field from its interior, causing the Meissner effect. This is because, at the critical temperature, the electrons in a superconductor form pairs called Cooper pairs, and these pairs can move through the material without any resistance. When an external magnetic field is applied, the Cooper pairs will experience a force that causes them to move in a circular motion, generating a current that creates an opposing magnetic field. This opposing field effectively cancels out the external field, causing the superconductor to expel the field from its interior.

So, why does a superconductor "want to stay" in the magnetic field? This is because, as long as the superconductor remains in the external magnetic field, the Cooper pairs can continue to move in a circular motion without any resistance. However, if the superconductor is moved out of the field, the Cooper pairs will no longer be able to maintain this circular motion, and the superconductor will lose its superconducting properties.

In summary, the Meissner effect in superconductors is a result of the perfect diamagnetism exhibited by these materials at their critical temperature. The Cooper pairs within the superconductor create an opposing magnetic field, which causes the material to expel the external magnetic field. The superconductor "wants to stay" in the field because it allows the Cooper pairs to continue their circular motion and maintain the super