Magnetic Levitation and the Meissner Effect
The Maglev train train is based on the principles of magnetic levitation
As the resistance of a superconductor is zero, the electric field inside it must also be zero; otherwise, the lack of resistance will allow the currents to be infinite. Therefore, the magnetic field inside a superconductor cannot change, it must be constant, since, as we know from Faraday’s Law, any change in the magnetic field produces an electric field.
Suppose then that a superconductor is placed in a region in which initially the magnetic field is zero. Then we bring a magnet close to this conductor. Since the magnetic field inside the superconductor is initially zero and cannot change, it must remain zero.
According to Faraday’s Law, the approach of the magnet induces currents in the superconductor, which induces a magnetic field that cancels the field of the magnet, that is, that generates a field opposite to that of the magnet, producing a repulsion. As it is a superconducting material, the current persists and the magnet levitates indefinitely.
We have already observed that the magnetic field inside a superconductor is constant. We can detail this information further, reporting that physicists WH Meissner and Robert Ochsenfeld showed that the magnetic field of the superconductor is not limited to being just constant, it is null. Thus, if a superconducting material, initially at a temperature higher than T c , that is, in a state in which it cannot be considered superconducting, is subjected to a magnetic field and then cooled to a temperature lower than T c , it expels all the magnetic field from its interior.
This effect, called the Meissner effect, explains a second type of levitation. Suppose a magnet is placed on a superconducting material above the temperature T c , that is, still in a situation where it is not a superconductor. When the temperature drops below T c , the material becomes superconducting and generates currents that expel the magnetic field, which is, the magnet rises, starting to levitate above the superconductor.