In 1842, British physicist Earnshow proposed the famous Earnshaw theory when he studied the stable static problem of point particle set under the action of electrostatic force.
Earnshaw pointed out that two conditions are required for an object to be in a stable state of rest, that the resultant force of the object at the equilibrium point is zero and the flux of the resultant force of the object at the equilibrium point is less than zero. The second condition is that the resultant force flux is less than zero, which ensures the dynamic stability of the object, that is, after the object is disturbed, the resultant force it receives still points to the equilibrium point.
The force between permanent magnets and ferromagnetic substances and permanent magnets satisfies the inverse square relationship, so according to Earnshaw theory, it is impossible to generate stable magnetic levitation between permanent magnets or between permanent magnets and ferromagnetic substances.
The two conditions in Earnshaw’s theory were later applied by Braunbeck in 1939 to the problem of the stability of magnetic or dielectric media in magnetic or electric fields. Brubeck pointed out that a magnetic medium with a relative permeability less than 1 can remain stable in a static magnetic field.
A magnetic material with a relative permeability less than 1 usually becomes a diamagnetic material, and this kind of magnetic levitation is usually called a diamagnetic levitation. However, the absolute value of the magnetic susceptibility of the diamagnetic substance is often very small. The diamagnetic substance with the largest magnetic susceptibility is pyrolytic carbon, and its magnetic susceptibility is -400e-6, so the diamagnetic substance suspension often requires a relatively strong magnetic field. Metals such as bismuth, antimony, water, and most organic substances are diamagnetic substances. Andre Geim, the 2010 Nobel Prize winner in physics, successfully levitated a frog with a 16T magnetic field for the first time in 1997.
Superconducting Repulsive Suspension
Except for the diamagnetism, when the superconductor is in the superconducting state, because the Meissner effect magnetic flux cannot penetrate the superconductor, its magnetic susceptibility is equal to -1, so it meets the conditions of Brubeck’s inference. This suspension is called superconductor repulsive suspension.
Superconducting Pinning Suspension
For the non-ideal type II superconductor, it has a pinning effect. When the non-ideal type II superconductor is in a mixed state, the non-ideal type II superconductor can capture and pin the magnetic field lines and generate a pinning force with the magnetic field. The pinning effect can also produce magnetic levitation, which is called pinning levitation or quantum levitation.
For conventional magnetic media, such as weak magnetic materials, the relative permeability is close to 1. When an eddy current is generated on its surface or inside, it is equivalent to a diamagnetic effect, so that the equivalent relative permeability of the object is less than 1. Of course, vortex suspension can also be used, and Lenz’s law can be explained. Eddy current levitation usually uses AC power, electromagnets and conductive weak magnetic materials such as copper and aluminum as levitating objects.
Electric levitation is also the principle of eddy current, but the eddy current is not generated because of the alternating magnetic source, but because of the relative movement between the magnet and the weak magnetic material. For example, if a magnet (permanent magnet or superconducting magnet) moves quickly on an aluminum plate or copper plate, eddy currents will be induced on the aluminum plate or copper plate, and the eddy current will hinder the relative movement of the magnet and the copper plate and the aluminum plate. Objects can be levitated by buoyancy.
Active control suspension
Actively controlled suspension is a suspension technology that uses closed-loop control for the suspension system that cannot be self-stabilized. Electromagnetic suction suspension is a kind of active control suspension.
In addition, there are various forms of suspension technology such as resonance suspension and spin-stabilized suspension.
Post time: Aug-06-2022