One, two, three, levitate!
A magician places a blanket over a person. Three swings of the wand, the blanket off, and the person is slowly rising above the table, levitating.
Sure enough, most of us have seen this trick at least once in our lives. It is not a secret that, in reality, the whole magic of the wand is hidden in a combination of high skill, durable preparation, and inattention of a beholder.
However, does levitation exist in the real world? It will be a pity if people can see levitation only with the help of trained magicians. Fortunately, there is nothing to worry about as long as physics is around! In this article, we will discuss three ways of producing levitation using magnets and magnetic fields.
The phrase “it’s just magic!” is way too simple to explain the complex mechanisms behind real levitation. The real explanation requires us to go deeper in a jungle of the portion of physics called magnetism and electricity.
The first step in getting through this wild but breathtaking journey is to expose what an electric current is. An electric current is an ordered stream of charged particles: ions or electrons (electrons have a negative charge) flowing, for instance, through a conductor. The basic copper wire is considered a conductor — a type of material that has unbounded outer electrons moving freely in a material. Sometimes, these free electrons are even referred to as an electron gas because of how much freedom they have. Electrons are responsible for the appearance of an electric current in a conductor.
Notice that an electric current is not just “a stream of charged particles” but an “ordered one.” The order comes not from electrons’ whim, from an electric field, which is created by either electric potential or a changing magnetic field, which is going to be one of the most crucial aspects of magnetic levitation.
When an electric field is acted on electrons in a conductor, they start moving in a particular direction, acquiring a so-called drift velocity. Furthermore, our small charge carriers have another velocity — thermal velocity. This velocity is much higher than the drift one, but it does not have a particular direction, making electrons move chaotically. So, electrons are moving very slowly to their “destination” (drift velocity), determined by an electric field, and, at the same time, they are going crazy (thermal velocity). Thus, the lower the drift velocity, the lesser the electric charge will pass through a chosen segment of a wire over time or, in other words, the smaller an electric current will be.
For instance, increasing the temperature of a conducting wire will increase the thermal velocity of electrons. Apart from electrons, conductors have atoms, ions. With the increase of a thermal velocity, electrons are going to “bump into” those atoms more frequently, and the flow will slow down. (Imagine a car trying to ride at a high speed through the road filled with other vehicles: it is going to hit and slow down every time, decreasing the overall velocity to the desired destination.) Hence, the resistance of material will increase, leading to a decrease in the current. (It should be noted that in this model we imagine electrons, atoms, ions as small little spheres, but, in real life, it is not pretty accurate).
However, what if we cool down a material? We cannot decrease the temperature to any possible value — there is a limit of the lowest point possible: 0 Kelvin or -273,15 Celsius. By approaching this temperature, the resistivity of a conductor will approach zero as well (almost removing it). This type of material is superconductors. Even its name “super” and “conductor” gives a hint of the nature of the material.
It was mentioned that a changing magnetic field is a crucial aspect of creating levitation. A magnetic field is produced whenever a charged particle is moving. The most mind-blowing thing about a magnetic field is that a changing magnetic field will create a changing electric field and vice versa — a changing electric field will create a changing magnetic field. Without this incredible property of two fields, we could not survive since light, an electromagnetic wave, would not exist. However, the change in magnetic flux (not in a magnetic field) will be used when we are going to talk about inducing an electric current. Magnetic flux is the same as a magnetic field but with two exceptions — it takes into consideration the area of the circuit/material through which a magnetic field passes as well as the angle between a vector area of a circuit and magnetic field lines.
Thus, when magnetic flux is going to change, it will be because of the change whether in the angle between magnetic field lines and a vector area of a circuit, in the area through which magnetic lines go through, or in the magnetic field itself. Either way, the amount of a magnetic field going through the circuit will be changing as well. That is why it is important to be on the lookout for the change not only in a magnetic field but in the magnetic flux overall.
It brings us to the most significant law — Lenz’s law. It is saying that if we have a circuit (or a material) in changing magnetic flux, it will induce an electric current in a circuit which will move in such direction as to create its magnetic field, which will oppose the initial change in magnetic flux. It sounds heavy, but, in other words, we can say that materials/circuits can’t stand changing the amount of a magnetic field that passes through them, so they try their best to compensate for this change. The way they do this is by producing their magnetic field, which opposes the initial change of magnetic flux.
The one demonstrative example of this phenomenon is the simplest generator. Take one loop and bring it into a constant magnetic field. Wait, wait! If a magnetic field is constant, how are we going to change it? Remember, when we talk about circuits/materials, we are going to look at the change in magnetic flux, not a magnetic field itself. So, how does it work? When the loop is in a constant magnetic field, we start rotating it. The amount of a magnetic field that now passes through the open area of the loop is going to change because of the change in angle.
We can view it as if our loop did not move and the magnetic field itself just changed. Because of the change in magnetic flux, an electric current is induced and, therefore, its magnetic field is created in a way as to oppose the change in magnetic flux. Therefore, we get electricity.
Now, finally, we can get magnetic levitation.
The first way of producing magnetic levitation involves a superconductor. If a permanent magnet is placed above a superconductor, it will levitate. Why? Even though a magnetic field of a permanent magnet does not change, when we put it above a superconductor, the magnetic flux would change, increasing from zero to some particular value. However, superconductors cannot have any (not only the change) magnetic flux inside them. The reason is that the electromotive force (ε) of a superconductor is zero (ε=I*R=I*0=0), which means that magnetic flux is zero as well (ε=dФ/dt=0 => dФ=0 — the change in magnetic flux). Thus, the magnetic field of the magnet induces an electric current inside the superconductor to repel any magnetic flux. Because there is no resistance — no waste of energy — the initially induced current will not decrease. Thus, even though the magnetic field will be steady, the current is still going to flow in such a direction as to create an opposing magnetic field, repelling the initial magnetic field and keeping the magnet steady.
The second way is to use a conductor and a permanent magnet. We place a conducting plate and move a permanent magnet right above it (that is how some magnetic trains work). Imagine the magnet is only starting to enter the space, which is right above a small portion of the conducting plate. On its way, it changes the magnetic flux of the portion of the plate which is exposed to the magnetic field of the permanent magnet. According to Lenz’s law, a current inside of this part of the plate occurs, producing an opposing magnetic field. This field will try to cancel the change of the magnetic flux, therefore, it will be headed in the opposite direction comparing to the initial magnetic field, pushing our permanent magnet away. Throughout the whole area of the conductor, the permanent magnet will change the magnetic flux of the plates’ new portion, inducing the current and keeping itself in the air.
The last way which we are going to discuss involves a conducting plate and an electromagnet instead of a permanent magnet. An electromagnet consists of a ferromagnetic material (a magnet) that is wrapped in a coil with a changing electric current running inside it. We know that an electric current, being moving charged particles, produces a magnetic field. Thus, a changing electric current will produce a changing magnetic field. If we place this electromagnet above a conductive plate, according to Lenz’s law, a changing magnetic field will induce a current inside the conductor, which will create an opposing magnetic field, keeping the electromagnet floating.
Now, when the next time you see a magic trick with levitation, you will be certain that this magic trick can be turned into reality with a pinch of physics.
