Category: magnetically levitated trains

What is a magnet?

A magnet is an object that has magnetic poles and therefore exerts forces or torques (twists) on other magnets. There are two types of these magnetic poles—called, for historical reasons, north and south. Like poles repel (north repels north and south repels south) while opposite poles attract (north attracts south). Since isolated north and south magnetic poles have never been found in nature, magnets always have equal amounts of north and south magnetic poles, making them magnetically neutral overall. In a permanent magnet, the magnetism originates in the electrons from which the magnet is formed. Electrons are intrinsically magnetic, each with its own north and south magnetic poles, and they give the permanent magnet its overall north and south poles.

What would be a legitimate form of propulsion for magnetic trains? — DS, Kenton, OH and MB, Willows, CA

The most sensible propulsion system for a magnetically levitated train would be a linear electric motor. This motor would consist of electromagnets on the train and electromagnets on the track. By turning these electromagnets on and off at carefully chosen moments, they can be used to pull or push the train forward for propulsion or backward for breaking. The timing is important because, for propulsion, the magnet on the train must always be attracted toward the track magnet in front of it and repelled by the track magnet behind it. For breaking, this relationship must be reversed.

What is a superconducting magnet? — JS, Montreal, Quebec

Electric currents are magnetic. That’s the basis for electromagnets—if you run an electric current around a coil of wire, that coil of wire will develop a north magnetic pole at one end and a south magnetic pole at the other end. But an electromagnet made with normal copper wires consumes electric power all the time. The current passing through those wires wastes energy because of friction-like effects in the copper and the wires become hot. The electromagnet also needs a power source to keep its current flowing.

However, a superconducting electromagnet is one in which the wires are superconducting—the current passing through them doesn’t waste any power. Once a current has been started in a coil of superconducting wire, it flows forever. Since it doesn’t waste any power, that current needs no source of power and produces no thermal energy. In fact, you can buy superconducting magnets with the current already started at the factory. As long as the wires are kept cold (as they must be to remain superconducting), the current will continue to flow and the coil will remain magnetic forever.

Does magnetism affect the growth of plants? If so, how? — JA, Somerville, MA

I am not aware of any effects of magnetism on plant growth. The effects of magnetism on most molecular processes are incredible slight and I don’t see how any but the most extreme magnetic fields could affect plant growth.

What is the physical nature of magnetism? Is it a wave or particle phenomenon or an undefined energy like gravity? — GA, Paisley, Scotland

Magnetism is one sector of the electromagnetic interactions of matter. From a classical perspective, magnetism consists of an energy-containing field that surrounds magnetic poles and that exerts forces on other magnetic poles. At a higher classical level, magnetism and magnetic fields are part of the full electromagnetic interaction, meaning that they are inextricably mixed with electricity and electric fields. Finally, from a full quantum mechanical perspective, magnetism is associated with energy-containing quantum fields, the fields of quantum electrodynamics, that govern the electric and magnetic interactions of matter. These quantum electrodynamic interactions are mediated by virtual photons, cousins of the real photons that include light and radio waves. From this quantum viewpoint, magnets interact with one another by exchanging virtual photons and, like all quantum objects, these photons are emitted and absorbed like particles but travel as waves. Thus magnetism is both a wave and particle phenomenon. It isn’t undefined at all; in fact, quantum electrodynamics is probably the most well-established and precise theory in modern physics.

Are there any levitated trains in the world? – BP

At present, I believe that the only magnetically levitated trains are those undergoing development and testing.

How does a magnetic train work? How can I make an experiment with it for a school project? — AASE, Quito, Ecuador

There are many techniques for supporting a train on magnetic forces, but the simplest and most promising involves electrodynamic levitation. In this technique, the train has a strong magnet under it and it rides on an aluminum track. The train leaves the station on rubber wheels and then begins to fly on a cushion of magnetic forces when its speed is high enough. Its moving magnet induces electric currents in the aluminum track and these currents are themselves magnetic. The train and track repel one another so strongly with magnetic forces that the train hovers tens of centimeters above the track.

To demonstration this effect, you can lower a very strong magnet above a rapidly spinning aluminum disk. In my class, I spin a sturdy aluminum disk with a motor and lower a 5 cm diameter disk magnet onto its surface. I hold the magnet firmly with a strap made of duct tape, so that the magnet won’t fly across the room or flip over as it descends. Instead of touching the spinning disk, the magnet floats about 2 cm above it. If you try this experiment, don’t spin the aluminum disk too fast or it will tear itself apart. It should spin about as fast as an electric fan on high speed. Also, be careful with the magnet, because it will experience magnetic drag forces as well as the magnetic lift force. If you don’t hold tight, it will be yanked out of your hand.

For a simpler experiment that anyone can do, float an aluminum pie plate in a basin of water and circle one pole of a strong magnet just above its surface. The pie plate will begin to spin with the magnet. You are again inducing currents in the aluminum, making it magnetic. While the forces here are too weak to lift the magnet in your hand, they are enough to cause the pie plate to begin spinning, even though you never actually touch it. This technique is used in many electric motors. That’s physics for you—the same principles just keep showing up in seemingly different machines.

Can we create an invisible wall of electromagnetic fields? – AW

Not really. While you could make an electromagnetic “wall” of laser beams or X-ray beams, it wouldn’t really be “invisible” and it wouldn’t feel like a solid wall. It would just cause injury if you put your hand through it. For a surface to feel like a wall, it would have to push your hand backward if you tried to move your hand through it. A real wall does just that and it does so with electromagnetic forces—when you touch a wall, electromagnetic forces that the wall’s atoms exert on your atoms push your hand back and prevent it from penetrating the wall. So a clear window could be described as an “invisible wall of electromagnetic fields,” but that isn’t really what you want. A freestanding electromagnetic field, one that doesn’t involve atoms yet prevents your hand from penetrating it, just isn’t possible.

I have heard of a magnetic top that will spin on top of another magnetic field because of the gyroscopic effect. If that is put into a vacuum chamber, would it spin perpetually? — JH, Visalia, CA

Probably not. The magnetic top that you mention is a wonderful invention, sold under the name “Levitron”. It uses gyroscopic precession to stabilize what is normally an unstable arrangement: two oppositely aligned magnets, one supporting the other. In air, you can get the Levitron top to stay aloft for a couple of minutes before its spin rate declines to the point where it stops being stable. In a vacuum, I’d expect it to last much longer but not forever. Thermodynamics overwhelms just about everything sooner or later and the Levitron won’t be an exception. Even if you get rid of air resistance, the spinning top’s strong magnetic field will interact with its environment and will allow the top to exchange energy with that environment. While there is always the possibility that these exchanges will make the top spin faster, such favorable exchanges are relatively unlikely. Instead, the energy exchanges are much more likely to extract energy from the top and slow it down. For example, any conducting surfaces near the Levitron top will exert a magnetic drag force on the top and will convert its energy into thermal energy in those conducting surfaces. Forever is a long time and the top will certainly slow to a stop eventually. Still, it might be interesting to see how long it can stay spinning. I’ll bet 10 minutes is the realistic maximum. If I have a chance to test it out, I’ll let you know what happens.

My son and I are building an electromagnet for a science project. We know that if we wrap the wire around the nail and connect the battery to the wire…presto, a magnet is born. But what is it about flowing current that allows this to happen? — GG, Westfield, NJ

Moving electric charges are inherently magnetic. That’s because electricity and magnetism are intimately related and aren’t really separate phenomena. To see why this is true, imagine two electrons sitting motionless in front of you—they push one another away with electric forces. But now imagine that you and those two electrons are moving northward in a train and someone standing beside the track is watching all of you pass. From that person’s perspective, the two electrons are moving and they exert both electric and magnetic forces on one another. What appears to you to be a purely electric effect appears to the person near the track to involve both electricity and magnetism. Without the appearance of magnetic effects in moving charges, grave inconsistencies would appear in the dynamics of objects view from different perspectives.

So the current in the wire of your electromagnet is inherently magnetic. The magnetic field it produces aligns the tiny magnetic domains in the steel nail so that the nail’s magnetic field greatly strengthens that of the current in the wire.