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Magnetically Levitated Trains
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512. How does a magnet work and is there a way that I can determine which end of the magnet is north and which end is south?
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The magnetic fields that are responsible for the interesting behaviors of magnets can be created either by (1) moving electric charge or (2) changing electric fields. We can ignore the second process because it has very little to do with permanent magnets. Instead, let's focus our attention on the first process: moving electric charge producing magnetic fields. Whenever electric charges flow through a wire, a phenomenon that we call an electric current, they create magnetism. Many appliances use electricity and electric currents to create magnetism, notably televisions, motors, and audio speakers. But a permanent magnet doesn't use an obvious electric current to create its magnetic field. Instead, it uses the spinning character of the electrons inside the material from which that magnet is made. Electrons are electrically charged and they have an intrinsic spinning character. A simplistic view of an electron is as a spinning, electrically charged ball. Since its charge is in motion, an electron acts as a magnet and has both a north pole and a south pole. In most materials, the magnetic electrons are turned in opposite directions, canceling out one another's magnetism so that the overall material is non-magnetic. But in a few special materials, including most steels, the cancellation is imperfect and some magnetism remains. In a permanent magnet, this remaining magnetism is particularly apparent. The material is, in effect, a big collection of magnetic electrons that all work together to create a large magnet.

To determine which end of a permanent magnet is its north pole and which is its south, take a compass and hold it a reasonable distance from one end of the magnet. If the north end (often the red end) of the compass needle points toward this end of the magnet, you know that this end of the magnet is a south pole! That's because opposite poles attract and the "north" end of the compass needle, a north pole, is attracted to south poles. Interestingly enough, the magnetic pole near the earth's geographic north pole is actually a south magnetic pole. That's why the north pole of the compass needle points toward the earth's north geographic pole. When you use a compass to detect which pole of the magnet is north, be careful not to bring the compass needle too close to the permanent magnet. A strong permanent magnet can remagnetize the compass needle and reverse its poles. To make sure that this hasn't occurred, check to see whether the compass still points toward the north pole after you bring it near strong permanent magnets.


579. Can you suspend an object in midair with magnetism? — JA, Holmen, WI
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Yes. However, you can't suspend a stationary object in midair with permanent magnets. Instead, you must either use a moving object or you must use electromagnets that can be adjusted in strength in order to balance the object. Such magnetic suspension is an important issue because people are trying to suspend trains above tracks using magnetic forces. Magnetic levitation is useful because it eliminates the friction and wear that occur between wheels and track. Some of these schemes are based on electronic feedback that turns electromagnets on or off in order to keep the train floating properly. Other schemes use electromagnetic induction to turn the metal track into a magnet so that the moving magnetic train automatically hovers above the track. I should also note that there is a wonderful toy called a Levitron that's a spinning permanent magnet that hovers above a permanent magnet in its base. The spinning behavior of the magnetic top keeps it stably suspended about an inch above the base. It's a fantastic invention.

590. How does a magnetically levitated transit vehicle work? — LB, West Palm Beach, FL
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Although there are a variety of schemes for magnetically levitating trains, perhaps the most promising is a technique called electrodynamic levitation. In this scheme, the train contains very strong magnets (probably superconducting magnets like those used in MRI medical imaging systems) and it travels along an aluminum track. The train starts out rolling forward on wheels but as its speed increases, the track begins to become magnetic. That's because whenever a magnet moves past a conducting surface, electric currents begin to flow in that surface and electric currents are magnetic. Thus the moving magnetic train makes the aluminum track magnetic. For complicated reasons having to do with electromagnetic induction, the track's magnetic poles are oriented so that they repel the magnetic poles of the train—the two push apart. While the track can't move, the train can and it floats upward as much as 25 cm (10 inches) above the track. Once the magnetic forces can support the train, the wheels are retracted and the train floats forward on its magnetic cushion. To keep the train moving forward against air resistance (and a small magnetic drag force), there is also a linear electric motor built into the train and track. This motor uses additional electromagnets in the train and track to push and pull on one another and to propel the train forward (or backward during braking).

604. How do electromagnets work? — HL, Kurtistown, HI
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Whenever an electric current—a current of moving electric charges—flows through a wire, that wire becomes magnetic. This phenomenon is an example of the wonderful interconnectedness of electric and magnetic effects—electricity often produces magnetism and vice versa. Because of its magnetic character, a current carrying wire will exert magnetic forces on another current carrying wire and they are both effectively electromagnets.

A more effective electromagnet uses a coil of wire and a core of very pure iron. Wrapping the wire into a coil gives it specific north and south magnetic poles and adding the iron strengthens those magnetic poles dramatically. Iron is a ferromagnetic material, meaning that it's intrinsically magnetic. All materials contain electrons and an electron has a spinning character that makes it magnetic. But the electron magnetism in most materials cancels completely and only a few materials such as iron retain the magnetism of their electrons. While iron's magnetism is hidden as long as its tiny internal magnets are randomly orientated, its magnetic character becomes obvious when it's inserted in an electromagnet or placed near one. When current flows through the wire coil of the electromagnet, the iron's magnetic poles align with those of the electromagnet and the electromagnet becomes extremely strong.


648. 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
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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.

649. 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
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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.


712. Can we create an invisible wall of electromagnetic fields? - AW
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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.

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


888. Are there any levitated trains in the world? - BP
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At present, I believe that the only magnetically levitated trains are those undergoing development and testing.

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

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