Category: magnetically levitated trains

How do electromagnets work? — HL, Kurtistown, HI

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.

How does a magnetically levitated transit vehicle work? — LB, West Palm Beach, FL

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).

Can you suspend an object in midair with magnetism? — JA, Holmen, WI

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.

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?

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.

Can magnetic energy be used to power a vehicle?

When you talk about “magnetic energy,” you are referring to magnetic potential energy. A potential energy is energy stored in the forces between objects. In the case of magnetic potential energy, that energy is stored in the forces between magnetic poles. But there is only so much potential energy in any given collection of objects. Potential energy is released by allowing the forces between objects to push the objects around and once it is used up, there isn’t any more available. That’s because energy is a conserved quantity—something that can’t be created or destroyed and that can only be transferred between objects or changed from one form to another. While you can store energy in a collection of magnets, that potential energy is limited by how much was put in in the first place. So to answer to your question: yes, magnetic energy can be used to power a vehicle, but not indefinitely. The only practical magnetic energy storage proposals I’m aware of are ones that suggest using huge superconducting magnets to store electric power. While such devices might be practical for an stationary power company, they would be impractical or even dangerous in a vehicle—picture cars containing incredibly strong magnets driving down the road, repelling or attracting one another as they pass.

How can currents and electromagnets encounter frictional effects without touching?

When you slide a strong magnet quickly above a metal surface, there is a friction-like magnetic drag effect. This effect occurs even when the two objects don’t touch. The origin of this effect lies in the repulsions between the metal and magnet: it’s strongest slightly in front of the moving magnet so the magnet encounters some difficulty heading forward. The reason why the magnetization of the metal is strongest slightly in front of the moving magnet is related to the loss of energy by current moving in the metal. The magnetization (of the metal surface) in front of the moving magnet is fresher than the magnetization behind it. The current responsible for the magnetization behind the magnet has been flowing for long enough to have lost energy. But the faster you move the magnet across the metal surface, the less time the currents in it have to lose energy and the less friction-like force the magnet experiences.

How does a magnet induce a metal to become attracted to the magnet? Does the metal become a magnet also?

A steady, motionless magnet can’t induce a piece of normal metal (not iron, cobalt, or nickel) to become magnetic. Only a moving or changing magnet can do that. When a metal is exposed to a changing or moving magnet, it does become magnetic. That metal becomes a type of magnet; an electromagnet. The metal itself isn’t really the magnet; the electric charges inside it are. These charges move in response to the changing or moving magnet nearby and they become magnetic, too. The effect is always repulsive, not attractive. The temporarily magnetic metal repels the magnet that is making it magnetic.

How does running current through a coil cause a magnetic field?

Electricity and magnetism are interrelated in a great many ways. At the very basic levels, they are manifestations of the same fundamental physical concepts. As a result, electricity can produce magnetism and magnetism can produce electricity. One way in which electricity can produce magnetism is for charged particles to move. When an electric current passes through a coil (or any wire, for that matter), it creates a magnetic field. The coil develops a north magnetic pole and a south magnetic pole. I can’t really explain why because the answer is simply that moving charges create magnetic fields; that’s the way our universe works and no one has ever seen otherwise.

If magnetic trains are to work, wouldn’t friction on the bottom of the train create thermal energy which would destroy the magnetism of the train?

When a magnetically levitated train is operating properly, it doesn’t touch the track and experiences no friction. In principle, it shouldn’t get hot at all. The magnetic drag effect will warm the track slightly, but that won’t matter to the train’s magnets. Actually, the train’s magnets will almost certainly be superconducting wire coils with currents flowing in them. That type of magnet doesn’t depend on the magnetic order of permanent magnets. It’s the magnetic order of permanent magnets that is destroyed by heat. An electromagnetic coil will stay magnetic as long as current flows through it, even if it’s so hot that it’s ready to melt.

If you have more volts is it more energy (like a stun gun—is it better to have one with more current or volts or both)?

Volts is a measure of energy per charge. Thus if you tell me how much charge you have and the voltage of that charge, I can tell you have much energy that charge contains. I simply multiply the voltage by the amount of charge. Current is a measure of how many charges are moving through a wire each second. If you tell me how much current a wire is carrying and for how long that current flows, I can tell you how much charge has gone by. I just multiply the current by the time. To figure out how much energy electricity delivers to something (such as a person zapped by a stun gun), I need to know the voltage, the current, and the time. If I multiply all three together, the product is the energy delivered. In a stun gun, the voltage is important because skin is insulating and it takes high voltage to push charge through skin and into a person’s body. But current is also important because the more charge that passes by, the more energy it will carry. And time is important because the longer the current flows, the more energy it delivers. So all voltage and current are both important. I can’t guess which one is most important.