## I am currently working on a physics project, the magnetic levitation train. How …

#### I am currently working on a physics project, the magnetic levitation train. How can I make this train move on the track without it crashing? I only have a few days to make it work so I can present it in the science fair. – VC

I’m afraid that you’re facing a difficult problem. Magnetic levitation involving permanent magnets is inherently and unavoidably unstable for fundamental reasons. One permanent magnet suspended above another permanent magnet will always crash. That’s why all practical maglev trains use either electromagnets with feedback circuitry (magnets that can be changed electronically to correct for their tendencies to crash) or magnetoelectrodynamic levitation (induced magnetism in a conducting track, created by a very fast moving (>100 mph) magnetized train). There are no simple fixes if what you have built so far is based on permanent magnets alone. Unfortunately, you have chosen a very challenging science fair project.

## How do the sizes of two magnets determine how much paper can be held between the…

#### How do the sizes of two magnets determine how much paper can be held between them? — D

While the full answer to this question is complicated, the most important issues are the strengths and locations of the magnetic poles in each magnet. Since each magnet has north poles and south poles of equal strengths, there are always attractive and repulsive forces at work between a pair of magnets—their opposite poles always attract and their like poles always repel. You can make two magnets attract one another by turning them so that their opposite poles are closer together than their like poles (e.g. by turning a north pole toward a south pole).

To maximize the attraction between the magnets, opposite magnetic poles should be as near together as possible while like magnetic poles are as far apart as possible. With long bar magnets, you align the magnets head to toe so that you have the north pole of one magnet opposite the south pole of the other magnet and vice versa. But long magnets also tend to have weaker poles than short stubby magnets because it takes energy to separate a magnet’s north pole from its south pole. With short stubby magnets, the best you can do is to bring the north pole of one magnet close to the south pole of the other magnet while leaving their other poles pointing away from one another. Horseshoe magnets combine some of the best of both magnets—they can have the strong poles of short stubby magnets with more distance separating those poles.

Returning to the paper question, size is less important than pole strength and separation. The stronger the magnets and the farther apart their poles, the more paper you can hold between them.

## Is the earth a huge magnet? If so, how does it do this without being made out of…

#### Is the earth a huge magnet? If so, how does it do this without being made out of metal?

The earth is a huge magnet and it is made out of metal. The earth’s core is mostly iron and nickel, both of which can be magnetic metals. However, the earth’s magnetism doesn’t appear to come from the metal itself. Current theories attribute the earth’s magnetism to movements in and around the core. There are either electric currents associated with this movement or some effects that orient the local magnetization of the metal. I don’t think that there is any general consensus on the matter.

## What keeps the earth stable so that it doesn’t get pulled up into the “magnet”…

#### What keeps the earth stable so that it doesn’t get pulled up into the “magnet”?

If you are asking why doesn’t the earth itself get pulled up toward a large magnet or electromagnet that I’m holding in my hand, the answer is that the magnetic forces just aren’t strong enough to pull the magnet and earth together. I’m holding the two apart with other forces and preventing them from pulling together. The forces between poles diminish with distance. Those forces are proportional to the inverse square of the distance between poles, so they fall off very quickly as the poles move apart. Moreover, each north pole is connected to a south pole on the same magnet, so the attraction between opposite poles on two separate magnets is mitigated by the repulsions of the other poles on those same magnets. As a result, the forces between two bar magnets fall over even faster than the simple inverse square law predicts. It would take an incredible magnet, something like a spinning neutron star, to exert magnet forces strong enough to damage the earth. But then a neutron star would exert gravitational forces that would damage the earth, too, so you’d hardly notice the magnetic effects.

## Are all metals magnetically charged?

#### Are all metals magnetically charged?

First, magnets don’t involve charges, they involve poles. So the question should probably be “are all metals magnetically poled?” The answer to this question is that they are never poled—they never have a net pole. They always have an even balance of north and south pole. However, there are some metals that have their north and south poles separated from one another. A magnetized piece of steel is that way. Only a few metals can support such separated poles and we will study those metals in a few weeks.

## If the train track gets bumpier in effect with increasing speed, why is it that …

#### If the train track gets bumpier in effect with increasing speed, why is it that your car bumps less when you go over a speed bump fast instead of slow?

Actually, if you drive fast over a real speed bump, it’s not good for your wheels and suspension. The springs in your car do protect the car from some of the effects of the bump, but not all of them. However, imagine driving over a speed bump on a traditional bicycle—one that has no spring suspension. The faster you drive over that bump, the more it will throw you into the air.

## Is it possible to create a magnet with more north poles than south poles?

#### Is it possible to create a magnet with more north poles than south poles? — GS

Yes, but only if some of the poles are weaker than other so that when you sum up the total north pole strength and the total south pole strength, those two sums are equal. For example, you can make a magnet that has two north poles and one south pole if the north poles are each half as strong as the south pole. All magnets that we know of have exactly equal amounts of north and south pole. That’s because we have never observed a pure north or a pure south pole in nature and you’d need such a pure north or south pole to unbalance the poles of a magnet. A

The absence of such “monopoles” is an interesting puzzle and scientists haven’t given up hope of finding them. Some theories predict that they should exist, but be very difficult to form artificially. There may be magnetic monopoles left over from the big bang, but we haven’t found any yet.

## I’ve used metal detectors that only pick up gold signals. How does that work?

#### I’ve used metal detectors that only pick up gold signals. How does that work? — MB

While metal detectors can easily distinguish between ferromagnetic metals such as steel and non-ferromagnetic metals such as aluminum, gold, silver, and copper, it is difficult for them to distinguish between the particular members of those two classes. Ferromagnetic metals are ones that have intrinsic magnetic structure and respond very strongly to outside magnetic fields. The non-ferromagnetic metals have no intrinsic magnetic structure but can be made magnetic when electric currents are driven through them.

Good metal detectors produce electromagnetic fields that cause currents to flow through nearby metal objects and then detect the magnetism that results. Unfortunately, identifying what type of non-ferromagnetic metal is responding to a metal detector is hard. Mark Rowan, Chief Engineer at White’s Electronics of Sweet Home, Oregon, a manufacturer of consumer metal detecting equipment, notes that their detectors are able to classify non-ferromagnetic metal objects based on the ratio of an object’s inductance to its resistivity. They can reliably distinguish between all denominations of U.S. coins—for example, nickels are relatively more resistive than copper and clad coins, and quarters are more inductive than smaller dimes. The primary mechanism they use in these measurements is to look at the phase shift between transmitted and received signals (signals typically at, or slightly above, audio frequencies). However, they are unable to identify objects like gold nuggets where the size, shape, and alloy composition are unknown.

## How does a rail gun work?

#### How does a rail gun work?

A rail gun is a device that uses an electromagnetic force to accelerate a projectile to very high speeds. This acceleration technique is based on the fact that whenever an electrically charged particle moves in the presence of a magnetic field, it experiences a force that pushes it perpendicular to both its direction of travel and the magnetic field. In a rail gun, this perpendicular magnetic force—known as the Lorentz force—pushes the projectile along two metal rails and can accelerate it to almost limitless speeds.

The rail gun’s projectile must conduct electricity and it completes the electric circuit formed by two parallel metal rails and a high current power source. During the rail gun’s operation, current flows out of the power source through one rail, passes through the projectile, and returns to the power source through the other rail. As it passes through the two rails, the electric current produces an intense magnetic field between the rails. The projectile is exposed to this magnetic field and as charged particles pass through the projectile, they experience a Lorentz force that pushes them and the projectile in one direction along the rails. The projectile picks up speed as it travels along the rails and doesn’t stop accelerating until the current ceases or it leaves the rails. In practice, the power sources used in most rail guns is a large bank of capacitors. These devices store separated electric charge and supply enormous currents to the rails for a brief period of time.

## How fast can maglev trains travel?

#### How fast can maglev trains travel? — AP

As long as the track is straight enough that the train doesn’t experience severe accelerations up, down, left, or right, there is no limit to how fast it can go. In fact, the levitation process becomes more and more energy efficient as the speed increases. However, the moving train does experience a pressure drag force (a type of air resistance) that increases roughly as the square of the train’s speed. The power needed to overcome this drag force increases as the cube of the train’s speed, making it impractical to propel the train forward above a certain speed.