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# Electromagnetism

## A magnet and a solenoid can be used to generate electric current.

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Levitating Trains

### Levitating Trains

Credit: Alex Needham
Source: http://en.wikipedia.org/wiki/File:A_maglev_train_coming_out,_Pudong_International_Airport,_Shanghai.jpg
License: CC BY-NC 3.0

In Maglev trains, magnetic forces are used to remove the frictional forces between the track and the train. By allowing the train to float on a cushion of air, these levitating trains are able to reach speeds of more than 310 mph.

#### Amazing But True

• There are two types of magnetic levitation technology:
1. Electromagnetic suspension: electromagnets inside the train are attracted to a magnetically conductive track.
2. Electrodynamic suspension: magnetic fields are used to induce currents in metallic conductors.
• In electromagnetic suspension systems, electromagnets that are attached to the train allow the train to levitate above a steel rail. More unstable than an electrodynamic suspension system, this systems major advantage is that it works at all speeds. Its major drawback though is how precise the distance between the train and the track needs to be controlled. Because the magnetic forces vary inversely with the cube of the distance, the smallest change in the distance between the track and the electromagnets produces greatly different forces.
• In electrodynamics suspension systems, the train is levitated by a combination of attractive and repulsive magnetic forces. The magnetic fields used in an EDS system is created by using either permanent magnets or superconducting magnetics. The major advantage of this system is that the design allows for the correct distance between the train and the track to be held in place by the competing attractive and repulsive forces. While this is a major advantage, the biggest drawback is that the system does not work at slow speeds. Because of this, wheels must be used as the system slows down or accelerates to the point where the magnetic fields can support the weight of the train.
• Watch Japan's Maglev train in action:

#### Explore More

Using the information provided above, answer the following questions.

1. When superconductors are used, why must they be cooled to such cold temperatures?
2. How do the frictional forces felt by the train change as the speed increases?
3. What direction does the magnetic field lines flow, North to South or South to North?

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