We know that two dipole magnets attract or repel (see Figure 1.2) depending on whether the two adjacent magnet polarities are different or alike. Recall that field directions are defined as the direction that an isolated north pole would move. Draw the field directions for the two adjacent dipole magnets and see how the direction of the force on the poles means that similarly directed magnetic fields repel and two oppositely directed magnetic fields link together to attract the dipole magnets. The force of magnetic repulsion can be used to overcome the force of gravity so that a heavy magnetized object can be suspended in air over a magnetized base when the fields from the two are similarly directed. This suspension is called magnetic levitation, or maglev for short.
Electric current flow causes magnetic fields. The creation of extremely strong electromagnets depends on super-high electric currents. Wire resistance limits the current flow. However, at the very lowest temperatures (called cryogenic temperatures) wire resistance fades away, allowing engineers to design electromagnets with immense fields. In recent years, using these field techniques, maglev trains have been developed. Fields from cryogenic electromagnets are used to support the weight of the train and provide lateral guidance along its channel guideway so that it experiences frictionless movement. For propulsion, electromagnets spaced along the sides of the guideway provide attracting fields to pull and repelling fields to push the train along. Master controls excite the necessary electromagnets as the trains moves. Five-car maglev trains have obtained speeds of over 340 mph (550 km/hr). NASA is experimenting with maglev propulsion for initial track launch of its space vehicles, to obtain a high speed before the ignition of the rockets.
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