1. An eddy current is a closed loop current that flows in a conductor, such as the iron core of a coil of an electromagnetic brake plate, when there is relative motion between the object and a magnetic field. The eddy current, flowing in a closed loop, acts like the current in a coil or solenoid and produces its own magnetic field. The polarity of this magnetic field depends on the direction in which the eddy current circulates
Lenz's Law says if an induced current flows, its direction is always such that it will oppose the change of flux that produces it. That is, the polarity of the magnetic field produced by the eddy current is such that it opposes the relative motion of the magnetic field that induced the eddy current.
2. Eddy currents are used in electromagnetic braking, especially in high speed trains. An electromagnet can be placed near the rotating wheel. This will result in eddy currents being induced, which will oppose the motion of the wheel. This produces a braking effect. Consequently, a smooth stop can be obtained by using electromagnetic braking. The rotational kinetic energy is converted to heat energy.
Advantages of electromagnetic braking are that there is no friction involved and there is no wear and tear due to moving parts. Electromagnetic braking works very well for high speed roller coasters and trains.
