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Eddy currents in the metallic rails ahead of a moving train can be used for magnetic braking because:
AThey create a magnetic field that levitates the train
BThey convert kinetic energy into electrical energy fed back to the train
CThey exert a force on the train in the direction of motion
DThey oppose the change in flux and therefore the motion, dissipating energy as heat
Answer & Solution
Correct answer: D. They oppose the change in flux and therefore the motion, dissipating energy as heat
By Lenz's law the eddy currents induced in the rails by the train's electromagnet flow in a direction that opposes the change in flux. The associated $\vec{I}\times\vec{B}$ force opposes the motion of the train, while the eddy currents themselves dissipate energy as Joule heat in the rails. This is non-contact braking with no mechanical wear.
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