A manifestation of eddy current is in damping the motion of a moving metal piece

Question

A manifestation of eddy current is in damping the motion of a moving metal piece in a changing magnetic field. This exploration involves understanding how the magnitude of the eddy current and the corresponding induced magnetic field and force depends on how large the eddies are. You are provided with three aluminum pieces one of which has no teeth, one has combs and one has slits. Predict in which of these three aluminum pieces will the largest and smallest eddies be produced under the same changing magnetic field. Explain. Also predict the aluminum piece whose motion will be damped fastest if you hang it from the support provided and swing it in an external magnetic field by lifting to one side and releasing. Explain the origin of the force which is responsible for damping the motion (ignore air resistance).

Explanation / Answer

Aluminium piece with comb has least surface area available, hence magnetic flux is least. therefore, in turn, amount of eddy current will be least.

while,

Aluminium piece with no teeths, has maximum surface area, hence maximum magnetic flux. therefore maximum eddy current.

Damping is more in the piece with no teeths, since it has maximum Eddy current. Eddy current dissipates the energy of the system, causing damping.

the damping force is a consequence of Lenz's law, since Eddy current produces its own magnetic dipole, which experience magnetic force due to external magnetic field.

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