The figure shows two metal disks of the same size and material oscillating in an

Question

The figure shows two metal disks of the same size and material oscillating in and out of a region with a magnetic field. One disk is solid; the other has a series of slots Is the retarding effect of eddy currents on the solid disk greater than, less than, or equal to the retarding effect on the slotted disk? Choose the best explanation from among the following: The slotted disk experiences the greater retarding force because the slots allow more magnetic field to penetrate the disk The solid disk experiences a greater retarding force because eddy currents in it flow freely and are not interrupted by the slots The disks are the same size and made of the same material, therefore, they experience ' the same retarding force.

Explanation / Answer

Part A:

The slots in the second disk will tend to suppress the clockwise and counterclockwise currents that are induced by the change in the magnetic field through the disk. The diminished currents through the disk will result in a diminished magnetic force on the disk, so we conclude that the retarding effect of eddy currents on the solid disk is greater than the retarding effect on the slotted disk.

Part B:

   The best explanation is

Statement I. The solid disk experiences a greater retarding force because eddy currents in it flow freely and are not interrupted by the slots.

Statement II .does not recognize that the field penetration through the disk is not affected by the slots, and statement III. ignores the importance of the eddy currents in explaining the motion of the disks.

In this common demonstration the motion of the two disks are dramatically different. The motion of the solid disk is quickly damped to zero while the slotted disk continues to swing nearly unimpeded.

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