VISUAL PHYSICS ONLINE

6 ELECTROMAGNETISM

P60 016

(A)

You have a task of designing the mechanism for adjusting level of effort required to rotate the spinning wheel of an exercise bike. Explain how this could be achieved using a set of strong permanent magnets.

(B)

A non-ferromagnetic metal disk is at initially at rest and balanced on a support so that it is free to rotate. A magnet is moved in a circular path above the surface of the disk. Explain why the disk starts to rotate and in what direction will it rotate? What device operates on this principle?

View solution below only after you have completed the answering the question.

Solution

(A)

The conductor (wheel) is moving through a magnetic field – induced emf in metal wheel – induced currents (eddy currents) – produce a magnetic field to oppose the rotation of the wheel. The closer the magnets are to the wheel the greater the opposition to the motion. Therefore, the levels can be changed by varying the spacing between the wheel and magnets.

(B)

Changing magnetic field due to motion of magnetic field Þ changing magnetic flux Þinduced emf Þ induced eddy currents Þ induced magnetic field to oppose the motion of the magnet: N pole induced at top of disk attracts S pole of rotating magnet Þ disk spins chasing the magnet, i.e., disk spins in same direction as magnet. This is an example of Lenz’s law – the induced currents in the disk produce an induced magnetic field in a direction which will oppose the rotation of the disk.

This is an example of a simple form of an induction motor.

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Ian Cooper School of Physics University of Sydney