*38.  The drawing shows a bar magnet falling through a metal ring. In part a the ring is solid all the way around, but in part b it has been cut through. (a) Explain why the motion of the magnet in part a is retarded when the magnet is above the ring and below the ring as well. Draw any induced currents that appear in the ring. (b) Explain why the motion of the magnet is unaffected by the ring in part b.

 

w1189

 

In the ring in (a) when the magnet is above the ring, the magnetic field strength is increasing and pointing down as the magnet falls towards the ring.  The increase in magnetic field strength means an increase in magnetic flux down, so the induced current will go counter-clock wise and produce a magnetic field going up.  This makes the ring producing a “North” pole above the ring and “Like Poles, Repel” will cause the retardation.

When the magnet is below the ring in (a), the field is getting weaker and still pointing down when it falls so the decreasing field strength means a decreasing magnetic flux, so the induced current will now want to create a field in the same direction, so pointing down.  The induced current will go clockwise.  Now below the ring you have a “North” pole and this time it is interacting with the south pole of the magnet, so you have “Opposite Poles, Attract” going on and that continues to retard the motion.

For (b) the ring is not continuous, so you cannot set up a current and thus you cannot have an induced magnetic field.  So the magnet falls without any slowing due to retarding forces acting.

 

 

Please send any comments or questions about this page to ddonovan@nmu.edu

This page last updated on June 25, 2018