A bar magnet is held vertically with its upper end a little bit below the center of a horizontal metal ring. The upper end of the magnet is its north pole, as shown in the figure. The bar magnet is now dropped. An observer views the ring from above its center. To this observer, how will the induced current in the ring behave as the magnet falls?
A) The current will flow clockwise and be increasing.
B) The current will flow clockwise and be decreasing.
C) The current will flow counter-clockwise and be increasing.
D) The current will flow counter-clockwise and be decreasing.
E) The induced current will be zero.

Question

A bar magnet is held vertically with its upper end a little bit below the center of a horizontal metal ring. The upper end of the magnet is its north pole, as shown in the figure. The bar magnet is now dropped. An observer views the ring from above its center. To this observer, how will the induced current in the ring behave as the magnet falls? 
A) The current will flow clockwise and be increasing. 
B) The current will flow clockwise and be decreasing. 
C) The current will flow counter-clockwise and be increasing. 
D) The current will flow counter-clockwise and be decreasing. 
E) The induced current will be zero.

Quizplus · Accepted Answer

As the magnet falls, the magnetic field passing through the metal ring changes. The north pole of the magnet induces a magnetic field in the ring in a direction opposite to the falling magnet. Therefore, the induced current flows in the opposite direction to the magnet's motion. Using Lenz's law, we can conclude that the direction of the induced current in the ring is counterclockwise. While the magnet falls, the magnetic field through the ring continuously increases as more sections of the magnet pass through the ring. Therefore, the current induced in the ring decreases, which means that the correct option is D.

A Bar Magnet Is Held Vertically with Its Upper End