PURPOSE: To show Lenz's Law in a dramatic way.
DESCRIPTION: A magnet is dropped through
a vertical copper tube. The motion of the magnet induces a current in the
copper tube. According to Lenz's law, the direction of the magnetic field
by the induced current will oppose the the motion of the falling magnet. It
takes 30 seconds for the magnet to fall one meter! Yes, that's pretty slow- imagine pressing the slow motion button on your movie player.
This demonstration is sometimes called Newton's Nightmare (Besides having bad dreams about apples falling on his head, he also thought about falling magnets.....or maybe just all falling objects in particular. This is what happens when you study too much physics- not to discourage you or anything...). So what exactly plagued Newton at night regarding this simple demonstration?
First, let's state two tenants of classical electromagnetism.
As the magnet falls through the copper tube, the magnetic flux increases in the tube. Remember that there is a magnetic field surrounding the magnet itself. Based on this, Faraday was able to discover that a changing magnetic field leads to a flow of electric charges or current. Lenz was able to specifically state the direction of the current. (Remember Lenz's Law? For a current induced in a conductor, the current flows in a direction that its own magnetic field opposes the change that produced it.)
We will use the convention that magnetic fields flow from the North pole to the South pole. Consider when the magnet is falling through the tube with the North side pointing down. A counterclockwise current is therefore produced and since a current has a magnetic field associated with it, a magnetic field is produced from this induced current. This induced magnetic field goes against the magnetic field of the magnet itself.
A video camera looks down the
tube and projects the image of the free falling magnet to the class.
EQUIPMENT: Eddy current tube, magnet, lamp, cover, camera.
SETUP TIME: 1/2 hour. See the photo below.
Click on the movie reel to watch the magnet falling through the tube.
Updated by Jun Qi in 3/13/2000