When a non-magnetic conductor (for example copper or aluminum) is exposed to a varying magnetic field, eddy currents are produced in the conductor. The currents obey Lenz Law and oppose the direction of the external changing magnetic field.
A classic demonstration of this is shown in the MIT Physics Demo -- Pendulum and Magnet. During the video, the first paddle released is a solid copper sheet. It swings freely between the poles of an electromagnet until the electromagnet is turned on. Then it abruptly stops because of the interactions between the eddy currents produced within its solid surface and the magnetic field between the poles of the strong electromagnet. Later, a perforated paddle is used which, when released between the poles, does not immediately stop when the electromagnet is turned on. Instead it displays a damped oscillation. This change in behavior is because the slits no longer allowed the larger eddy currents to form. Instead weaker currents can only be created inside the smaller copper strips. A nice explanation of this effect (with diagrams and captions) is given on the Science Ready page from New South Wales Higher School Certificate (HSC) program.
A second classic demonstration is shown inn the MIT Physics Demo -- Jumping Ring. It begins with a solid aluminum ring being placed over the soft-iron core of an electromagnet. When the electromagnet is turned on the eddy currents induced in the aluminum ring cause it to be repelled from the electromagnet and "jump" up into the air. A second ring with a slit down on side is then placed on the electromagnet. This time the ring does not "jump" since the slit has broken the conducting surface and no eddy currents can be sustained.
The third classic demonstration on eddy current involves letting a neodymium magnet fall through a copper tube. Fortunately, we have been granted permission by Mr. Cavini to use a series of screen captures from his YouTube video Lenz's Law at Work to perform an in-depth investigation of this phenomena. Please view his video several times BEFORE proceeding through the lab simultion. Here is a second video of this phenomena also using a copper tube.
In his demonstration, a non-magnet is first dropped through a copper tube. As expected it emerges without any time delay; just as it would if released from rest through the air without the tube. Then a strong magnet is dropped through the tube. The magnet does not touch the sides of the tube, but takes a significantly longer amount of time to re-emerge at the base. Just as in the other examples, eddy currents are responsible for the extra time needed by the magnet to traverse the length of the copper tube. The eddy currents in the copper tube create a drag force on the magnet. If the magnet is sufficiently light and strong, the magnetic drag force will balance the magnet's weight, and it will reach terminal speed during its fall. Included in the video are graphs of the two situations: a non-magnet free falling through the tube and the magnet falling through the tube. |