2006 AAPT Summer Meeting
LenzŐs Law Demonstration
University of Louisville
Louisville, KY 40292
pjouse01 ÔatŐ louisville ÔdotŐ edu
LenzŐs law is interpreted in two different ways: 1) induced current opposes pole movement and 2) induced current opposes flux change (page 176 Fundamentals of Physics, Sixth edition, by Halliday, Resnick, and Walker). This apparatus demonstrates both aspects of LenzŐs law. It can also be used to illustrate the principle of conservation of angular momentum and is rather easy to construct.
Construction of Apparatus:
Figure 1 Top Plexiglas sheet is supported by o-rings and the bottom Plexiglas sheet (black) is supported by Plexiglas rings. At the center of the top sheet is a 3-cm long rod. The rod, with paper tape on the side, is to move the top disc along with top Plexiglas sheet up and down. On the right hand side of the central rod is the aluminum pin when inserted into the top disc (as shown in the figure) locks the top disc. One of the four magnets fixed to the bottom disc can be seen in the figure.
The apparatus shown in figure1 has two aluminum discs of 15-cm radius and 0.6-cm thickness. Two 0.7-cm diameter rods are fixed to the centers of the discs. These rods are fixed to ball bearings glued to two Plexiglas sheets (17.8 cm x 10 cm). The bottom Plexiglas sheet is supported by four Plexiglas rings glued to Plexiglas rods. The top Plexiglas sheet is supported by o-rings slipped over the four Plexiglas rods. The closest distance between the discs can be changed by adjusting the positions of the o-rings. The top disc and the top plastic sheet can be moved up and down within the limits set by the o-rings and four adjustable rings fixed to the four rods. Four ceramic magnets are glued to the bottom disc at 6.5 cm from the center of the disc. There is an aluminum pin that can be inserted in to a small hole in the top disk through the top Plexiglas sheet. Inserting the pin in to the top disc keeps it from rotating.
Use of Apparatus:
Two aspects of LenzŐs law, discussed in the abstract, and conservation of angular momentum can be demonstrated by this apparatus.
Insert the pin into the top disc and lift it about 2 cm along with the Plexiglas sheet using the central rod. Rotate the bottom disc and bring the top disc down till it is stopped by the o-rings. The distance between the discs will only be a few millimeters. As soon as the top disc is brought down close to the bottom disc it slows down and stops in a few revolutions. This demonstration illustrates the principle of magnetic brake.
Remove the pin from top disk and lift it up about 2 cm. Rotate the bottom disc and bring the top disc down close to the bottom one as in the first demonstration. This time you will observe that the top disk rotates in step with the bottom disk to eliminate flux change in the top disk.
In the second demonstration, when the two discs are brought closer we notice the angular speed of the bottom disc decreasing. Since the dimensions of the discs are the same their moments of inertia are equal. Therefore, we expect the angular speed to decrease by a factor of two. Notice that this demonstration is extremely qualitative. However, with additional equipment the angular speeds can be measured to quantitatively prove the principle of conservation of angular momentum.