Name: Don Fahnline
Institution: Penn
State Altoona
Address: Penn
State Altoona
3000
Ivyside Park
Altoona,
PA 16601
Apparatus Title: Eddy Currents and Lenz’s Law
Demonstrations
Abstract (50-75 words)
A magnet spun by hand induces eddy
currents in non-magnetic, conducting, metallic objects placed over it on a platform,
and it simultaneously exerts magnetic forces on the objects via the eddy
currents in accordance with Lenz’s Law.
These forces cause a top to spin in the same direction as the magnet,
but a cone to role in the opposite direction.
Description:
The base of the apparatus consists of a
permanent horseshoe magnet mounted on a vertical axle made from a brass bolt
and mounted beneath a transparent plastic platform so it spins easily by hand
in either direction. (The Edmund
Scientific magnet #D30419-49 is convenient because it comes with a centered
hole ready for a 3-inch long 12-24 brass machine screw cut and pointed on both
ends for the pivots.)
The top is a 3-inch diameter circular
copper or aluminum disk mounted on another pointed brass bolt.
The cone is machined from a 1-inch
diameter aluminum bar and is 1.25 inches in height.
Demonstration 1: Spinning the top. Center the top on the platform over the magnet and spin the magnet by hand. The top will start spinning in the same direction as the magnet. Try spinning the magnet in the opposite direction to check that it works in reverse. Try spinning the magnet without the top to see if it slows down and stops more quickly with the top or without the top.
Questions: Why does the top start spinning in the same direction as the magnet? How does this illustrate Lenz’s Law? How is this consistent with conservation of angular momentum? (There are several variations on this demonstration, such as holding the magnet fixed and spinning the top by hand, etc.)
Demonstration
2: Rolling the cone: Place the cone on the platform with its apex
over the center of the magnet and spin the magnet by hand. The cone will start rolling in the direction
opposite the direction of the magnet.
Try spinning the magnet in the opposite direction to check that it works
in reverse.
Questions: Why does the cone roll in the direction opposite the direction of
the magnet? How does this illustrate
Lenz’s Law? How is this consistent with
conservation of angular momentum?
Equipment and
costs required to construct apparatus:
|
Item |
Source |
Part number |
Cost |
|
Horseshoe
magnet |
Edmund
Scientific |
# D30419-49 |
$29.35 |
|
Top |
|
|
5.00 |
|
Cone |
|
|
5.00 |
|
Miscellaneous |
|
|
5.00 |
|
|
|
|
|
Total Cost |
$44.35 |
||