PASCO MG-8600 Lenz's Law Demonstrator - Manual

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Phone (916) 786-3800 • FAX (916) 786-8905 • email: [email protected]
10101 Foothills Blvd. • Roseville, CA 95747-7100 USA
®
Instruction Sheet
for the PASCO
Model MG-8600
012-03319C
6/90
$.50
The Model MG-8600 Lenz's Law Demonstrator provides a
dramatic demonstration of Lenz's Law. A mass is dropped
through a 1.5 meter long aluminum tube. It falls through the
tube in about 0.5 seconds. Then an identical mass (actually a
magnet) is dropped through the tube. The falling magnet
produces a current in the tube, which in turn produces a
magnetic field that opposes the field of the falling magnet.
This opposing field slows the motion of the magnet, so it
takes more than ten times as long to fall through.
The equipment and setup are shown in Figure 1. Two falling
masses are included with the apparatus. They are seemingly
identical, but one is a magnet, the other is not. A spring
scale is included so you can show that the mass shown on
the scale increases as the magnet falls through the tube, but
not as the unmagnetized mass falls through.
To perform the demonstration:
1. Set up the equipment as shown in Figure 1.
2. Hold the unmagnetized mass over the opening in the
tube, then drop it.
3. Now drop the magnet through the tube.
4. You may want to repeat the demonstration, allowing
your students to observe the reading of the spring scale.
Theory
According to Faraday's Law of Induction, a changing
magnetic field induces an electric field. According to
Ampere's Law, a circulating current induces a magnetic
field. So, if a magnetic field changes within a conductor, a
current can be produced which in turn produces a secondary
magnetic field. Lenz's Law states that this secondary
magnetic field always opposes the change in the original
field.
Figure 2a shows a diagram of the magnet falling through the
tube. The N-pole of the falling magnet is facing down.
Three cross sections of the tube, A, B, and C, are shown.
The magnetic field through all three cross sections points
down. In cross section A, the magnetic field decreases as the
magnet falls. Lenz's Law says that the induced field will
therefore point down, reducing the rate at which the total
field decreases. In section B, the field from the falling
magnet is relatively constant. There is therefore no induced
field in cross section B. In cross section C, the field from the
falling magnet increases as the magnet falls. According to
Lenz's Law, the induced field will point up, reducing the rate
at which the total field increases.
An easy way to conceptualize the effect of these fields on the
falling magnet is to imagine the fields as if they are produced
by tiny magnets, as shown in Figure 2b. The direction of the
field of the upper magnet is the same as that of the falling
magnet, so the N-pole of the induced magnet is adjacent to
the S-pole of the falling magnet. The falling magnet is
attracted, which slows its motion. The field of the lower
Mounting arm
with velcro pad
Two falling masses
(one of which is a
magnet)
Figure 1 Equipment and Setup
Spring scale
Tube with
velcro pad for
mounting arm
Lenz's Law Demonstrator