PASCO SE-8657 MOTOR ACCESSORY - Manual (Page 18)

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Motor Accessory 012-06247A
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Surprisingly, the direction of induced current can be determined from the law of conserva-
tion of energy. Due simply to friction, work must be done to rotate a generator. If the
generator is connected to a load and producing electric current, the law of conservation of
energy dictates that additional work must be done to turn the shaft. This is an example of the
reasoning that led to Lenz’s law: the induced current is in such a direction as to produce a
magnetic field that opposes the original magnetic field.
You can demonstrate Lenz’s law to yourself by determining the direction of the magnetic
field of the Variable Gap Magnet and by detecting the direction of the induced electric
current with a galvanometer (or multimeter) as you move the armature through the magnetic
field.
Setup
➀ Be sure you have the flat iron pole pieces placed on the two neodymium magnets of the
Variable Gap Magnet. (The iron pole pieces spread the magnetic field over a wider area.)
Screw the larger threaded portion of the shaft into the threaded hole in the magnet base.
Insert the threaded end of the shaft from above, screwing it in until 1 mm, or slightly less, of
the threaded portion remains above the upper surface of the base.
➁
Turn the magnet over and screw the retaining nut onto the smaller diameter threaded portion
of the shaft that protrudes through the bottom of the magnet base. Use firm finger pressure.
Do not over tighten.
Figure 1. Experimental Setup
voltmeter or galvanometer