Science Theatre's
Recipes for Science
The Leo Stick
A Faraday's Induction Law Demonstration
Equipment Needed:Leo Stick (see diagram)
The Leo Stick Diagram
WARNINGS!!! None.
What to Do: The Leo Stick consists of a clear plastic 5/8
inch diameter tube around which are placed five coils of coated wire. Each
coil of wire has one thousand turns of # 28 gauge enamel coated wire.
Connected to each coil are two LED (Light Emitting Diodes), one yellow and one
red. The connection of the diodes to the coils is also on the diagram. They
are connected in parallel so that as current flows in each direction through
the coil only one diode lights. Tip the tube over to let the magnet drop down
through the LED's. You should see that the LED's light when the magnet drops
through.
Why it Is: The magnet slides down the tube as the tube is
tipped, causing the field penetrating the coil to vary and thus the flux
through the coil changes in time. This causes a voltage to be induced in the
wire by Faraday's Law of Induction:
E = - N (dF/dt),
where E is the electric potential, N is the number of turns of wire, dF is the
change in flux and dt is the change in time. The flux is increasing as the
magnet approaches the coil, after the magnet passes the center of the coil, the
flux is decreasing. This generates a sinusoidal voltage which first causes
one LED to light and then the other. With a sufficiently long magnet (approx.
1") the change in voltage is slow enough that the eye can easily see two LED's
flash in sequence as opposed to both flashing simultaneously.
What to Say: Explain that current will only flow in a loop
if it is in a changing magnetic field. The magnetic field from the magnet gets
stronger the closer the loop gets to it. As the magnet falls in the tube, its
magnetic field changes and current flows in the loops. We can see this as the
LED's light up.
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