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In an electric power plant, steam or water power is used to move huge coils of wire past extremely strong magnets, generating megawatts of electricity to light whole towns. This Snack uses your muscles to move ordinary magnets past a small coil of wire, generating milliwatts of electricity—just enough to light an LED. The two generators work at very different scales, but they are both based on the same physics principles.
Grasp the tube at both ends to hold the film cans in place. Shake the tube rapidly back and forth so the magnet stack slides back and forth inside. Shake the tube as rapidly as you can.
Each time the magnet stack passes through the coil of wire, the LED should flash. The flashes are more dramatic in a darkened room.
Remove the original LED, and hook up the bipolar LED. Again, shake the tube so that the magnet stack slides back and forth. What happens now?
Whenever a wire and a magnetic field move perpendicular to each other, a voltage is induced in the wire. If the wire is part of a complete electrical circuit, the voltage will cause a current to flow in the circuit. If a coil of wire is used instead of a single wire, the voltage obtained is the single-wire voltage multiplied by the number of turns in the coil.
In this Snack, every time the magnet stack moves through the coil of wire, the coil experiences a changing magnetic field, inducing a voltage in the coil. Since the coil is part of a complete circuit that includes the LED, current flows through the LED, and it lights.
When the magnet stack passes through the coil, two pulses of voltage of opposite polarity are produced. This happens because the direction of the voltage induced in a coil is related to both the orientation of the magnetic field lines relative to the coil and their direction of movement relative to the coil. When the north pole of the magnet stack, for example, is moving to the left through the coil, the magnetic field lines point away from the center of the coil, and the current flows in one direction in the coil. Then, when the south pole of the magnet stack passes through the coil, the magnetic field lines point toward the center of the coil, and the currents flows in the opposite direction (see diagram below).
The normal LED lights up once each time the magnet stack passes through the coil, regardless of which way the stack is traveling, even though two pulses of current are produced. There is only one flash because the LED is a normal diode, and a normal diode only allows current to flow in one direction. When the magnet stack passes by the coil in one direction, the first pulse of current lights the LED, and when it passes by in the other direction, the second pulse of current does the job.
The bipolar LED is actually two LEDs in one casing that allow current to flow in both directions. When current flows in one direction, one of the LEDs emits red light; when current flows in the opposite direction, the other LED emits green light. When you hook up this LED, you see two flashes of light each time the magnet passes through the coil—one when the north pole of the magnet stack passes through the coil and the other when the south pole passes through.
Current that always flows in the same direction is called direct current (DC). Current that flows first in one direction and then the other is called alternating current (AC). The generators that provide the electricity you use every day produce AC current just like the Stripped-down Generator and operate on the same general principle. Most of the appliances and lights in your home can use AC current straight from a wall outlet. Some electrical devices, however, require DC for their operation. These either have a built-in electrical circuit that converts the AC to DC or use an adapter that accomplishes this task. Many devices that can be run on batteries, such as laptop computers and cell phones, come equipped with AC adapters, which convert the AC from the wall outlet into DC for the device.
Attach an ordinary LED to the alligator clips. Note which clip holds the longest leg of the LED. Then hook up a second ordinary LED, with its longest leg in the other clip. The two LEDs now have opposite polarities. Predict what will happen when you shake the tube.
Curt Gabrielson developed this Snack based on an idea proposed by Van, a sixth-grade student in an afterschool workshop.
Macaulay, David. The Way Things Work. Boston: Houghton Mifflin, 1988. See LED, page 293, and Electric Generator, pages 304–305.
Make a simple mini-motor.
Build a simple wind generator.
Iron filings trace out magnetic field lines in three dimensions.
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Attribution: Exploratorium Teacher Institute