AAPT Apparatus Competition, Entry Description

Low Cost Category

And

Introductory Laboratory Category

Apparatus Title: Apparatus to Demonstrate Induced Voltage/Current In Wire Coils

Abstract (50-75 words)

In this demonstration of Faraday’s Law, a number of movable coils encircle a length of PVC pipe at various positions. A very strong magnet is dropped vertically through the pipe. As the magnet passes through each coil, a bicolor LED attached to the coil flashes red, green, or yellow depending on the position of the coil and the number of turns of wire on the coil.

Equipment and costs required to construct apparatus:

Item	Source	Part number	Cost
1 Neodymium Magnet	Educational Innovations	M-195	$14.95
20 Fahnestock Clips	Electronix Express	N2330FC10	4.30
Magnet Wire 28ga.	Electronix Express	N2700MG28	10.50
1 ¾” PVC Pipe X 10 ft.	Lowe’s		.79
5 ¾” PVC Couplings	Lowe’s	25532 (Pkg of 10)	1.12
4 1” PVC Couplings	Lowe’s	23852	.96
1pkg of 3 Magnet Wire	Radio Shack	278-1345B	4.69
9 Bicolor LEDs	Radio Shack	276-012	13.41
1 red LED	Radio Shack	276-209	1.29
1 green LED	Radio Shack	276-022	1.29
Assorted supplies:
Glue, solder, wire, etc.			5.00
Total Cost			58.30

Description:

In a popular demonstration of Lenz’s Law, a magnet is allowed to fall freely through a metal pipe. The rate of descent of the magnet depends on the strength of the magnet, the weight of the magnet, the diameter of the pipe, the wall thickness of the pipe, and type of metal that the pipe is made from. Induced eddy currents create a magnetic force that opposes the motion of the magnet.

To help students visualize what is happening in the above demonstration, a 1.5 meter length of PVC pipe was fitted with a series of coils that wrap around the pipe. (Figure 1.) The terminal ends of the coil are attached to small Fahenstock clips. A bicolor (red-green) LED, individual red or green LEDs, or a combination of red and a green LED aligned with opposite polarities can be used as detectors for the voltage induced as a very strong 7/8” X 1” Neodymium magnet passes through the coil. (Figure 2.)

Ten coils were constructed with the following characteristics:

Coil 1 2 3 4 5 6 7 8 9 10

Turns 50 100 150 200 200 200 200 200 350 100

Gauge 28 28 28 28 28 28 26 30 30 22

Individual coils can be added or removed from the pipe. The relationship between velocity of the magnet and voltage necessary to cause the LED to flash can be studied by changing the position of the coil relative to the point of release of the magnet. If the velocity is too low, the light will not flash. The coil can then be moved down until the LED flashes. With a bicolor LED as an indicator, the direction of the current is indicated by the color of the flash. If the magnet is inverted the opposite color will be seen. Slower speeds of descent can be achieved by tilting the pipe at an angle.

The relationship between number of turns of wire on the coil and the distance that the magnet must fall to light the LED can be studied in a similar manner.

The LED indicators can be replaced with either voltage or current probes attached to a computer interface system such as the Vernier Lab Pro or a PASCO Science Workshop. With these devices a detailed study of voltages and currents generated can be made.

The device can be used in large classrooms as a demonstration. In this context it is best to dim the lights so that the LEDs are more visible.

The apparatus could also be used as the basis for individual laboratory experiments at the high school or introductory college level.

Figure 2. — Coil with Bicolor LED Attached

Construction Notes:

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[EW1] The construction of the coils was done with hand tools using the following procedure:

3. A piece of ¾ inch PVC pipe was purchased that was slightly larger than the available magnet. The pipe was cut to a length of approximately 1.5 meters. The ends were smoothed with sand paper.

4. A ¾ inch coupling that is used to join two pieces of PVC pipe together is slightly larger than the pipe on the outside end but tapers to a smaller diameter in the center. There is a ridge in the center of the coupling. The material in the center was removed using a rotary tool and a curved rasp so that the coupling will slide freely over the pipe.

5. The coupling was then cut into two pieces with a hacksaw. To produce a fairly even cut it was necessary to rotate the coupling making a shallow cut all of the way around then continuing to deepen the cut until the coupling was cut all of the way through. Each coupling produces two coil spools.

6. To produce the rims of the spools, it was found that the inside of a 1 inch coupling was slightly larger than the outside of the ¾ inch coupling. Narrow rings (approximately 4 millimeter) were cut from this coupling using the method described above. Each coupling will produce about eight rims.

7. The rims were then glued to the spool using household adhesive and allowed to dry. There was a little play between the rims and the spool body. The glue filled the space.

8. Two small holes were drilled through the rim of the spool and the Fahenstock clips were held in place by a piece of bare wire bent over the top of the clip. The clip was then soldered to the wire. The clip was held with a needle nose pliers which acted as a heat sink. Care must be used not to melt the plastic.

9. The end of the enameled magnet wire was bared and was soldered to the lug on the Fahenstock clip. The appropriate number of turns was then wrapped around the coil and the other end attached to the second Fahenstock clip.

AAPT Apparatus Competition, Entry Description

Low Cost Category

And

Introductory Laboratory Category

Cost

Total Cost