High School Mathematics-Physics SMILE Meeting 1997-2006 Academic Years Miscellaneous Electrical

11 November 1997: Walter McDonald [CPS Substitute; X-ray Technician at Great Lakes Veterans Hospital]
He showed us a way a converting a physics [electrical] problem involving internal-external resistance to an experiment. Also some of the problems of using reactive loads on an AC circuits with DC formulas were touched on, and labeling truths (an UV labeled lamp in which we could not detect any UV radiation). Lee Slick and Larry Alofs wheeled out the band saw and showed the power surge at the start because of the windings [inductance] in the motor.

20 January 1998 Larry Alofs [Kenwood Academy (HS)]
He promised when he taught the chemistry after splitting H₂O into component hydrogen and oxygen molecules, that he would show how they came back together. He used a battery charger to split H₂0 by electricity. Now he had two test tubes, one with (since energy was added we suspect that there might also be some O₃ ozone) with half the volume of H₂. He used an electrode with insulation located in the separation liquid which had 1/2 mole of NaOH. He took a plastic pop bottle top and a tray of water, He placed the gases in the pop bottle top (under H₂O); the top had a rubber stopper that had 2 wires in a "V" configuration pushed through the stopper. He clipped one to the gas pipe, and the other to a spark generator, and showed how the mixture exploded. CAUTION: in his trials at school the wires were blown out of the rubber stoppers. He used a small test tube that was approximately 3/4 full of H₂.

13 October 1998: Alex Junievicz [CPS Substitute]
He was upset by an ISPP presentation that used video tape to analyze an experiment used 30 frames a second, but the video gave a vertical rate of 60 cycles per second. Video has 60 raster interlaced; thus it is 30 full frames per second. 1/2 inch video tape units typically have 2 video heads, and they are offset in azimuth as a scheme to conserve tape and enhance performance in slow motion. Thus the 30 per second rate was displayed. This applied to only certain dual head video tape recorders, and in certain others the 60 cps rate would apply. Actually video has a mathematical phase relationship of 15 frames per second. This is due to the color sub-carrier. This is why there are noise cancellations, and provides the basis for the fact that comb filters work.

14 September 1999: Therese Donatello (ST Edwards School)
passed out magnets and common objects, and we explored which were attracted to the magnets. We tried the magnets on stuff in our pockets, etc, and learned that some metals are attracted. None of the non-metals we tried were attracted.

Next, we tried to generate static electricity by rubbing. If the rubbed object became charged, then we found the puffed wheat could be attracted; there was no case of repulsion found. We carefully discussed the results and got an idea of how scientists look at the world. Good ideas!

06 November 2001: Leticia Rodriguez (Ruben Salazar Bilingual Center) Science Fair Projects
Leticia described what she learned at a recent in-service day concerning science projects.  She learned the following points:

• The science project should not be just a "show and tell" demo.
• The project should address and solve a problem.
• It is important to develop a procedure that permits isolation of the effect of a single variable.

She described a project involving the effect of magnets on paper clips.  The strength of the magnet can be shown to depend on the size or magnet, or the number of magnets affecting a given paper clip.  The more magnets, the greater the distance from which the paper clip can be moved.

As another example she talked about determining which of three brands of popcorn gives bigger popcorn kernels, by counting how many kernels can be fit into a container of a given size.  The more kernels, the smaller the volume per popcorn.  It is simple and convenient to draw graphs and charts to illustrate the findings, and students can begin doing this in the primary grades.

05 February 2002: Frana Allen (Skinner School, grades 1-5) Circuits
Frana brought in some very neat battery-operated kits (costing about $50 each). The kits, which are called SWITCH ON,  require two AA batteries for operation.  You can order them by email, snail-mail, FAX, or telephone:

SWITCH-ON!
P.O. Box 705
Bellevue, WA 98009
U. S. A.
Phone: (425) 747-7766
Fax: (425) 957-9384
E-mail address: switchon@concentric.net

07 May 2002: Tyrethis Penrice (Oak Park School System) -- Electricity and Magnetism
Tyrethis gave a handout on Electrostatics, which covers some of the same material as the SCETV National Teacher Training Institute website on Static Electricity [ http://www.scetv.org/ntti/lessons/1999lessons/staticelec.html], an excerpt of which is given below:

1. "Tie about 10 inches of thread around a piece of cereal. Suspend the cereal so that it does not hang close to anything else. (Your partner can hold (it) or attach (it) to the side of the desk.)"
   
2. "Charge the comb by rubbing it vigorously with the wool."
   
3. "Slowly bring the comb near the cereal. It will swing to touch the comb. Hold it still until the cereal jumps away by itself."
   
4. "Now try to touch the comb to the cereal again. It will move away as the comb approaches."

"Rubbing the comb with the wool moved the electrons from the wool to the comb. The comb had a negative charge. The neutral cereal was attracted to it. When they touched, electrons slowly moved from the comb to the cereal. Now both objects have the same negative charge, and the cereal is repelled."

18 November 2003: Fred Farnell [Lane Tech  HS, physics]     Electric Tennis Shoes 
Fred showed a pair of heavily worn tennis shoes on temporary loan from his daughter, on which lights flashed whenever the shoe experienced a strong impulse.  How come?  It was generally agreed that not much current would be required to set off the LEDs in the shoes.  There was no definitive answer as to how this was done, and it was felt that the shoes should be taken apart to determine how they function.  The following hypotheses were suggested to explain the operation of the shoes:

• Piezoelectric film inside to produce voltage, firing an LED?
• Battery plus triggering spring?
• Magnet and coil to produce current when triggered by spring?

24 February 2004: Fred Farnell [Lane Tech, Physics]           The Dissection of the Twinkling Shoes
Fred held up his daughter's tennis shoes, the same ones that he had showed us at the 18 November 2003 SMILE meeting [mp111803.html].  The lights embedded in the shoes still flashed on and off when he struck them.. His daughter's shoes had outgrown their usefulness, and Fred was allowed to study / destroy  them to determine how they work. He polled the audience on hypotheses as to how they work. Here is the "official tally":

You really got to the sole of the issue! Very nice, Fred.

09 March 2004: Fred Farnell [Lane Tech, Physics] Twinkling Shoes - Circuitry
At our last meeting [mp022404.html], Fred had dissected his daughter's twinkling tennis shoes in an effort to find out how they worked. He had found -- near the center of the sole -- a hard, transparent, plastic box, about 3 cm (square) and 1 cm thick, with wires connected to various LEDs imbedded in the shoe. But we ran out of time to figure out how it worked.

Now Fred held up a cardboard sheet (about 20 cm ´ 30 cm) on which he displayed to us the complete circuit, consisting of the LEDs connected to the box. He tapped the back of the cardboard against his hand (analogous to a shoe striking the ground), and sure enough! The LEDs twinkled on-and-off in some sort of sequential pattern, as they evidently had done in the shoe! "Inside the box," Fred told us, "you can identify a battery, a capacitor, a kind of switch, and transistor circuitry." Next, Fred brought a small (about 2.5 cm square, 6mm thick) permanent disk magnet near the box. Depending on where he placed the magnet, we watched the LEDs flash through a sequential pattern once, or to continually do so, or to light a single LED continuously! Fascinating! The switch appeared to be some sort of reed switch. Our speculations as to the basic mode of operation at the last meeting were only partially correct; it was more complex than most of us had assumed. But who would guess that such electronics would be found embedded in tennis shoes? From the patent number [US Patent Number 5,9969,479] on the box, Fred's colleague, Don Kanner, looked up the patent on the website of the United States Patent and Trademark Office: http://www.uspto.gov/patft/index.html. (Don has had experience in obtaining a patent a few years ago.) The following abstract describes this Light Flashing System, patented by Wai Kai Wong:

"A light-flashing system for flashing lights on and off and for generating a pattern of illumination for a plurality of lights in response to intermittent switch closures. The system includes a battery, a plurality of light-emitting elements, a plurality of transistors which enable the illumination of the light-emitting elements, a switch, a capacitor, and a pattern-generation circuit. The battery powers the light-emitting elements and the pattern-generation circuit. The switch intermittently clocks the pattern-generation circuit and enables the flow of current in certain of the transistors, allowing illumination of certain of the light-emitting elements in response to changes in inertial forces caused by movement of the flashing light system. The capacitor is connected in parallel to the battery such that the capacitor stores electrical charge when the switch is closed and continues to enable the flow of current through certain of the transistors after the switch is opened. The pattern-generation circuit then causes at least one, but not necessarily all, of the plurality of light-emitting elements to illuminate by enabling the flow of current through certain of the transistors. As the switch intermittently opens and closes, the pattern-generation circuit is clocked through various states, and the outputs of the pattern-generation circuit enable the flow of current through certain of the transistors, allowing illumination of at least one, but not necessarily all, of the light-emitting elements in a pattern."

You really held our feet to the fire for this one! Great job, Fred.

04 May 2004: Richard Goberville [Joliet Central HS, physics]           Lightning Reaction
Richard recently purchased the Lightning Reaction toy [http://shockingfun.zoovy.com/product/REACTION. Here is a description taken from The Stupid Store page:

"Here's how Lightning Reaction works -- Anywhere from two to four people can play at once. You remove a handle from the base and get ready. When you press the button in the center, a red light pulses and suspenseful music plays. As soon as the red light turns green, you press the red trigger button as quickly as possible. If you're the slowest player, you will get rewarded with a painful electric shock. If you were faster than your opponents, you can simply laugh as the loser screams in pain.

07 February 2006: Larry Alofs (Kenwood Academy, retired)             Pan Pipes, Fresnel Lenses, and Hall Effect Sensors
Larry had made a Pan pipe by taping together 8 PVC  pipes (about 1 cm in diameter) and varying, in length, the shortest being about 6.5 cm  = 0.65m. Larry noted that the length of the tube should be one quarter of the wavelength (l) of the fundamental tone. For the 0.065 m tube, the wavelength would be l = 4 * 0.065m = 0.26 m. Now the frequency f of the tone will be given by f = V/l, where V = 350 meters/second is the speed of sound. Thus the frequency should be f = 1350 Hz.

Next, Larry held up - for all to see - a transparent plastic sheet, about 35 cm square. He showed us that it magnifies like a convex lens, despite being flat. Called a Fresnel lens, it has the advantages of being
flat, lighter weight, and less expensive than an equivalent convex lens of glass. A Fresnel lens may be thought of as formed from a convex glass lens. Imagine removing from its surface a narrow and thin ring of glass, concentric with its optical axis. Place the ring flat on a flat, transparent surface. Then remove the next larger ring and place it to surround the first ring. Continue this process until the entire glass lens surface has been placed on the flat, transparent surface as a series of thin glass rings, each having the curvature of the original convex lens surface from which it was removed. This would then be a Fresnel lens, and would focus light like the original convex glass lens.  The Fresnel lens was common in old light houses to make a focused, intense beam. For details see the Michigan Lighthouse  Conservatory website:  http://www.michiganlights.com/fresnel.htm.

Porter noted that Augustin Fresnel  1788-1827 [http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Fresnel.html] developed the wave theory of diffraction that led to the following striking prediction:

"Let parallel light impinge on an opaque disk, the surrounding being perfectly transparent. The disk casts a shadow - of course - but the very centre of the shadow will be bright. Succinctly, there is no darkness anywhere along the central perpendicular behind an opaque disk (except immediately behind the disk)."

When the existence of the bright spot was experimentally confirmed, the wave theory of light became accepted by virtually everybody.

Finally, Larry held up what we saw as a small (about a cubic inch) black object with wires coming out. "This is a Hall Effect Sensor," Larry  told us. He explained that he had a problem with a car that was hard to start once it had been warmed up and turned off, and he had traced the problem to the Sensor. Larry made a sketch on the board and explained how  the Sensor works. Suppose a strip of semiconductor conducts a current along  its length. If a magnetic field is produced transversely to the current, electrons are diverted toward one side of the strip, producing an electric field across the strip. This is the Hall Effect. For details see http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/hall.html.  In the car, a shaft (synchronized with the engine's crankshaft) rotates and moves a magnetic disk into a slot of the semiconductor strip, producing a Hall Effect electric field. A transistor detects the field and triggers other circuitry to fire a spark plug and to activate a fuel injector.  This Hall Effect Sensor replaces the points in old fashioned engines in delivering high voltage sparks; in addition, it controls the fuel injectors. For details see this Wikipedia website:  http://en.wikipedia.org/wiki/Hall_effect_sensor. The faulty sensor in Larry’s Saab apparently was misbehaving only when it got too hot, so that the semiconductor behaved more like a conductor. Larry worked very hard to take out the sensor from the Saab and made a circuit to test it.

Porter pointed out that modern Hall Effect Sensors use semiconductors rather than conductors because the rate of flow of individual current carriers (electrons) is really slow in conductors, and it is much faster in the semiconductors  -- because the latter have so many fewer current carriers, which travel at much greater drift speeds. This produces a "Hall voltage" that is large enough to detect. 

Fascinating stuff!  Thanks, Larry.