High School Mathematics-Physics SMILE Meeting
1997-2006 Academic Years
Electric Circuits

28 October 1997: Roberta Williams [IIT Physics Graduate Student]

         --Power of a Grape--- Fruit Powered Clock
                    Zany Brainy $13
                    Cu (+) and Zn electrodes
         Cut grape in half, constructing 2 cells

Grape has 0.92 V and 0.058 mA when shorted [internal resistance thus 16000 Ohm].  Lifetime of grape as battery to drive LCD clock = 4 days [the grape is pretty well dried up at the end].

Caution: One must use a Voltmeter with very high internal resistance!

20 April 1999: Arlyn Van Ek [Iliana Christian High School]
He tried to show the effects of different power light bulbs in a parallel and series circuit.
With different wattage, why is the larger wattage (lower resistance) dimmer when in series, yet brighter when in parallel?? A way of explaining is to use V=I R to solve for the unknown. If V is fixed [parallel] we solve for I, whereas if I if the same we solve for V.

It was pointed out that when the wattage was close the higher wattage bulb would gain brightness more slowly than the brighter one. This is because a Tungsten bulb has a lower resistance when cold, thus the higher resistance would get a surge while the lower resistance (large wattage) would take longer...A flicker of brightness surge in the lower wattage. (Edison's original bulbs had carbon filaments and had the reverse where it would slowly get bright, where tungsten would cause a surge to quickly get to brightness. Why do bulbs burn out when they are turned on ???)

Other matters

14 September 1999: Walter McDonald [CPS Sub; Great Lakes Vet Ctr]
showed us a small apparatus with a wire coil and variable capacitor, and a 5.7 V 60 Hz power supply. He passed out copies of a lab experiment: The Q Meter and Measuring Inductance. Then he described some results he obtained. The series R-L-C circuit was to be tuned to resonance and voltage measured across the C.

27 February 2001 Bill Shanks (Joliet Central retired; Joliet Junior College Music Student)
took advantage of the after-Christmas sales to purchase a string of 70 Christmas lights for $1.50, reduced from $5.99. By removing bulbs he was able to determine that there were 2 strings of 35 bulbs hooked in series. The current through the bulbs was 120 mA, and the rms Voltage across each bulb was 120 Volts/ 35 = 3.5 Volts. Thus, the internal resistance of each bulb would be r = 3.5 Volts / 0.120 Amperes = 30 Ohms.

   
Bill next pulled out a light emitting diode (LED), which required a Voltage of 2.0 Volts and current of 20 mA to fire. From these numbers, one might estimate its [variable] forward internal resistance to be R = 2.0 V /.020 A = 100 Ohms.

What happens when one of the bulbs is replaced by the LED [Symbol:  ] ?  There were some dire predictions that the LED would burn out, because it would block the flow of AC current in the "back" direction, and thus get a maximum voltage of about 170 Volts across it every 1/60 second.  A counter argument would be that "current" rather than "voltage" is needed to burn up the LED, and there is no current flowing during reverse bias.  What really happens?

   
The LED did work well in the circuit, and did not burn out.  The current passing through the LED should have been over 100 mA, since the internal resistance of the LED is only slightly greater than that of one of the bulbs. Apparently, these LED's are tough little critters. Very interesting, Bill.

14 March 2001 Marilynn Stone (Lane Tech HS, Physics)
showed a circuit board system containing a battery, 3 light bulbs, and 4 single switches, and one double switch [S5] attached to a plywood sheet [roughly 2 feet by 2 feet], as shown.

She then asked the students to accomplish these tasks with the battery:

Connect B1 in series. Connect B2 and B3 in series. Connect B1, B2, and B3 in series.
Connect B1 and B2 in parallel Connect B1, B2, and B3 in parallel. Connect B2 in Series.
Connect B3 in series. Connect B1 and B3 in series. Connect B1 and B3 in parallel
Connect B1 and B2 in series.    
The last exercise may be rather difficult.

10 April 2001 Ann Brandon (Joliet West HS, Physics)
passed out 4 sheets with various circuit problems designed to emphasize the basic approach in analysis of circuits involving resistors in series and in parallel. These sheets, which she prepared in the "draw" program in Microsoft™ Word /Office 95 or 97, had the following titles:

Here is an image of one of the sample circuit problems from the third sheet:
The full set can be obtained from Ann Brandon: llbrandon@aol.com

10 April 2001 Earnest Garrison (Jones Academic Magnet HS)
showed some Discovery Kits on electromagnetism that he had obtained from the source:

Science Kit & Boreal Laboratories
P.O. Box 5003
Tonawanda, NY 14151-5003
http://sciencekit.com/  [order a free CD-ROM catalogue]
Phone: 1-800-828-7777
Fax: 1-800-828-3299
He passed around booklet #65305-05, Electric Motor, written by Dr Lawrence Lowery, University of California, Berkeley, which described several experiments. Then Earnest showed us the transparent box with a tunnel passing through it, containing iron filings suspended in mineral oil, for showing the magnetic field in 3 dimensions when a magnet is inserted into the tunnel. Then he showed us a simple kit for making an electric motor, which simplified the process of winding the wire and putting the loop into the armature.

11 October 2001 Marilynn Stone (Lane Tech HS, Physics) Home Made System to Illustrate Circuits
Marilynn showed us a system consisting of a 4-plug convenience outlet box, with cord to house current, and two different bulbs {#1 and #2) screwed into plug sockets.  When the bulbs were plugged into the back two plugs, they both lit, with #1 being brighter than #2.  When the two bulbs were plugged into the front two plugs, then #2 is brighter than #1 How come?  As an additional hint for the wiring impaired, she unscrewed bulb #2 in the latter case, and we noted that #1 also went out.  By contrast, with bulbs in the back two sockets, #1 was lit, whether or not #2 had been unscrewed.  The group drew the following conclusions:

Can you explain everything that was seen under these assumptions?? Materials for assembly were estimated to cost $10 - $15.  However, it is important to practice safe science, and to label this device as a piece of laboratory equipment, and not an ordinary box extension cord.

Very nice, Marilynn!.

23 October 2001: Marilynn Stone (Lane Tech HS, Physics) Home Made System to Illustrate Circuits
Marilynn gave us the following diagram for her circuit from the last meeting,  09 October 2001, with directions:


Click here for a larger image.

03 December 2002: Karlene Joseph [Lane Tech HS, Physics]    A Story of Science Excitement
Karlene
 showed her 4th grade daughter a variation of the SMILE demonstration of resistors in series and resistors in parallel, which her daughter described as way cool! A few days later her daughter went into the hospital for tests involving a pulse oxygen monitor, a heart monitor, and a a neural monitor.  The daughter was quite surprised when she learned that humans ALSO conduct electricity.  Karlene got her a CHIRPING CHICK toy, on which a chirping noise is produced whenever you touch the terminals and make a closed circuit.  At school the daughter's class got 26 people to form a "series circuit", through which electricity passed so that the bird would chirp. The resistance between two points of dry human skin is of order 100,000 Ohms, so that a current of 0.1 milli-Ampères would flow with a potential difference of 10 Volts. Children are never too young to begin to appreciate science.  Really way cool, Karlene!

11 March 2003: Arlyn van Ek [Illiana Christian HS, Physics]      Power to the People
Arlyn
  illustrated that Power P = Voltage V ´ Current I, using Fred Schaal as his hapless volunteer / victim.  Fred was instructed to hold on to a small resistor until it became too hot to hold.  With a current of 1.0 Amp and Voltage  of 3.1 Volts, corresponding to a power of 3.1 Watts, Fred was able to hold on for about 12 seconds.  With a Voltage of 4.8 Volts and a current of 1.5 Amp, corresponding to a power of 7.2 Watts, Fred released the wire after only 5 seconds.  Interestingly, the total amount of heat [H] generated in the wire was about the same in the two cases:  36 Joules.

H = P t = (3.1 W) (12 s) » 36 J » (7.2 W) ( 5 s)    ---    WOW!
Arlyn then showed an array involving 3 identical light bulbs, with two placed in series and hooked in parallel with the third one.  We guessed correctly that the single bulb would be much brighter than the pair in series, when we hooked the array to the Voltage source.  In fact, because the two bulbs in parallel each have half the Voltage drop and half the current, each one is 1/4 as bright as the bulb in series.  Do you believe that?

Arlyn next brought out the Genecon Generator [http://www.arborsci.com/detail.aspx?ID=543], obtained from Arbor Scientific, which he had showed us at the 11 April 2000 SMILE meeting [mp041100.htm].  By turning a crank, we convert mechanical energy into electrical energy, providing current to light the bulbs.  When current passes through one bulb, it becomes much more difficult to turn the crank --- the more electrical energy one makes, the more difficult it is to turn the crank to supply mechanical energy. We cannot readily see the stress placed on a battery when it converts chemical energy into electrical energy, but we can feel the stress in our own hands when we we turn the crank to make electricity!. Beautiful!

An electrifying demo on a hot topic, Arlyn!

23 September 2003: Bill Shanks [Shanks Math-Science Academy, lifetime student]        Miraculous Battery Recharger
Bill
showed his latest toy, a novel Nickel Hydride battery charger made by Rayovac Corporationhttp://www.rayovac.com/.  The following information is excerpted from a more complete description.

"Rayovac’s I-C3 Technology Makes 15-Minute Charger the First of its Kind.
New rechargeable system allows for battery charging in 15 minutes or less and up to 1,000 times.
Recharged batteries last up to up four times longer than alkaline in certain devices.
No other battery recharging system is faster"

The charger appeared to work as claimed, but Bill noticed a problem with the specifications on the package.  It required a power source with an E.M.F. of 14.5 Volts DC and a current of 4.5 Ampères, and yet it produced 2 Amp hours of charge in 15 minutes.  It seems to produce 8 Amps of charging current, and yet it requires only 4.5 Amps from the power source. Very odd!   After all, a battery, like a water pump, merely lifts the potential of the circulating fluid, and is not itself the source of the fluid. Does anybody understand whether their specifications are correct?  If so, how can they be?

Bill also showed his latest Ultra-bright High Intensity LED's from L.E.D. CLUB:  http://www.ledclub.com/products.htm, in Green, Blue, and White [two 3-Volt batteries required], as well as Red and Yellow [one 3-Volt battery required].  They were quite powerful, and he used them to show us color addition in the darkened room.

That was intense, man! Thanks for sharing your physics gadgets with us, Bill!

09 December 2003: Bill Shanks [retired physics teacher & member, Joliet Junior Chorale]        Clothes Pins that Light Up
Bill
showed us the perfect "party gag" gift --- a pack of 50 plastic clothes pins, complete with (LED) bulbs, which light when you use the clothes pin to clamp the LED leads to make contact with electrodes on a small "dime shaped" Lithium cell, such as CL 2016, 2025, or 2032, which are rated at 2.8 Volts.
PJ Comment:
A red or green LED can be lighted with a single cell, since a photon of energy 2.8 eV corresponds to a wavelength

l =  c / n =  h c /(h n) =  hc / E =  1240 eV nm / 2.8 eV =  442 nm.

How could you use this in class?

Bill, you must be the life of the party! Very nice!

24 February 2004: Ann Brandon [Joliet West HS, Physics]           Figuring Physics:  Light Bulbs
Ann called our attention to the March 2004 issue of The Physics Teacher [http://scitation.aip.org/tpt/], an official publication of the AAPT: American Association of Physics Teachers:[ http://www.aapt.org]/. The monthly column Figuring Physics by Paul Hewitt contained questions about two light bulbs, A and B. They were in sockets connected in series across a DC source (battery).  Bulb A is definitely brighter than bulb B.  What happens when the positions of the two light bulbs are switched?  We took the following "straw poll":

A will be brighter        17 Votes
B will be brighter   1  Vote
No Idea   4 Votes
Ann then switched the bulbs. Behold!  Bulb A was still definitely brighter than bulb B, in agreement with Hewitt's answer page.  Why?

Ann then asked which bulb would burn brighter when they were each placed directly across the battery? Curiously enough, bulb B burned brighter than bulb A in that situation. Why? 

The result can be understood with the formulas relating the voltage V, current I, resistance R, and power P for a resistor:

P = I2 R = V2 /R
The lower wattage bulb thus corresponds to higher electrical resistance. Tres simple!

You really lit us up! Thanks, Ann.

06 April 2004: Peter Smagacz [Paul Robeson HS, Physics]         Drift Velocity
Peter
asked how fast electrons are traveling when a large electric current is passing through a wire.  Some people might guess that they move at the speed of light.  Surprisingly, the electrons are slowly drifting through the wire, at less than one millimeter per second, when, say, current is passing through the starter motor in an automobile.  How come? The answer lies in the fact that the density of electrons in a conductor is very large (n = about 1029 per cubic meter), so that a very large current per unit area J is produced even when the drift velocity is fairly modest.  Specifically, J = I / A = n e0 vD. Thus, when vD = 0.001 m/sec, we get

J = 1029 ´ (1.6 ´ 10-19) ´ 0.001 = 1.6 ´ 107 amp/m2.

If the battery cable has a cross sectional area A= 2´10-5 m2, the current flow would be I = J A = 320 amps. Peter illustrated this with an analogy by lining up some small stones (electrons) in a trough. When he pushed another stone into the trough at one end, a stone at the other end fell out.  Thus, while the stones did not move rapidly, the effect of a stone entering at one end was rapidly communicated to a stone at the other end.

Thanks for sharing this, Peter!

06 April 2004: Roy Coleman [Morgan Park HS, Physics]           Diodes and Bulbs 
Roy took 40 watt and 75 watt light bulbs, and showed us that the 40 watt bulb produced less light than the 75 watt bulb, when screwed into a 110 volt socket -- presumably because of the higher resistance of the 40 watt bulb.   Then he reminded us of Ann Brandon's demo from the last SMILE meeting, in which the lower wattage bulb produced less light than the higher wattage bulb, when placed in series across the 110 volt supply.  He then asked us whether the same thing would happen here.  Curiously enough, when he placed two knife switches in series with the bulbs, one bulb would light only when the first switch was closed, whereas the other bulb would light only when the second was closed.  How can that possibly be true??  Roy explained that he had "improved" the switches and the bulb sockets by placing diodes under them, as shown in the circuit below (handout).


NOTE: diodes are wired in opposing directions 
The same principles apply to low voltage lights, but the diodes involved should have higher current ratings than those in the high voltage case.

We see the light(s)!  Thanks, Roy!

04 May 2004: Bill Blunk [Joliet Central, physics]           Series Circuits  --- or What?
Bill showed us a simple-looking circuit that consisted of two identical light bulbs in sockets, hooked together in series and attached to wires with a plug on the end.  When the circuit was plugged into the 115 VAC line, both lights went on with equal intensity, as expected.  However, when Bill unscrewed one of the bulbs, the other one continued to burn.  How come?  A similar thing happened when he screwed that bulb back in and unscrewed the other bulb.  Although Bill claimed to be a magician who would not reveal his secrets, we suspected that he had slipped diodes under the sockets.  Are we right, or are we right? Better luck next time! Thanks for the show, Bill!

12 October 2004: Bill Shanks [Joliet Central HS,  happily retired]           Sale on Volt - Ohm Meters at Harbor Freight
Bill called attention to a sale on VOMs for about $2.00 each, at local franchise stores of Harbor Freight Tools [http://www.harborfreight.com/] in Arlington Heights IL, (940 W Dundee Rd);  phone: 847 - 392-1400 and West Aurora IL (904 N Lake St in Westgate Mall); phone: 630 - 966-9008.
Thanks for the heads-up, Bill!

14 December 2004: Larry Alofs [Kenwood HS, physics]           Inductance
Larry
set up his mini video-camera, attached it to our small TV, and focused it upon a rather sophisticated TVM:  Transistor Voltmeter.  Now we could all see the readings on the small TV.  The TVM could be used to measure voltage, current, and resistance --- in addition it could be used to determine Capacitance and Inductance, using special plugs called C and CX for Capacitance, and L and LX for Inductance.  He set the meter to its most sensitive scale for inductance (mH: milliHenry), and attached a long piece of wire to the special plugs L and LX.  When the wire formed only one loop, the meter read an inductance L = 0.001 mH.  When he looped the wire around several times in the same direction (lasso style), the inductance reading increased, up to 0.016 mH.  When he folded these loops at the middle, so as to double the number of loops, the inductance again increased. Then he formed a smaller loop with the same number of turns, and we saw that the inductance decreased to 0.007 mH.  He then put an aluminum bar inside the coil, and there was no observable difference in the inductance. But when he placed a (soft) iron bar inside the coil, we saw the inductance increase from 0.016 mH to  0.022 mH.  As he put more iron bars inside the coil, the inductance steadily increased.  Here are the data for the inductance versus the number of turns of wire, with one iron bar inside:

Number of Turns     Inductance (mH)
1 0.001
5 0.001
10 0.003
15 0.010
20 0.016
25 0.022
30 0.030
35 0.037

Larry next took a large air core solenoid, consisting of about 500 turns of wire with inner radius about 5 cm.  He measured the inductance of the coil.  When he placed an iron bar inside the coil, the inductance increased -- the more bars, the greater the inductance.  Here are the data:

Number of bars inside    Inductance
0 4.83 mH
1 8.22 mH
2 11.67 mH
3 14.75 mH
Larry then took a much heavier coil of wire and repeated the experiment of measuring its Inductance with and without iron bars in its core.  Here are the data:
Number of bars inside       Inductance
0 0.812 H
1 1.28 H
2 1.69 H
3 2.05 H
Larry added an aluminum bar into that coil, and we saw that its measured inductance decreased slightly How come?  Could diamagnetism be at work? Or is it Lenz's Law? Eddy currents?

Larry next passed around a 1.5 Volt dry cell battery, along with wires coming from one side of a transformer.  Larry mentioned that there is no problem in connecting the leads from the primary coil in the transformer to the battery, but that when the leads are removed a spark often develops.  The effect is explained by Faraday's Law, relating the induced electromotive force EMF to the time rate of change in F, the magnetic flux:

EMF = - DF / Dt = - L DI / Dt.
We passed the device around the room, and were occasionally able to draw a spark. Much to our surprise, we got quite a shock as a high voltage pulse passed through our bodies. Amazing!

For his next encore, Larry took an ordinary 40 Watt light bulb, and calculated its internal resistance from the formula relating (RMS) power P to (RMS) Voltage V and resistance R:

P = V2/ R ... or ... R = V2/ P = 1202 / 40 = 360 W.
First Larry put the light bulb into a socket, and plugged the leads of the socket into the house current. The bulb lit normally. Next Larry hooked the big coil in series with the light bulb, and then plugged it into the house current. The bulb was much dimmer than before. How come?! Larry told us that the big coil had a DC electrical resistance of 80 W -- which would not be enough to explain the dramatic change in brightness of the bulb, since 80 W is a small fraction of 360 W.

We decided that the villain here was Inductive Reactance, a term in the Complex Impedance [ http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/impedance.htm] produced by inductance. Inductive Reactance is the resistance to alternating current caused by the Inductance of a coil.  The Inductive Reactance X =  2pfL. Since L = 0.812 H for the coil and f = 60 Hz, then X = 2p (60 Hz) (0.812 H) = 310  W, so that the inductive reactance is more important than the DC resistance in this case.  Larry showed that, as he stuffed more and more and more iron bars into the coil, the bulb became dimmer and dimmer and dimmer, because of the steady increase in Inductance, and therefore Inductive Reactance.

A superb phenomenological exercise, from which we all enjoyed and learned a great deal! Thanks, Larry!

29 March 2005: Charlotte Wood-Harrington [Brooks HS, physics]              Series and Parallel Resistances
Charlotte
gave each of us three Christmas Tree bulbs with wire leads that she had cannibalized from old sets, as well as tape and a stick of chewing gum.  We were permitted to chew the gum, with strict instructions to use its metallic wrapper to make electrical contact with the lights.  We were able to make various series and parallel combinations of lights, which we tested with a 9 Volt battery.  The lights could be made to burn dim or brightly, with various combinations.

As an extension of this lesson, Roy Coleman asked the following question:

Q: Why should batteries be purchased using a credit card?
A: So that they get charged.
Enthusiastic and cheap! Thanks, Charlotte.

12 April 2005: Arlyn Van Ek [Illiana Christian HS, physics]              AC-DC
Arlyn
brought in a set of 4 transformers that convert house current  [120 V AC] into about 10 V AC. He showed the output Voltage from the First transformer on a small oscilloscope.  We saw the graph of Voltage versus time, and became convinced that AC was coming out of the transformer. The Second transformer had a diode in series with an output lead.  Since the diode permits the flow of current only in one direction, we would expect the output signal to be "pulsed DC", or "half-wave rectified AC". The Third transformer was connected to a two-diode configuration, which gave full-wave rectified AC, but with only about 70% of the maximum voltage obtained with the first transformer. Finally, the Fourth transformer was connected to a configuration of four diodes, giving full-wave rectified AC with the same maximum voltage as with the first transformer.  Finally, Arlyn connected a capacitor across the secondary leads of the Fourth transformer (with four diodes).  The oscilloscope trace showed a rather constant Voltage -- we obtained DC at last! For more details see the All about Circuits website http://www.allaboutcircuits.com/vol_3/chpt_3/4.html.

Very nifty stuff, Arlyn! 

02 May 2006: Erik Jurgens (Joliet West HS, physics)                            Going From a Circuit Schematic to the Breadboard
Erik made “components of circuits” molded out of toilet paper rolls and boxes (to show resistors, voltage sources, etc. with colorful yarn at either end to represent wires. With magnets attached to the yarn and the components, circuits could be illustrated on the blackboard. The students could then visualize the circuit before setting it up in the laboratory.  An example of "roll playing"!
Very interesting and useful! Thanks, Erik.