High School Mathematics-Physics SMILE Meeting
1997-2006 Academic Years
Static Electricity

10 March 1998  Bill Blunk [Joliet Central HS]
Electrostatics - a way of calculating electrostatic forces in the spirit of charging "pith balls", using balloons charged with fur, which repel one other. Then measure the distance of separation at equilibrium and the mass of each balloon. With d = 0.8 meters and masses of 2.5 g (.0025 kg), we get about 1.3 ´ 10-6 Coulombs.

Roy Coleman commented that two one Coulomb charges separated by one meter repel at 1010 Newtons --to give a feel for the effect of a one Coulomb charge.

[Another comment was that students are quick to state that a solution is impossible because of missing facts, whereas the missing data is not necessary when it cancels out in the process of doing the math.]

02 February 1999: Bill Blunk [Joliet Central HS]
Another modeling presentation
Using flat magnets (either from Radio Shack [http://www.radioshack.com/], or American Science and Surplus [http://sciplus.com/])
Using red tape put a+ on one side and a - on the other, then static charges can be modeled. He also pointed out the no charge was actually both + and - 's

05 February 2002: Bill Blunk (Joliet Central, Physics) Garbage
Bill has given up trying to make garbage attractive, so he showed a method to make your garbage repulsive to everybody else.  Specifically, he used rabbit fur [the remains of Poor Thumper, who gave his/her all to science], a hard plastic rod [for electrostatics experiments], and plastic foam packing material in sheet form.  He formed a ring out of the packing material, and used the Poor Thumper [rabbit fur] to charge both the rod and the ring.  Then he threw the ring into the air, and it "floated" in the air above the rod.   Since the ring and the rod contained charges of the same sign, courtesy of Poor Thumper, the ring was held aloft by the repulsive electric force between them.  The ring could conveniently [or inconveniently] be dropped on a nearby person's head by taking the rod away. [Note for animal activists:  artificial fur works pretty well also!]  If you don't have a rod, it will work very well with an inflated and electrically charged rubber balloon, as Bill showed us.  You made a repulsive display attractive to us, Bill!  Thanks!

19 March 2002: Ann Brandon (Joliet West HS Physics) -- Static Electricity with Ping-pong Balls
Ann held up two ping-pong balls for us to see. They had been painted in "copper print", a conducting paint once extensively used for repairing printed circuit boards, so their surfaces were electrical conductors. She proposed to put an equal charge on each ball, and then determine the amount of that charge by measuring the Coulomb repulsion between the balls. The balls were connected by a piece of insulating nylon thread about 1 meter long,  so that when  Ann stuck the center of  the thread to a ringstand support, the two balls became suspended below and dangled in contact with each other.  Next,  she charged the balls, touching them together while charging to be sure that they held equal charges. Since like charges repel, the balls were repelled away from each other, and when they soon came to rest,  the situation looked like this:


The forces producing equilibrium on, say, the left ball are

Ann measured the mass of the balls to be 2.8 grams, and the apex angle 2q to be about 10°, so that the Coulomb repulsion of the balls is  F = mg tan q = 2.4 ´ 10-3 Nt. If the charge on each of the balls is q, and the separation distance is d = L sin q = 0.038 m, then the charge q can be calculated from Coulomb's law for the repulsive force:  F = k q2/d2, where k = 9.0 ´ 109; thus q = 4 ´ 10-8 Coulomb! Very nice!.

Ann then estimated the electrostatic potential of the charged balls, which we calculated according to the formula V = k q /R, where the ball radius is R = 1.55 cm. Thus we get V = 9.0 ´ 109 ´ 4 ´ 10-8 / (.0155) = 22,000 Volts.

Ann then showed how to put a charge on objects, using a clear (vinyl) ruler and a black (acetate) ruler.  She rubbed each ruler in turn with a piece of paper, and then used an electroscope to show that each charged on rubbing (triboelectric effect), and that they were given opposite charges by this process---positive for the clear ruler and negative for the black ruler.  She also created a charge using an electrophorous, an apparatus made from a small slab of stiff Styrofoam® insulation and  a conducting pie pan with an insulating handle (a Styrofoam cup} at its center.  She charged the insulation by rubbing with a piece of cloth and placed it on the table. {Let's assume the charge is positive.} Then, holding the pie plate by its insulating handle, she placed it on the charged insulation. When she touched the pie plate with her finger, negative charge flowed from her to the pie plate, since opposite charges attract. This gave the pie plate a net negative charge, and this process of charging is called "charging by induction." None of the positive charge on the insulation was removed, since - by definition - charge cannot move in an insulator. Ann then touched the pie plate to the electroscope so that negative charge on the pie plate was conducted to the electroscope, giving it a negative charge also. She showed that you could impart the the opposite charge by holding the object near the electroscope, and touching its frame with your finger (charging by induction). 

We got a charge out of this subject, which has great potential, Ann!

19 March 2002: Roy Coleman (Morgan Park HS Physics) --Electric Ding-Dong [A Harald Jensen Original!]
Roy described an apparatus in which a pair of parallel, conducting plates (assumed infinite in area) were charged to a potential difference of, say,  V = 5000 Volts.  A conducting ping-pong ball is suspended by a long, insulating thread so that it hangs about midway between the plates, where it is free to swing. When the ball is moved into contact with one of the plates, it acquires a charge (by conduction). It then oscillates back-and-forth between the plates, striking first one, then the other, making a "Ding-Dong" sound. This is a fascinating phenomenon to observe! The electric field between the plates is E = V/d, where d is the distance between the plates. Problem: Given the radius R of the ball and the distance d between the plates, estimate the time required for the ball to go back and forth.

Solution outline:  When the ball touches a plate with potential V, it acquires a charge q, where V = k q / R, or q = RV / k.  The ball then experiences a force, F= qE , due to the electric field E = V / d , so it is pushed toward the other plate.  Its center travels a distance d - 2R, so, by Newton's Second Law, F = ma , it experiences an acceleration a = q E / m. Assuming simple harmonic motion and the equations that follow, the time required for the trip is given by Ö (2 (d-2R)/a). End of story.

19 November 2002: Barbara Lorde [Attucks Elementary]       Static Electricity
Barbara passed around information from the websites Humans and Sparks [The Cause, Stopping the Pain, and "Electric People"] http://www.amasci.com/emotor/zapped.html, and Your Admirer is a Balloon! http://www.mos.org/sln/toe/admirer.html.  We carried out an exercise, Salty Sounds of Static, concerning the creation of static electricity by friction, as well as attraction through static electricity.  She gave us small inflated balloons, and passed around a salt shaker.  We sprinkled a little salt on our desks, and then rubbed the balloons briskly about on our heads, arms, clothing, etc. --- doing whatever was necessary to generate some static electricity.  We then brought the charged balloons near the salt, but not touching the salt.  We found that the balloon attracted a little salt, and studied whether "more rubs of the balloon" led to "more salt", etc.  We also studied "Styrofoam® attraction", as described in the website What Will a Charged Balloon Attract?; http://www.mos.org/sln/toe/balloon.html, and found that Styrofoam® leaps onto the balloon, and then jumps off after a few minutes, as explained there.  You shocked us with your knowledge, Barbara!

28 January 2003: Bill Blunk [Joliet Central HS, Physics]     Electrostatics for the Follically Challenged
Bill began by lamenting that electrostatics experiments are ineffective on people who have certain  types and styles of hair, or who use certain conditioners --- not to mention those who have practically no hair at all on their heads!  How do we bring these people into the inclusive electrostatics fold?  He found an interesting answer --- pom-poms!  He obtained some extra pom-poms after an event, and found that they were quite effective as pom-pom wigs. Just for fun, he put the pom-pom on his head. He then placed it on the table, and charged it by rubbing it with rabbit fur [poor Thumper, who gave his all to science!].  The plastic strands of the pom-pom spread apart in a lovely radial pattern when he held it up. He too, of little hair, could experience the joy of electrostatics!  Next, Bill  lit his BIC® lighter, and when he brought it under and near the pom-pom without touching it, the pom-pom rapidly drooped down, losing its charge.  [Bill had been limited to making charged balloons fall off the wall by bringing his lighter near, but this is definitely more dramatic!]  How come Bill ended up with "flat hair" once again?  Why did the pom-pom discharge?   

These pom-poms make interesting and potentially fashionable hairpieces, and fashion moguls should pick it up!  You're on to something here, Bill! Great!

09 March 2004: Bill Shanks [Joliet Central, Physics -- retired]           Pop Can Electroscope: Construction and Operation
Holding a home-made apparatus up for us to see, Bill asked, "Does anybody know what this is?" Somebody guessed, "An electroscope?" In response, Bill rubbed a small, inflated balloon on his head and held it near the apparatus. We saw a small, gold-colored, metallic strip pivot back-and-forth as he moved the balloon toward-and-away from the apparatus. "We're all going to to build an electroscope like this, to take home," said Bill. Then he gave us the following items:

We turned the Styrofoam® cup upside down on a level surface, where it served as an insulating base.  The tape was placed along the cylindrical surface of the can, in rough alignment with the bottom of the tab opening on its end. The tape was folded back on itself, with its adhesive holding the horizontal can to the top end of the inverted cup.  A strip of foil  (about 1 cm ´ 4 cm) was cut, and an end of the narrow side was wrapped partially around the toothpick, in order to shape the strip like a hook.  The strip was then hooked onto the top end of the tap, so that it could pivot.  It took some patience and careful attention to get the right configuration. We then blew up the balloon, knotted it shut, and rubbed it against our hair or clothing to build up a static charge on it.  When the charged balloon was brought close to one end of the can, the strip on the other end moved away from the metallic surface of the can.  Like charges repel! Using Bill's electroscope as a model, it was fairly easy for each us to make his/her own.  We used our apparatus to explore and observe electrostatic interactions, including the following: Bill made a sketch of the pop can electroscope on the blackboard. On it, he placed small, round (about 2 cm diameter) red magnets to represent positive charges, and gray magnets to represent equal negative charges. [These magnets are available at office supply houses] Each red magnet was paired with a gray one to represent a net charge of "0". Then he drew a picture of the balloon with positive (red) charge on it, close to one end of the electroscope. To illustrate the resulting redistribution of charge on the pop can, Bill moved the negative (gray) charges on the pop can closer to the balloon (unlike charges attract!), and the positive (red) charges away from it (like charges repel!), resulting in a positive charge on the other end of the pop can and on the pivoting, foil strip. Bill pointed out that he gets his students to the blackboard to show electrostatic interactions using such magnets. 

For related information on Home-made Electroscopes see the page by Niles Township HS (IL) physics teacher Joe Serpico: http://www.niles-hs.k12.il.us/joeser/notes/electrostatics/electroscope.html.

Beautiful phenomenological physics, Bill!

25 January 2005: Ann Brandon [Joliet West HS,  physics]           Scotch Tape Electrostatics
Ann passed out materials as described in a handout containing the following instructions, and we soon were doing all these things:

A very nice phenomenological experience! Thanks, Ann!

10 May 2005: Bud Schultz [West Aurora HS, physics]              Dots and Lights 
Bud detected the large electric fields that are produced in the vicinity of a Vandergraaf Generator by bringing gaseous discharge tubes, which contain gases such as neon, close to the generator. We saw a very impressive display of red light from an ordinary neon tube! Fluorescent tubes (which contain mercury vapor) are available in 2 feet and 18 inch lengths, for more convenient experimentation.  For more information see The History of Electrostatic Generators:  http://www.hp-gramatke.net/history/english/page4000.htm. Bud showed us how to produce both positive and negative charges by rubbing objects together.

21 February 2006: Bill Blunk (Joliet Central HS, retired)                       Ping-pong Electrostatics
Bill  had bought a gross of ping pong balls and sprayed them with silver conductive paint. He made a pair for everyone! With a  monofilament string attached to connect the pair of balls, he hung the balls from the ceiling -- like a pair of pendulums -- (hanging by about 2 meters) so that they were next to each other in contact. With this setup there are a lot of fun things to do!

Before experimenting with this setup, Bill rubbed a plastic rod with a piece of fur, and then rubbed a loop (about 25 cm diameter) made from a strip of light, plastic, packing foam. The resulting charges on the two pieces of plastic permitted Bill to levitate the plastic ring above the rod and move it around the room! Then he rubbed the  rod again and touched both ping-pong balls to it, giving them like charges so that they repelled each other, and served as an electroscope -- unlike most other electroscopes, the charge on the ping pong balls could be determined! The separation of the balls in equilibrium was 14 cm.

We can calculate the charge on the ping pong balls.

F = k Q1 Q2 / r2,
where F is the force, Q1 and Q2 are the charges, and r is the distance between the centers of  the two balls (r =14 cm = 0.14 m).  The other forces on the balls are their weights and the tensions in the strings. The three forces on a given ball sum to zero, as he illustrated in a free body diagram. This includes the known distance from the ceiling to the balls (about  2.0 m) and the assumption that that Q1 = Q2, which is reasonable since they are identical and were in contact with the same charged rod.

Bill constructed a homemade balance from a meter stick for arms and a block of wood for a fulcrum. He taped the ping pong ball to one end of the meter stick, and moved a nickel (mass =  5 gm) along the other arm of the meter stick until the balance was achieved; the nickel was a distance X = 29 cm from the fulcrum.  Bill determined the mass of the ping pong ball as the mass of the nickel multiplied by the ratio of distances X / 50 cm, obtaining 2.9 grams.  Then he calculated the charge on each ball,  Q1 = Q2, obtaining 46 nanoCoulombs.

An amazing tour de force!  Thanks, Bill.

21 March 2006: Larry Alofs (Kenwood HS, retired)             Piezoelectricity
Larry first showed us a little piezoelectric igniter, like those used in gas grills. Larry then showed us another version of the igniter (the "matchless pilot light"), and a third example (a cigarette lighter; it would also need butane as a fuel to ignite and burn). Each uses a crystal (of something Larry couldn't remember) which is encapsulated and can be squeezed at either end. Larry took the lighter apart for us, projecting the (small) parts on the screen using the overhead projector. A tiny hammer mechanism hits the crystal and causes the spark.

Larry then showed, using a homemade electroscope apparatus, that the crystal from one of the igniters, when it is compressed, can transfer charge to another object. It was done by transferring charge to two small strips of aluminum foil which were hanging together, suspended from a paper clip, causing them to repel each other. The harder Larry compressed the crystal, the more charge was transferred and the greater the repelling of the two aluminum foil strips.

For additional information on Piezoelectricity see the Wikipedia website:  http://en.wikipedia.org/wiki/Piezoelectricity#Crystal_classes, from which the following has been excerpted:

"Many materials exhibit the effect, including quartz analogue crystals like berlinite, gallium orthophosphate, ceramics, tungsten, barium titanate, strontium titanate, lead zirconate titanate, lithium niobate, lithium tantalate, sodium tungstate, ... . Materials like rubber, wool, wood fiber, and silk often behave as electrets. Although this phenomenon is often confused with piezoelectricity, the two phenomena are distinct. The orientation of polarization in a piezoelectric is limited by the symmetry, whereas the polarization direction in an electret is not. The polymer polyvinylidene fluoride, PVDF exhibits piezoelectricity several times larger than quartz. Bone exhibits some piezoelectric properties, due to the apatite crystals: it has been hypothesized that this is part of the mechanism of bone remodeling in response to stress, as the electric fields on the apatite crystals stimulate further bone growth."
Quite amazing and put together from stuff around the house! Thanks, Larry. 

21 March 2006: Bud Schultz (Aurora Middle School)                        Leyden Jar
Bud brought in a Van de Graaff generator [http://www.amasci.com/emotor/vdg.html] and a homemade capacitor -- a Leyden Jar made from a plastic soup container, aluminum foil, and a bit of wire. For details see the website  http://www.alaska.net/~natnkell/leyden.htm.  Bud used the generator to charge the capacitor.  Next he disassembled the Leyden Jar, and gave the parts (aluminum foil, wife, plastic container) to Fred to examine.  He then put Leyden Jar back together and -- to our surprise -- it was still charged. Fred then discharged the capacitor through his finger.
Quite stimulating!  Thanks, Bud.