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RED AND GREEN "ELECTRICITY"

(c)1994 William J. Beaty
Electrical Engineer


Here's a simple technique for demonstrating some basic electricity concepts. For classroom use, the colored shapes can be placed on an overhead projector. Also try using a white desktop, or a whiteboard. This demonstration is like an animated diagram, rather than a demonstration of any actual electrostatic effects. It is probably best used for grades five and above. I suspect that this demo is very effective for teaching basic electricity, because while I was working with these colored sheets, I discovered many new concepts myself, and I am supposed to be an electricity expert! Therefor don't be shy in using these with adults as well as kids. By making "static electricity" visible, anyone can gain insights which we never had before.

You'll need some red plastic, green plastic, tape, and scissors. The colored film must be transparent if you intend to use it on an overhead projector. For use upon a desktop, translucent film is fine. I used some red and green clear plastic report covers from Fred Meyer stores. Larger sheets are available for about $8 as "filter gel" from theatrical supply stores. Ask for Primary Red and Primary Green filters.

First, let's make a model of ordinary matter:

  • Roughly cut out a couple of red and green pieces, about 3" across.
  • Place a bit of folded tape in the center of one sheet, sticky sides facing out.
  • Stick the sheets together with the folded tape between them.
  • Carefully cut around the edges so you end up with a perfectly overlapped BLACK shape, which is actually a sandwich containing red and green layers.
  • Remove the tape.
[OVERLAPPED SHEETS, CUT AROUND EDGE]

Fig. 1 Red and green sheets stuck together, cut around the edge

This is your model of a piece of ordinary matter. Matter is composed of atoms, and atoms are composed of positive nuclei surrounded by negative electrons. So, ordinary matter is actually composed of equal quantities of positive and negative charge. The red plastic sheet represents the positive part of matter, and the green represents the negative:

[BLACK 'BLOB' SHOWS RED/GREEN COLOR IF 
SKEWED]

Fig. 2 Matter is composed of positive (red) and negative (green)
in equal proportions

In everyday matter, the positive and negative charges are equal, so they cancel each other out, and the matter has an overall electric charge of zero. The plastic sheets illustrate this: when we combine the red and green, the result is colorless black. Normal "uncharged" matter is actually made of positive and negative charge, just as the black plastic sheet is actually made of bright red and green colors. Matter is entirely MADE of 'electricity', yet because the positives and negatives cancel out, we rarely encounter electrical effects in everyday life. Think about it: even our own bodies are made out of electric charge, yet it took mankind until the 1700's before we became curious enough to start seriously investigating electricity.

Make several more red/green cutout sandwiches of various shapes, so you have a collection of everyday "objects" with which to work. Or have your students make their own.

[MAKE VARIOUS RED/GREEN SHAPES, CAR, TREE, HAND]

Fig. 3 Various everyday objects, all made with red and
green plastic sheets (and all made of opposite charge.)

Hint: For a summer lecture I stamped out several hundred red and green circles 2 inches across, this gave me a give-away item to include with the lecture. A die-cutting print shop can do this for you. Or cut out some perfect squares using a paper shear.


DEMONSTRATE ELECTROSTATIC CHARGING

Cut a sliver out of the edge of the green part of one of your objects, while leaving the red sheet uncut. You now have a sliver made out of green "negative charge." You also have a black object that has a red stripe of "positive charge" on its edge.

[CUT OFF A GREEN SLIVER, THEN PUT IT BACK]

Fig. 4 Remove some green "negative" from your object.
This leaves behind some red "positive."

Place the object, green side up, on the overhead projector, and carefully replace the green sliver, so you end up with a black object again.

To tell the story of how "static electricity" can arise, touch a second black object against the one you have modified. Touch it only against the part of the object having the cut sliver of green. Use a finger to hold the sliver against this new object. This shows how one object can steal some "electricity" from the surface of another object.

[AN OBJECT GRABS THE SLIVER, 'CREATES' RED & GREEN AREAS]

Fig. 5 Here's how "static electricity" is caused by friction. It would be less misleading if
instead we said: "Here's how CHARGE SEPARATION is caused by CONTACT."

Now separate the two objects. You'll end up with the original object with a red stripe on its "surface" or outer edge, while the new object has a green stripe being held against its "surface." This process is called "SURFACE CHARGING," or "ELECTRIFICATION BY CONTACT." Or in somewhat misleading everyday language, we call it "static" electricity.

The demonstration above illustrates a number of distinct concepts:

  • Conservation of Charge: separated positive and negative charges always appear in equal proportions when they are pulled from matter.
  • Electron transfer: it's most often the green "negative" stuff that is exchanged between objects, not the red "positive" stuff.
  • Existence of cancelled charge within matter: it shows that matter is actually made of vast quantities of positive and negative charge in near-perfect balance. It shows that charges are not CREATED, so much as SEPARATED and un-cancelled.
  • The role of contact: it shows that charging arises through contact between surfaces, which implies that the surfaces must have had dissimilar electrical characteristics. (If the two objects in the above figure have identical composition, there would be no reason for one object to steal the green from the other.)
  • We commonly state that "static electricity is caused by friction." This is not quite correct. While friction sometimes plays a secondary role in surface charging, its the contact and the electron-stealing which are the fundamental elements in the process. I try to avoid saying that "static electricity is caused by friction." It would be misleading. Instead I say: "charge imbalance is caused by contact and peeling!"
  • A bit about the misleading label "static electricity"... What's being created here is not an invisible substance called "static." Instead, we are creating an imbalance between the quantities of negative and positive electrical "stuff" which were already there in the matter. Once they are separated, it's not necessary that the charges remain "static" upon the surfaces. The charge does not need to remain unmoving. It's not the static-ness of the charge which creates all the interesting effects, it's the imbalancing and separation of the plus and minus. Also, "Static electricity" is not the complement of "current electricity," since "imbalance" is not the opposite of "motion." Charge-flow and charge-imbalance are actually two different phenomena which can even occur together.
    How can "static" electricity flow? How can "static" and "current" be simultaneous? Easy: "static" is not static, it is imbalanced, and the imbalance can actually flow along. Is this a FLOW of STATIC electricity?!
    It would be less misleading if we called the imbalance by some other name besides "static." Maybe "Voltage electricity?" "Imbalanced electricity?"

DEMONSTRATE INDUCTIVE CHARGING

Place one of your "black" objects on the overhead projector. Place a small piece of green plastic on the overhead an inch away from it. Since alike charges repel, and green repels green, demonstrate this by slightly sliding the green part of your large "black" object away from the small green piece. Slide it just a tiny amount, just enough to expose some bands of color on the black object:

[SMALL GREEN OBJECT 'INDUCES' TWO AREAS OF OPPOSITE CHARGE]

Fig. 6 CHARGING BY INDUCTION: the negative (green) object causes the
charges in the neutral ball to separate.

This illustrates charging by induction. The small green piece represents a negatively charged object. It pushes upon the "green stuff" within the black object. After you slide the green a bit, you'll see a band of red appear on the side of the black object towards the small green piece, while a band of green appears on the side facing away. These bands of color represent the areas of "induced charge." Notice why they must be equal and opposite? Can you see how a charge-imbalance can be induced upon any neutral object, even though another charged object has not actually touched the neutral one?

"STATIC" MEETS CURRENT

Cut out two perfect circles of red and green plastic. (It helps to use a compass to mark them.) Remove any tape, superimpose them upon the overhead projector green side up, place a pencil point against their exact centers as a pivot, then carefully rotate the green disk while leaving the red disk stationary. This demonstrates electric current. See any colors? No, since electric current is a flow of the *neutralized* charge within a metal. The green can flow through the red, yet the two colors remain mixed together. Now un-overlap the two disks a bit to expose some red/green colors and to demonstrate "static" electricity. Can you see the difference between "static" and "current?" "Static" is when the red and green (the plus and minus) is imbalanced. "Current" is when the red and green move relative to each other. Obviously "static" and "current" are not opposite kinds of electricity. Instead they are two independent phenomena. If your textbooks say that "static electricity" is the opposite of "current electricity," then they are propagating a misconception which must be UNlearned if students wish to make good progress in later understanding electrical science. An imbalance need not be "static", and an imbalance is not the opposite of a flow. In the same way, "static" electricity is not necessarily unmoving, and imbalanced charge is not the opposite of charge-flows.


More Ideas:

Once the red/green plastic gets boring, you can put together some other demonstrations based on the same effect...

MICROSCOPIC VIEW: Giant Atoms

To crudely illustrate the nature of the red and green plastic analogy, I made an overhead projector demo which depicts the red and green atoms in the plastic. To do this you need two transparent, uncolored sheets, a red marker, and a green marker.

In the center of one of the sheets draw a bunch of red dots spaced randomly 1/2" apart. Draw your "dots" large enough that the audience can easily see them when projected. These dots are protons or simple positive nuclei.

Now place your second transparent sheet over the first, align them perfectly, and draw one green dot on top of every single red dot. Make the green dots large enough so that the red and green overlap to produce black.

Now move the red and green dot sheets so that the dots only overlap partially. This illustrates induced dipoles, as well as piezoelectric charge separation. (When you squeeze a quartz crystal, the electrons and protons separate a bit, and opposite ends of the crystal will mysteriously display opposite charges.)

Now rotate the "green" sheet over the "red," and you've shown how a metal conductor operates, with the negative charge-sea able to flow along while every red metal atom still always has a green electron nearby.

Now rotate the "red" sheet and "green" sheet in opposite directions, and you've shown how electrolytic conductors (batteries and human bodies, for example,) can support electric current via opposite flows of positive and negative atoms. In salt water, electric current is made from moving sodium and chlorine ions, and no free electrons are present.

I admit it, I stole the above idea from Monty Python's "Holy Grail", where the hundred-eyed animated monster attacks our heroes in the caves. Crazily-swerving eyes, rather than nucleus-orbiting electrons.

SPIFFY SELF-ATTRACTIVE DYNAMIC FLUIDS

If electric charge was REALLY large and colored, we could do all sorts of visible demonstrations which show how electrostatic fields and forces work. The connections between electric fields and electric charge might then be intuitively obvious to students. Here's a way to accomplish this by using gravity fields and colored water.

[TWO BOWLS WITH RED COMING UP THE SIDES]

Fig. 7 CONDUCTOR DEMONSTRATION: Slope-sided bowls with the
lower half painted red

Obtain two light-colored ceramic chili-bowls having sloped sides. Paint the insides partially red as follows: place the bowls on a very level surface. Fill each bowl half-full of red paint. Use latex or oil paint, since it will be in contact with water.


  \                                 /
    \      Side view of bowl      /
      \\                       //
        \\                   //  inside is 
          \\_______________//   painted red

Now carefully spoon all the excess paint out, taking care not to disturb the "high water mark" left by the paint. Use a small brush to smooth out the red coating and to remove the last bits of excess. Allow the paint to dry thoroughly. You should end up with a bowl having a red area inside, and with the red part surrounded by a light, unpainted border.

[HALF FILL THE BOWLS WITH GREEN WATER]

Fig. 8 Fill both bowls with green water so the red part is exactly covered

Next, fill both bowls with water so the water exactly covers the red paint. Mix drops of green food coloring into the water, using just enough so that the red paint and green coloring cancel out and appear dark grey. You have just created electric fluid analogy devices which represent two metal balls, along with their internal electron seas and positive ion arrays.

Here's the simplest demonstration. Scoop some green water out of one bowl and dump it into the other. The bowl which has some green water missing now has a red border uncovered.

[TRANSFERRING GREEN CAUSES A RED/GREEN IMBALANCE]

Fig. 9 To "Charge" a pair of metal objects, scoop some negative
green water from one "object" and drop it into the other.

The bowl with extra green water now has a green border. Transferring electrons between neutral conductive objects causes them to acquire a charge imbalance. And the self-repelling imbalances move to the outside of the objects, while the inside remains neutral (black.)

INDUCTION: take one bowl, fill it with green water to cover the red. Now tilt the bowl.

[TILTED WATER LEVEL EXPOSES EQUAL RED AND GREEN]

Fig. 10 An electric field (tilted table) can slosh the negative liquid and
create areas of opposite charge on a conductor.

This will produce areas of red and green on opposite sides of the bowl. The tilting is an analogy for an electric field coming from outside. The red and green areas are the induced charge imbalances. You have demonstrated how induced charges might look if we could see them.

A SPREADING IMBALANCE: take one bowl, fill it with green water to cover the red paint. Now carefully pour some green water into the bowl from its edge, so the water runs down the tilted incline.

[ADDING GREEN IN ONE SPOT CAUSES GREEN TO APPEAR EVERYWHERE]

Fig. 11 Charge seems to spread instantly over the conductor, even
though it was deposited in one small place.

Although you add the "charge" to the object in *one small spot*, and although that particular bit of green water remains in the place where you put it, the excess charge increases everywhere (the water level rises.) Try adding blue-colored water instead of green, and you'll see that the blue "marked" charges stay in one part of the bowl, even though the green water level rises all around the rim. It APPEARS that the charge you've added has instantly distributed itself all over the entire surface of the object. If we could dump a bucket of electrons into a conductive object, then the excess charge will get bigger everywhere on the object's surface, even though the electrons stay approximately where we dumped them. Even if we could place just ONE electron on a metal object, that one electron would SEEM to distribute itself across the entire surface. It would raise the "water level" everywhere, but by a very tiny amount.

ELECTROMAGNETIC WAVES: take one bowl, fill it with green water to cover the red. Place it on a rickety table such as a portable card table. Now gently shake the table so that the green water in the bowl starts to slosh. See the alternating red and green areas? You are showing how an antenna can pick up electromagnetic energy. The moving table surface is an analogy for incoming EM waves. They make the electrons in your conductive object slosh back and forth, which creates an alternating current and an alternating plus/minus red/green charge signal. Wiggle the table too fast or very slow and the green water won't slosh. This shows how resonant frequencies are important with antennas: if you wiggle the table at exactly the right rate, you can build up a huge wave in the bowl.

RADIO TRANSMITTER: place the two bowls on a rickety card table. Fill them with green water to cover the red. Use a spoon to force the green water to slosh mightly in one bowl, and the sloshing bowl will shake the table, which can cause the other bowl to slosh a *tiny* bit.

CONDUCTORS AND INSULATORS: mix up some jello, color it green, fill one bowl with liquid jello-mix just enough to cover the red, then allow it to harden. Fill the other bowl with green water so you cover the red. You now have models of a conductive object (liquid) and an insulating object (solid.)

How is a conductor different than an insulator? In an insulator, the "green" charge does not flow when electric force is applied. In a conductor, when electric force is applied, the charges do start flowing. If you apply too much force to the jello (the insulator), cracks will shoot through the material and the green stuff suddenly moves for a moment, leaving a path of destruction. Jello-cracks are like electric sparks! Lightning happens when the "jello" in the air has "cracked" because of the strong electric forces in the space below the thundercloud.

 




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