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STICKY ELECTROSTATICS
(C)1996 William Beaty


ALSO:

Electrical experiments using plastic tape

There are several things which interfere with our understanding of "Static Electricity." Most demonstrations incorrectly focus on friction. Also, the nature of matter and the fundamental reasons for charge conservation are usually ignored. And the materials used in demonstrations (silk, fur) are hard to obtain and have a finicky dependence on humidity. The following demonstrations are my attempt to fix these problems.

"STATIC CHARGING" WITHOUT FRICTION

Get a spool of plastic tape. Pull a couple of long strips from the roll, about 20cm each. Hold them up by their ends so they hang downwards, then slowly bring them side by side. Notice that they repel each other? If you try to force the dangling lengths of tape to touch together, they'll swerve and gyrate to frustrate your efforts. You can stick the strips to a door jamb and on a dry day they will keep repelling each other for several minutes. They will also "attack" anyone who passes through the door. Obviously the tape has become electrically charged. But how? After all, no friction was involved. Something odd is going on.

These demonstrations won't work when the relative humidity is high. Try the first one above. If the lengths of tape don't repel each other, then the humidity in the room is probably too high, and none of the other demonstrations will work either. Move yourself into an air-conditioned building, then try again!

Also, 3M SCOTCH Magic(tm) brand tape doesn't work as well as similar tape from other companies. Perhaps 3M puts "anti-static" chemicals in the adhesive?

Next, pass the entire length of each of the hanging strips lightly between two fingers several times, then hold the two strips near each other again. This time they won't repel each other. You've managed to discharge them by fondling them, and the strips are now nearly neutral. (If your fingers are extremely dry, this might not work. Wet your fingers very slightly, but don't get the tape wet.)

Next, fold over a couple of cm of the top of the strips. This gives you a non-sticky tab on each strip. (It makes it easy to get the strips apart again in the next part.) Now carefully stick the two strips together so the sticky side of one strip adheres to the "dry" side of the other. To show that friction plays no part in the following, try to avoid rubbing the tape. You should end up with a double-thick layer of tape which is sticky on one side and has two tabs at one end. Grasp those tabs and rapidly pull the strips apart. Hold them distantly separated, then slowly bring them together. You'll find that this time they attract each other quite strongly. Before they repelled. Now they attract.

REPULSION AS WELL AS ATTRACTION

Next, do the same thing as above, but twice: take four pieces of tape and prepare two *pairs* of tape, each pair having one piece stuck to the back of the other as before. Pull both pairs apart, and either ask a friend for help, or stick a couple of the tapes to the edge of a table so they hang down. As before, you'll find that the lengths of tape which were stuck together now attract each other. But try holding a strip from one pair near each strip of the other pair. You'll find that your single strip will attract one of the other strips, but repel the other. When you peeled each pair apart, one the strips took on opposite charge polarities. The "sticky" strip now repels the other "sticky" strip, but it attracts the "dry" strip. When you have four strips, you can demonstrate that opposite charges attract, but also that alike charges repel.

UN-CANCELLED CHARGES

What's going on here? How did the strips of tape become electrified? There is a simple answer. Contrary to popular belief, "static electricity" is not caused by friction. It's actually caused by contact between dissimilar insulating materials, and is greatly amplified when those materials are forcibly separated. When you stuck the tape strips together, you instantly caused a separation of charges. When you peeled them apart, you pulled the oppositely-charged areas away from each other, causing "un-cancelling" of charges. Another name for this phenomena is "contact electrification." A less accurate description is "generate static electricity."

In explaining everyday electrostatic phenomena, most authors wrongly emphasize the need to rub materials together to generate separations of charge. They often directly state that the friction CREATES the charge separation. This is misleading, since friction really only plays a secondary role in the process. The physics behind "static" electrification usually doesn't involve friction, it involves chemistry.

When the surfaces of two everyday objects are touched together, they always adhere slightly. Chemical bonds form between the atoms which make up the adjacent surfaces, and this causes the adhesion. If the surfaces are not composed of the same sorts of material, then chances are the chemical bonds will be polar, and the bonding electrons will stay with the atoms of one surface more than with the other. The surfaces become oppositely electrified when they touch, because one surface immediately steals electrons from the other as the chemical bonds form. One surface ends up with more negative electrons than positive protons, and then has an overall negative charge. The other surface has fewer electrons than protons, so it has overall positive charge.

"CREATING" CHARGES?

I must take the opportunity here to point out something that bugs me. Books will often state that charges are "created" or "made" during static electrification. This is extremely misleading. Atoms are composed of positive and negative particles (protons and electrons.) The opposite charges are in intimate proximity so the atoms are normally electrically neutral. We cannot avoid the conclusion that all matter is composed of cancelled electric charge. If we define "electricity" to be that quantity carried by electrons, then we could also say that all matter is made of neutralized electricity. Strange, no? But true. Static electrification is a separating, an un-cancelling, of positive and negative particles which were already present in the materials involved. Static electrification is more properly called CHARGE SEPARATION. If you grab an atom by its protons and electrons and separate them far apart from each other, you create "static electricity" or charge separation.

Touch two dissimilar surfaces together and the pos/neg charges in their surfaces become separated. When you pull the surfaces apart again, the chemical bonds rupture, and one surface may end up with more electrons that it started with. The other surface has protons which now lack their nearby cancelling electrons. Oppositely charged particles which had once been adjacent to each other and "cancelled out" within the atoms have now been sorted out and separated by a great distance.

 

AN ATOM THE SIZE OF YOUR HEAD!

From another viewpoint, peeling the tape causes atoms to become enormously stretched, because the outer electrons of one set of atoms has been pulled far away from their protons. Weird fields of force are still connecting the separated protons and electrons, but these fields had originally existed only down within the microscopic world of the atoms. Stretching out the atoms in this way also "stretches" the tiny atomic force fields. This adds energy to them and causes them to balloon outwards and grow so large that they start to affect us here up in our "macro" world. The invisible attracting/repelling fields which surround electrified objects are the same force-fields normally only found inside of atoms.

So everyday "static electricity" has little to do with rubbing or friction. Instead it involves contact, chemistry, and imbalances in the electrical charges of which matter is made. Electrostatic attraction and repulsion between electrified objects is a feeble residue of the same immense forces which hold solid matter together. Our bodies are held together by "static electricity!" And when a huge crane lifts a steel beam, the immense force within the steel cable is actually an electrostatic force field between the atoms of the cable.

If the surfaces involved in contact electrification are rough or fiberous, then only a tiny part of the surfaces can be touched together at a time. If a balloon is touched against hair, the hair only touches the rubber in tiny spots. The "footprint" of contact area will be a tiny percentage of the total surface. In a situation like this, friction does play a role. If the balloon is DRAGGED across the hair, then the successive areas of contact add up to a much larger percentage. Rubbing a balloon on your head increases the total area of rubber and hair that's being touched, so it also increases the total amount of separated charges. Friction aids the charging effect, but friction does not create it.

ELECTRIFICATION BY "PEELING"

So why do strips of tape become charged? Adhesive tape is not a single material. The adhesive and the plastic backing are two different insulators. When they are touched together, one surface steals electrons from atoms of the other, and the surfaces become electrified. When they are peeled apart, atoms are torn open and opposite charges are separated. The tape can then attract and repel distant charges.

CONFUSE YOUR VICTIMS

There are other things you can try. Take two lengths of tape, discharge them between fingers so they no longer repel each other, then fold little tabs and stick them so the adhesive sides stick together. Adhesive to adhesive. Now peel them apart, then bring them near again. They will neither repel nor attract. No separation of charge occurred because the materials on both sides were the same. DISSIMILAR materials are required in order to create separated charge. (This trick can be used to fool people. If you stick YOUR tape strips back to front, but tell someone else to stick THEIRS front to front, they won't notice the difference. When then peeled apart, your strips will attract, but theirs will not! You can then explain your trickery, and teach them a bit of Electrostatic trivia at the same time.)

CHARGED ATTRACTS UNCHARGED

You may have noticed that your charged tape-strips don't only attract and repel other strips, they also attract everything else! Hold a charged strip near your arm, or the wall, near most any neutral object, etc., and the strip will be attracted. Regardless of whether your tape strip is positive or negative, it will attract a neutral object. A general rule: charged objects always attract uncharged objects. Why? Because the charged object causes the charges inside the uncharged object to separate a bit. If you hold a positively charged strip of tape near the wall, the charge on the tape strip will cause negative charges in the substance of the wall to move a bit toward the tape. At the same time, positive charges in the wall move away from the tape. The tape is then attracted to the negative charges in the wall. This is called "attraction by induction," since the charged tape "induces" a separation of charge to occur in the wall. Induction works better with conductors, since the charges in a conductor are free to move. If you hold your tape strip near a metal object such as a refridgerator door, it will be pulled a bit more strongly than the wall pulls it. Hold the strip near your arm, and the pull is strong. Your body is salty water, you are a conductor.

WHICH ONE IS WHAT?

How can you tell which tape strip is positively charged, and which is negative? Easy: by comparing them against a known polarity. An expensive and dangerous way to do this is to string 9-volt batteries together until the voltage adds up to several thousand volts. The positive end of the chain will attract negatively charged tape and repel the positive. Don't touch the battery chain, the high-current capability makes them lethal! A safer, easier way: When you rub a balloon on hair, the balloon's rubber always becomes negatively charged. To determine the polarity of a tape strip, hold it near a hair-charged balloon. If the strip is negative, the balloon will repel it. If the strip is positive, the balloon will attract it.

CHALK DUST PHOTOCOPIES

Here's a way to demonstrate part of the "Xerographic" process used by photocopiers and laser printers. Obtain a flat piece of clear or dark plastic 1/16" thick or thicker, talcum powder, a rag, and some tape. Peel off a strip of tape, discharge it between fingers, fold a tab at one end, and stick it securely over the surface of the plastic. Put down some newspaper so you don't get talcum powder all over, then sprinkle talcum powder on the rag and rub it in. Now peel the tape off the plastic, then shake the rag to make a cloud of talcum powder dust in the air near the plastic. The charged area on the plastic surface will attract the powder, and a "charge image" will appear. If your plastic was clear, try holding it against a dark background to make the white powder more visible. (This experiment works best when humidity is fairly low.)

If you can find a big piece of acrylic from a hardware store, try laying several pieces of tape on it to form your initials or to form a simple word. (Always fold little tabs at one end of each strip.) When you peel all the strips of tape and make a dust cloud, you should then be rewarded with a clear example of electric-charge writing.

Another demo: get some wide packaging tape, a marker, and a paperclip (as well as talcum powder, etc.) Stick the tape to the plastic, and unbend the paperclip to give you a sharp pointed tool. Use the marker to outline the tape (and where the charged area will be.) Peel the tape from the plastic, then lightly draw a large invisible "X" on the invisible charged area on the plastic. Flap the talcum-cloth, and you'll find that the dust cloud is attracted to the charged area as usual, but the "X" will be visible as a dust-free zone. The sharp point of the paperclip wire acted to discharge the plastic. Actually, a tiny corona discharge or "St. Elmo's Fire" was generated on the sharp wire point. Alike-charged air spewed out of the corona, and the opposite-charge air settled onto the plastic, cancelling out the surface charge. With skill (and a big piece of plastic,) you should be able to write several words on a long tape-charged area. Hint: paint one side of the plastic black for contrast. Another hint: try charging the plastic by rubbing it with fur or wool cloth, then write big invisible letters with the paperclip end. Clouds of talcum dust should make it visible.

In a copier, the talcum powder is replaced by black "toner" powder. The plastic plate is replaced by a light-sensitive coating on a metal drum, which discharges bits of itself wherever light lands on it. The charging device is a long thin wire with high-voltage corona on it that sweeps over the drum. The flapping cloth is replaced by a fuzzy brush made of iron filings stuck to a long magnet, and covered with black toner powder. And finally, the black powder melts when heated, so a red-hot "fuser" bar passes over the black dusty paper and makes the writing permanent.


  • Paul Hewitt has included an earlier version the above article in the lab manual of his excellent physics textbook CONCEPTUAL PHYSICS

 





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