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Engineering > Electrostatics > Sticky Electrostatics
 

Site: http://www.eskimo.com/~billb/emotor/sticky.html

Grade Category: High School

Subject Category: Engineering

Sub Category: Electrostatics

STICKY ELECTROSTATICS

(C)1996 William Beaty

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.

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