Why does plastic "cling wrap" cling?
Answer. This is probably a static electricity effect. When the wrap is pulled or stretched or pressed onto another surface, such as the rim of a food container, some of the (long organic) molecules out of which the wrap is made may be broken and the pieces may be charged. That is, the molecule becomes two oppositely charged ions. There may be some transfer of ions, perhaps even some electron transfer, when the wrap is pressed to a surface, or gets stuck on itself, and there is enough bonding between the broken molecule pieces (ions) to hold the wrap in place. In their book Electric and Magnetic Interactions Ruth Chabay and Bruce Sherwood discuss a related question: why a plastic comb that is run through your hair ends up with a negative charge. "Molecular breakage or electron transfer provides a rough explanation of...(why tapes and combs get charged), but such details as to why the comb rather than your hair becomes negative are the subject of continuing research by physicists and chemists. Part of the complexity of these phenomena is due to the fact that they are surface phenomena. The special nature of intermolecular interactions at the surface of a solid are less well understood than those in the interior."|
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Answer A spark forms when the finger gets close enough to the doorknob, a conductor which polarizes. Conduction electrons move away from the negatively charged finger, leaving the knob positive. The electric field strength increases as the finger approaches the doorknob. There are always a few free electrons around, knocked off air molecules (nitrogen or oxygen, mostly) by the products of ionization processes in the upper atmosphere. These products are the results of a chain of events that begin with collisions between "cosmic rays" (mostly high energy protons) from the sun and other extraterrestrial sources and molecules in the earth's upper atmosphere.
When one of these electrons acquires sufficient kinetic energy it can (by collision) ionize an N2 or an O2 molecule, freeing more electrons and an "avalanche" or "chain reaction" occurs. Positive ions move toward the finger and electrons move toward the doorknob. Sometimes an electron and an ion will recombine; the recombination energy is emitted in the form of light -- you "see" the spark. Since the speed (and kinetic energy) of the accelerated ions is higher than the average speed of neutral molecules in the vicinity, the average speed of the whole collection of particles increases. This corresponds to an increase in temperature and PV = nRT takes over. A small acoustic shock wave is formed and you hear the discharge.
You feel the effect because positive N2+ and O2+ ions react chemically with the salt water layer on your skin.
(The above is based on material in the Instructor's
Manual for Electric and Magnetic Interactions and material found on
William J. Beaty's web site:
http://www.eskimo.com/~billb/emotor/zapped.html )
May 5-12
Question
A car enters an intersection where there is a traffic
signal and moves into the left turn lane. The light is . A short
while later, a
indicates that a left turn can be made. This arrow does not go on unless there
is a car in the left turn lane. How does this system work?