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Name: Jim D.
Status: N/A
Age: 50s
Location: N/A
Country: N/A
Date: 2002


Question:
In The Energy Story Chapter 2: What is Electricity? at http://www.energy.ca.gov/education/story/story-html/chapter02.html

there are the statements: "Try this experiment.

Rub a balloon on a wool sweater or on your hair. Then hold it up to a wall. The balloon will stay there by itself. Now rub two balloons, hold them by strings at the end and put them next to each other. They'll move apart.

Rubbing the balloons gives them static electricity. When you rub the balloon it picks up extra electrons from the sweater or your hair and becomes slightly negatively charged."

How many electrons are on the balloon after rubbing it on your hair?


Replies:
Jim,

I cannot say exactly, but I would roughly estimate the charge to be about 1 nanoCoulomb. This is not much of a charge, but it does work out to be about ten billion electrons. I cannot say for certain because things such as temperature, humidity, how hard you press, how fast you rub, and how the wool is sewn can all affect the number of electrons.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


The "short answer" is NOT VERY MANY. To compare the strength of the gravitational force to the electrical force, it is instructive to compare the ratio of the force of gravitational attraction / force of electrical repulsion of two electrons, since so far as we know, the charge on the electron is the "fundamental" unit of electrical charge. This ratio is useful because both forces are proportional to: 1/ R^2 so in the ratio, the effect of distance,R, cancels (where the distances are not too too large or too too small).

That ratio is 0.24x10^-42. This fantastically small number is not an accident of the units of measure, but is a "pure" number. I have no idea what its significance is -- neither does Richard Feynman -- see "Lectures on Physics" Vol. I, chapter 7. That chapter is humbling if you ever get sloppy and think that we "understand" the nature of the fundamental forces operating in nature.

Unknown



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