Expert: Expert - 1/26/2005

Question
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Thanks so much for your very, very clear explanation! I'd like to follow up with one other static electricity question. When I walk on a carpet and then generate a spark between my fingers and the wall, I'm wondering if the following explanation is correct.

1. Assuming that electrons are transferred,  by rubbing, from carpet to shoe bottom, since the rubber soles are an insulator the extra electrons will remain on the bottom of the sole.

2. The bottom of the sole is therefore charged negative, and this charge (i.e., this group of electrons unbalanced by protons) creates an electric field.

3. The conduction electrons in my body (a conductor) , under the influence of the field, tend to move away from the soles toward the upper parts of my body.

4. Because of this my body as a whole is polarized, positive below and negative above.

5. When I touch the wall the conduction electrons which have moved upward in my body jump to the electrically neutral wall.

If this IS correct, then I still have to ask, why do the electrons in (5) jump from my negative hand to an electrically neutral non-conducting object?  Is it like your explanation for the tissue, that the field produced by these electrons in my hand polarize the outer atoms in the wall, so the electrons then move from my negative hand to the positive side of the wall atoms?

Thanks again.

David







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Followup To
Question -
I rub a plastic pen on my shirt to charge it, and can then use it to  pick up bits of Kleenex.  Since the Kleenex is not charged to begin with, and does not have conduction electrons which would allow it to be charged by induction why is it attracted to the pen? Further, when I have one piece of Kleenex hanging from the pen, I find I can use it to pick up a second piece. How does the charge get from the pen through an insulator (the first piece of Kleenex) to the second piece of Kleenex?

Thanks.
David

Answer -
I don't know off-hand if rubbing a pen creates a negatively or positively charged surface. I'm going to assume it's positive. The following discussion does not change if instead the surface of the pen is negatively charged.


> Since the Kleenex ... does not have conduction electrons

This statement is correct.

> which would allow it to be charged by induction

This is where you go wrong. While it is true that no FREE electrons can go to the surface in response to the applied electric field of your pen, the electrons in the atoms of the tissue (note that Kleenex{R} is a registered trademark) will respond by being more likely to be on the "pen" of their nuclei than being equally present on both sides.  In other words, the setup will change from something this (with the pen above the atoms, and using random numbers to determine whether electrons are above or below the nuclei):

eeee..e..e.eee.e.e.e.e.
NNNNNNNNNNN
....ee.ee.e...e.e.e.e.e

To something like:

eeeee.e.ee.eee.e.e.e.e.
NNNNNNNNNNN
.....e.e..e...e.e.e.e.e

Note that I have VASTLY exaggerated the change to a 10% switch from equally likely. But I hope you get the point -- that electrons are now more likely to be closer to the pen. This creates enough excess negative charge to permit an attractive force between the pen.

Since the charge distribution in the tissue is now altered from purely random to electrons favoring being near the pen, the tissue now has an electric field, although much weaker than that of the pen. The process I noted above can now repeat itself until the force created on the piece of tissue is not strong enough to counter gravity.

Answer
Your explanation is not quite correct. You go wrong in Step 4. The excess electrons that your shoe has picked up move all over your body, as the outside of your skin (salt, water, and oil) is somewhat conductive. The excess charge thus spreads around your body fairly evenly, and all parts of your body are at about the same voltage.  You are NOT polarized, all parts of your skin have the same charge.

As this URL notes, you can build up a surprisingly high voltage in this manner.

http://amasci.com/emotor/statelec.html

Your skin is now at a high voltage relative to ground. Thus, when you come in close contact to even a local ground (like a door knob, or your brother's neck), a spark appears and the excess charge is neutralized (as you correctly note).

Your supposition is correct; an electric field is created by the excess electrons on your skin.

The URL above will probably answer any question on this subject better than I ever could.