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100 Day Challenge 2020

Clingy Charges

Have you ever left the house unwittingly wearing your laundry?

If so, then you've fallen victim to the buildup of electric charge. When electrons and protons — the two kinds of electrically charged particles within atoms — aren't evenly mixed within matter, common objects get extra clingy!

Electrons and protons feel electric forces because they carry electric charge. Most of the time, electric forces are so small that they are hidden from our senses. But when there's a big imbalance between the numbers of protons and electrons in an object, electric forces can have a visible effect.

One easy way to see electric forces is using plastic adhesive tape. If you have a clean, smooth countertop, scotch tape, and somewhere to hang it from, you can see electric forces at work in no time. The setup is described in the video below:

The action of ripping the sticky tape strip off a clean countertop allows the tape and the surface to exchange a large number of electrons (which are smaller and more mobile than protons). This exchange is not symmetric. The tape always gains electrons from the surface, disrupting the delicate balance between protons and electrons in both the tape and the surface.

Once the tape strip is charged, you can see electric forces acting on it.

With one end of the tape hanging free, try slowly moving your arm, a glass, or a piece of aluminum foil toward the tape strip. Even the non-sticky side of the tape should attract and cling to the object!

(If the tape-strip demo doesn't work for you, try cleaning your surface before you try again. If it still doesn't work, try moving the experiment outdoors.)

Electric forces can be attractive or repulsive. Attractive electric forces arise between electrons and protons. Repulsive forces arise between two electrons or two protons. Both kinds of interactions contribute to the effect you see.

The foil is neutral (i.e. it has roughly equal numbers of protons and electrons), and as it's brought closer, electrons and protons on the tape interact with charges in the foil. Electrons in the foil are repelled by electrons in the tape and attracted to protons. Since there are more electrons than protons on the tape, free electrons in the foil are pushed away.

The protons in the foil are more or less fixed in place. They repel protons in the tape and draw the electrons toward them. Because there are more electrons, the net force is attractive.

An excess of electrons on a material, like tape, always attracts other neutral matter, like the foil or air molecules. Consequently, materials don't hold onto their excess electrons for long. Electrons easily hitch a ride on air molecules (especially water because of its polar structure) and the material discharges in a few minutes.

Today's Challenge

In the introduction to this challenge, you learned how to charge a tape strip.

What kind of force do you observe if you make two identical, charged tape strips and bring them toward each other?

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