Nib asked
Why does friction cause static electricity?
Friction is not really the cause. Charge moves wherever two different surfaces touch, and rubbing just creates millions of fresh contacts. Insulators then trap the charge that crossed over, leaving each surface with a net charge.
5 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the puzzle
Rub a balloon on your hair, and it clings to a wall while your hair lifts toward it. The usual explanation says friction creates static electricity. But friction does not make charge. Charge is already there in every atom. What matters is that two different materials touch, and charge moves between them.
Friction doesn't create charge. It moves charge that was already there.
2touch is key
Surfaces that look smooth are rough at the microscopic scale, so two objects touch only at tiny points. Rubbing drags fresh patches into contact again and again. Experiments show that simply pressing two different materials together and separating them can charge them, with no sliding at all. Rubbing mostly multiplies the number of contacts, so more charge moves.
Contact moves the charge. Rubbing just multiplies the contacts.
a common mix-upFriction does not create static electricity. Contact between different materials moves charge, and rubbing adds contacts.
Can two surfaces charge each other without rubbing?
Show the answer
Yes. Pressing different materials together and pulling them apart can transfer charge too.
3electrons hop
Different materials hold their outer electrons with different strength. This ordering is listed in the triboelectric series, with human hair near the positive end and rubber near the negative end. When hair and a rubber balloon touch, electrons cross from the hair to the balloon. Exactly how they cross is still an active research question, and for some plastics tiny bits of material move too, but electron transfer is the standard picture.
At the contact, electrons cross from the material that holds them loosely to the one that holds them tightly.
Which way do electrons move between hair and a rubber balloon?
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From the hair to the balloon, since rubber holds electrons more tightly.
4charges stay
After separation the charge stays put, because neither hair nor rubber lets electrons move freely. The balloon has gained electrons and is negative. The hair has lost the same number and is positive. Total charge is unchanged, which is conservation of charge. A metal would let the electrons flow back at once, which is why we do not see static on a metal rod held in the hand.
Insulators trap the transferred electrons. Total charge is conserved: balloon minus, hair plus.
Why does the charge stay on the balloon instead of flowing back?
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Rubber is an insulator, so the extra electrons have no path to move through.
5the whole idea
Here is the whole chain. Two different materials touch, rubbing multiplies the contacts, and electrons cross to the material that holds them more tightly. Because both are insulators the charge cannot return, so each ends up with a net charge. The charged balloon pushes the wall's electrons slightly away and leaves its surface faintly positive, and since the positive surface is nearer than the negative side, the attraction wins and the balloon sticks. A spark happens when the built-up voltage can break down the air, roughly three million volts per metre.
Contact moves electrons, insulators trap them, and the charged object attracts a neutral wall by polarizing it.
Why does friction cause static electricity?
Show the answer
Rubbing brings many surface points into contact, charge moves across each one, and insulators keep it there.
Rubbing doesn't make charge, it moves it: millions of tiny contacts shuffle electrons, and an insulator holds them in place.