5-Minute STEM

5-Minute STEM

Remote-Control Roller: Wield electrons and protons to roll a can.


Rub a balloon on your head, then watch a soda can race across the floor or a table. As you observe the interplay between electrons and protons, you’ll also discover why clothes cling together in a dryer.










Tools and Materials


  • Empty soda can

  • A balloon

  • Your hair (dry, not-too-short hair with no hair products in it works best)

  • A flat surface such as a table or a floor






Assembly

Inflate the balloon and tie it off.



To Do and Notice


  1. Put the can on its side on a table or the floor—any place that's flat and smooth. Hold it with your finger until it stays still. Rub the balloon back and forth quickly on your hair. Hold the balloon about an inch (2.5 cm) from the side of the can. The can will start to roll, even though you're not touching it.

  2. Move the balloon away from the can—slowly—and the can will follow the balloon. If you move the balloon to the other side of the can, the can will roll in the other direction.

  3. How fast will the can roll?

  4. How far can you roll the can before it stops?

  5. Will it roll uphill?


What’s Going On?

When you rub a balloon on your hair, the balloon ends up loaded with electrons.

Your empty aluminum can is neutrally charged, meaning it has equal numbers of electrons and protons spread pretty evenly around it. When the negatively charged, electron-loaded part of the balloon is held near the can, it repels the electrons in the part of the can closest to the balloon because a negatively charged object repels other negatively charged objects (like charges repel each other).


Since there are now fewer electrons in that region of the can (and thus proportionally more protons), you have induced a positive charge in that area of the can.  The negatively charged balloon then attracts the positively charged part of the can, pulling it toward the can, because a positively charged object will attract a negatively charged object. (It's true that opposites attract!)

As it rolls, the electrons in the can closest to the balloon are constantly being forced away, causing the positively charged area of the can closest to the balloon to be attracted to the balloon. As a result, as long as the balloon is held just in front of the can, the can will roll towards it.


Going Further

The attraction between protons and electrons also makes clothes stick together in the dryer. When you dry clothes in the dryer, different fabrics rub together. As a result, electrons from a cotton sock (for instance) may rub off onto a polyester shirt, causing them to stick together and even make sparks when you pull them apart. 

You may have used anti-static sheets in your dryer. As these sheets bounce around with your clothes, they add a uniform anti-static coating to the fabric. Rather than cotton rubbing against polyester, you've got the anti-static coating on the cotton rubbing against the anti-static coating on the polyester. No electrons rub off—therefore, no static cling.

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