In this experiment, we will accomplish the seemingly impossible feat of separating salt from pepper after they have been mixed together.

Equipment

  • Salt
  • Pepper
  • Plate
  • Balloon (or another plastic object with large surface)
  • Hair or a sweater made of natural wool

Instructions
The experiment can be viewed in the following video:

Explanation
The materials we see in everyday life are all made up of molecules and atoms, where each atom is made up of negatively-charged electrons surrounding positively-charged protons that are in the center of the atom. Generally, materials we touch are electrically balanced, meaning that in total, the positive and negative charges balance each other out, so the material has no overall net charge.

Static electricity occurs when two materials, such as hair and plastic, are rubbed together, or repeatedly put together and separated. This movement "rips" electrons off one material and moves them to the other. This creates an electric charge in both materials – one receives extra electrons and becomes negatively charged, whereas the other loses electrons and thus is positively charged.

Opposing electric charges (plus and minus) attract each other, whereas identical charges (minus and minus, or plus and plus) repel each other. This fact explains why the hair is attracted to the balloon after rubbing – the balloon rips electrons from the hair. Rubber is generally negatively charged and attracts the now-positively-charged hair.

But why do small objects, that have no net electrical charge, like salt and pepper grains, attract to the balloon?

Well, this happens because they can be electrically polarized. When you put an electrically charged rubber balloon next to another material, the other becomes electrically polarized. The electrons move according to the force applied by the charged balloon. If the balloon is negatively charged, the electrons in the material are repelled. The part closer to the balloon is slightly poorer in electrons and is slightly positively charged, although the total net charge of the material is still zero.

The result is that an electrical force will attract the balloon and the section of the material nearest to it. Both salt and pepper grains are attracted to the balloon, and the closer the balloon gets to the grain, the more strongly they are attracted to each other.

The force of gravity also play a part in determining which grains stick to the balloon and which stay on the plate. The mass of pepper grains is smaller than salt grains, so they are lighter and more likely to be attracted to the balloon at a large distance.

However, at a smaller distance, the electric force may be quite large, it could also attract the salt grains too. So if we want to get a good separation, it's important to find the exact distance where pepper sticks but salt doesn't.
 

Additional Information
There are many more fun experiments with balloons and static electricity. They can stick to a ceiling as if they were full of helium, force cans to roll and even divert streams of water.

Dr. Avi Saig
Davidson Institute of Science Education
Weizmann Institute of Science

Article translated from Hebrew by Aviv J. Sharon, M.Sc. student at the Weizmann Institute of Science.
 

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