Header

  • עברית
  • English
  • العربية
  • Moodle
  • You Tube
  • facebook
  • Contact Us
  • GeoGebra
Home
  • About
    • Our vision
    • About William (Bill) Davidson
    • Maps and Directions
    • Management
    • Board Members
    • Support Us
  • Programs
    • After School Activities
    • Programs for High Achievers
    • International Programs
    • Lectures
    • Student Advancement
    • Competitions
    • Computerized programs
    • Continuing Professional Education Program for Teachers
    • Scientific Programs for Classes
  • Davidson Online
    • About Davidson Online
    • Ask Our Experts
    • Science Multimedia Archive
    • The Math Circle
    • Science at home
    • Teachers Room
  • Garden of Science
    • About
    • Visitors information
    • Events and Activities
    • Young Science Garden Guides
    • Exhibits in the Garden of Science
  • Perach
Davidson Online
  • About Davidson Online
  • Ask Our Experts
  • Science Multimedia Archive
  • The Math Circle
  • Science at home
    • Biology, Brain & Senses
    • Chemistry
    • Mathematical Experiments
    • Physics
    • Voices and Music
  • Teachers Room

Search

What's new

What would happen to an astronaut who leaves his spacecraft into outer space with an oxygen bottle but not wearing his space suit? Ofer
In answering this question, most science fiction films depict gory scenes in which the human body resembles a balloon that
To the article »
The Race Against the Clock: Antibiotic Resistance
For the last 80 years or so, antibiotics have repeatedly cured people of bacterial infections, from salmonella to pneumonia.
To the article »
How Does the Ebola Vaccine Work?
A year ago, West Africa was in crisis mode. The death toll from the Ebola virus had exceeded 800. Cases of the virus were
To the article »
Not Just DNA: Three Important Gene Regulators
In high school, most of us learned that DNA is the bible. It encodes all the information our bodies need to function. It is our
To the article »
Molecular Cooking: Flavor-Tripping
When Violet Beauregarde chewed the amazing gum in Willy Wonka’s chocolate factory, she felt like she was eating a three-
To the article »
Molecular Cooking: Gelification
I have a personal vendetta against gelled foods. Some of the most delicious sweets – Jell-O, marshmallows, custards, and
To the article »
Molecular Cooking: Spherification
If you’ve never thought to yourself, “I really wish I could eat apple juice that had the consistency of caviar with
To the article »

Separating a Mixture of Salt and Pepper

Share

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 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 lost 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 like salt and pepper grains attract to the balloon? They have no net electrical charge.

Well, this happens because they can be electrically polarized. When you put an electrically charged rubber balloon next to another material, the other is induced to be 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 following animation shows the phenomenon:

 

 

The result is that an electrical force will attract the balloon and the part of the material near 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 so 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 nice experiments with balloons and static electricity. They can be made to stick to a ceiling as if they were full of helium, make cans 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.

Note for Surfers

If you find the explanations unclear or have further questions, please drop us a line on the forum. We welcome your comments, suggestions and feedback.

  • This article has 0 comments
  • 07.06.14
חשמל, חשמל סטטי, כוח המשיכה, מטען חשמלי
Printer-friendly version
CodeOasis
  • Home
  • About
  • Programs
  • Davidson Online
  • Science Museum
  • Perach
  • Contact Us
  • Terms of Use
© 2012 All rights reserved to the Davidson Institute of Science Education.