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Monday, January 21, 2013

Jumping paper circles

This is a classic! A electric balloon full of static electricity
What we need:
  • 1 balloon,
  • 1 wool cloth, you can use your hair,
  • 1 paper hole puncher,
  • old paper, reuse some old paper.
How to:
  1. Blow the balloon;
  2. Punch some holes in the paper, fold the paper several times before you use the puncher, that will give you more circles;
  3. Rub the balloon in your hair or in the cloth;
  4. Approach the balloon and the paper circles.
What happens?
The circles "jump and glue" to the balloon walls.


Although this is a very simple and basic demonstration some of us never stop to think about what is really going on.
The paper and the balloon are made of atoms. This atoms have a positive core surrounded by negative electrons, these move around the core.
As we saw here several times everything tends towards an equilibrium and thats why almost everything that surround us is charged with a neutral charge. Is the same as saying that the sum of its charges is 0.

Repulsion and/or attraction are only possible if the charges of both materials are different in sign. That means: repulsion and/or attraction are only possible when the objects have excess or lack of electrons and because of that have a negative or positive charge.

When the balloon was rubbed it passed from a resting state to a excited state, and therefore electrically charged(in this case with excess negative charge).

Why the balloon become charged? This was possible because the cloth and the balloon have different characteristics, one can receive electrons and the other can give them, despite they are both in a rest state to begin with. This means one is electronegative (easier to receive negative charges, balloon), and the other is electro-positive (easier to donate negative charge, hair or cloth), when two such materials are rubbed, negative charges migrate from one material to another, when are removed one is positively charged and the other negatively. If you use the hair you will find that it "will glue to the balloon."

The paper was in a rest state, neutral charged. If that is true why does it jumps to the balloon wall?

Yes, the paper was not charged.

The attraction between a neutral charged material and another can be explained using the idea of electrical dipoles, a phenomenon commonly referred to as "charge separation" (in Figure). This separation happens when neutral object is subjected to the action of other electrical charges, in this case electrical charges of the balloon, thats why the paper circles "jump" to the balloon, attracted by the negative charges.

Note: This electrical phenomenon only occurs between insulating materials, conductive materials do not have the ability to retain electric charges, as they seep through the material.

A step further:

  • Change the rubbing time;
  • Change the friction material (cloths can try silk, cotton, wool ...);
  • Change the amount of air in the balloon.
  • Make a table to record your results.
Do not forget the first rule: do not change more than one variable at a time.


Et Voil√°!
So simple so scientific


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