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Name: Rocklady
Status: student
Age: N/A
Location: N/A
Country: Great Britain
Date: N/A


Question:
What happens when you rub a balloon with wool and hold it next to an object that is neutral - I am finishing a unit on static electricity with my class and this question was asked.



Replies:
This is actually very interesting. This attraction occurs because of two facts:

1. All objects, even those that are "neutral," are composed of many particles that are electrically charged. When they are neutral, the amounts of positive and negative charges are equal to each other, and they are closely mixed together.

2. The force of electric attraction or repulsion gets weaker as you move the charged objects apart.

When an object with a net electric charge is brought near an object with zero net charge, the smaller charges making up the neutral object become polarized. Let us say that the charged object has a positive charge. Then the negative charges in the uncharged object will be attracted to it and the positive charges in the uncharged object will be repelled from it. So, the uncharged object will have a slight negative charge on the side near the positively-charged object, and a slight positive charge on the side opposite the positively-charged object. (The more easily that charges can move in the uncharged object, the more complete this polarization will be. Try bringing your charged balloon near an empty aluminum soda can lying on its side. The aluminum is a good conductor, so charges polarize within it easily, and you can get the can to roll toward the balloon.)

Because the electric force is stronger at close distances and weaker at long distances, the positively-charged object will attract the nearby negative charges in the neutral object more strongly than it will repel the more distant positive charges. So, the net force will be an attraction.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming


Hi Rocklady,

You have got a couple of cool things going on. When you rub a balloon with wool, or your hair, you are transferring electrons to the balloon. The balloon becomes negatively charged due to the additional electrons and the wool or hair positively charged due to the stripping of electrons. When you hold this balloon that is now negatively charged, next to a neutral wall, charge polarization takes place. In other words, since there are no free electrons to move throughout the material, there is a rearrangement of the positions of charges within the atoms and molecules themselves. One side of the atom or molecule is induced to be more positive, (or negative), than the opposite side. This realignment and proximity of the balloon to the wall allows the balloon to stick to the wall. This charge polarization does not last. Once the balloon is removed, the molecules go back to their usual positions.

A Nice explanation of his is in Paul Hewitt's, Conceptual Physics in the chapter on Electrostatics; chapter 32 in the third edition.

Happy Science,

Martha Croll



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