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Senin, 09 Agustus 2010

Causes of Static Electricity

by Ron Kurtus (revised 28 November 2008)

Static electricity occurs when positive (+) or negative (−) electrical charges collect on an object's surface. There are several ways this condition can be caused.
One way is by rubbing certain materials together or pulling them apart. Another way that static electricity commonly seen is when a charged material is brought near a neutral material. This is called electrostatic induction. You can also cause static electric separation of charges by using pressure in certain materials or using heat in other materials.
Questions you may have include:
  • How does separating materials cause static electricity?
  • What is electrostatic induction?
  • How does pressure or heat cause static electricity?

Separating or rubbing materials

When two different materials are brought into contact and then separated, some outer shell electron may be captured by the material that is closer to filling its outer shell. This results in the atoms of one material having an excess of electrons and thus a negative (−) electrical charge and the other material missing electrons and having a positive (+) electrical charge.
Electrons exchanged when objects pulled apart
Electrons exchanged when objects pulled apart
Putting certain materials together and then pulling them apart or rubbing them together can cause excess electrical charges to be created on their surfaces.

Excess of charges

Most matter is electrically neutral. That means its atoms and molecules have the same number of electrons as protons. If a material somehow obtains extra electrons and attaches them to the atom's outer shells, that material has a negative (−) charge. Likewise, if a material loses electrons, it has an excess of positive (+) charges. The electric field from the excess of charges then causes the static electric effects of attraction, repulsion or a spark.

Adhesive force takes electrons

A force called the adhesive molecular force holds materials together. But when two different materials are pressed together and then pulled apart, the adhesive molecular force pulls electrons from one material and deposits unto the other. This creates the separation of charges or static electricity.
Although your can create static electricity by pressing materials together and pulling them apart, rubbing them together usually works even better.
One unfortunate result from saying that rubbing materials creates static electricity is that most people think that friction causes the charges to build up. It is not friction that causes static electricity, rather it is the adhesive forces that pull off electrons.

Materials

Certain combination of materials work better than others in creating static electricity. The Triboelectric Series lists materials according to how likely they are to let go of electrons or to take on electrons from other materials.
Rubbing a balloon on a wool sweater creates static electric 
charges on the surfaces
Rubbing a balloon on a wool sweater creates
static electric charges on the surfaces
Dry human skin and rabbit fur have the greatest tendency to give up electrons when rubbed on something and become positively (+) charged. Teflon and vinyl have the greatest tendency to become negatively charged (−) when rubbed. If you want to create static electricity, rubbing fur on Teflon should give the best results.
(See Materials Causing Static Electricity for more information.)

Electrostatic induction

When you bring an object with static electricity charges on its surface near another object, those charges will induce opposite charges to come to the surface. This works best with conductors, which allows charged particles to move around fairly freely, but also works with nonconductors to a lesser degree.
Charged plastic rod induces charges in metal plate
Charged plastic rod induces charges in metal plate
The electroscope—a device used to detect static electricity—uses this principle to charge its detecting parts. It also explains how a charged balloon can be made to stick to the wall.

Special pressure and heating causes

Pressure or mechanical stress and heat can cause electrical charges to separate and create a static electricity situation.

Piezoelectric effect

When pressure or stress is applied to certain crystals, opposite electric charges collect on the ends of the crystal. By maintaining the pressure on the crystal, the static electric effects can be demonstrated.
Piezoelectric effect creates charges near surfaces
Piezoelectric effect creates charges near surfaces
Although applying pressure to a piezoelectric crystal is considered causing static electricity, its use is in creating an electrical current for numerous applications.

Pyroelectric effect

Likewise, when heat is applied to certain materials, positive and negative charges move to opposite ends of the material. By maintaining the heat on the material, the static electric effects can be demonstrated.
Pyroelectric effect creates charges near surfaces
Pyroelectric effect creates charges near surfaces
Although the separation of charges is considered static electricity, the usual applications of the pyroelectric effect is to create electric current for various devices.

Summary

There are several ways separation positive (+) or negative (−) charges on an object's surface, causing static electricity can be caused. Rubbing certain materials together or pulling them apart, electrostatic induction, the piezoelectric effect and the pyroelectric effect are ways of causing static electricity.

http://www.school-for-champions.com/science/static_causes.htm

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