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Explanation of Electrostatic Induction by Ron Kurtus - Succeed in Understanding Physics. Key words: static electricity, charged object, conduction, free electrons, ions, positive, negative, grounding, nonconductor, dielectric, polarization, polar molecules, bend water, physical science, School for Champions. Copyright © Restrictions

Electrostatic Induction

by Ron Kurtus (revised 24 May 2009)

Electrostatic induction is a method to create or generate static electricity in a material by bringing an electrically charged object near it. This causes the electrical charges to be redistributed in the material, resulting in one side having an excess of either positive (+) or negative (−) charges.

This phenomenon is most effective when the objects are conducting materials, such as a metals. The only drawback is that once the electrically charged object is removed, the conductor loses its charge. This can be solved by temporarily grounding the conductor.

Certain non-conducting materials can also be given a static electric charge by electrostatic induction. In these cases, it is caused by polarization of their molecules.

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Induction in a conducting material

In its normal, neutral state, an electrically conducting object typically has an equal number of positive (+) and negative (−) electrical charges—such as positive ions, negative ions and electrons—intermingled within the material. When an static electrically charged object is brought near this conductor, the electrical charges on or near the surface of the object attract the opposite charges in the conductor and repel the like charges.

Plastic rod near metal plate

As an example, if a charged plastic rod is brought near a metal plate, the negative charges on the rod attract the positive charges in the plate and repel its negative charges. This creates a redistribution of electrical charges in the plate.

Electrical charges in the conductor are redistributed

Electrical charges in the conductor are redistributed

As long as the electrically charged rod is near the metal plate, the electrical charges in the plate will be redistributed. But once the charged object is removed, thermal motion of the atoms in the metal will cause the charges to intermingle again.

Bringing charge near electroscope

Another example is the electroscope. If you bring a charged object such as the plastic rod near an electroscope, opposite electrical charges will move toward the the metal end of the electroscope.

In this illustration, the rod has negative (−) electrical charges on its surface, which attract positive (+) charges in the metal shaft of the electroscope by means of electrostatic induction.

The electrical charges in the metal shaft are redistributed, with negative charges collecting on the leaves at the other end of the shaft. Since like charges repel, the electroscope leaves push part, due to the electrical force where opposite charges repel.

Electroscope leaves separate because of electrical charges

Electroscope leaves separate because of electrical charges

The electroscope has not gained any electric charges. They have just been redistributed, with positive charges near the top and negative charges by the leaves, as seen in the our example.

Removing charge from electroscope

When the charged rod is removed, the electrical charges in the electroscope intermingle again and the leaves fall back to a neutral position.

Electroscope leaves fall back after charged rod removed

Electroscope leaves fall back after charged rod removed

Ground to keep charges

You can cause the electroscope to have an excess of one type of electrical charge by drawing off the other type of charge.

This is done in our example by touching the negative (−) electrical charged rod to the shaft containing the positive (+) charges or by simply touching the shaft with your finger. This will result in drawing off many of the positive charges but allowing the negative charges in the leaves to remain. It is often called grounding, although the charges aren't really going into the ground.

You can tell the leaves are still charged, because they remain separated.

Electroscope remains charged after grounding

Electroscope remains charged after grounding

This method has allowed a conducting object to remain become electrically charged through electrostatic induction.

Induction in nonconducting materials

Electrostatic induction can also work in nonconducting or dielectric materials. However, movement of electrical charges is much more constrained in nonconductors than in conductors.

Electrons are allowed to move about in a conductor, and that is what allows the flow of electricity in a metal wire. In a nonconductor, the electrons are constrained within the atoms, so separation of charges particles does not work.

However, if the nonconductor consists of polar molecules—that is, molecule that have one side more positive than the other side—then electrostatic induction will cause those molecules to be aligned with positive charges on one side and negative charges on the other side.

Water (H2O) molecule can be polarized by electrostatic induction

Water (H2O) molecule can be polarized by electrostatic induction

For example, the water molecule has more positive charges on one side of the molecule and negative charges on the other side. Thus, water can be slightly attracted to a static electric charge.

A demonstration of that can be seen in bending a stream of water with a charged plastic comb.

Bending water with a charged comb

Bending water with a charged comb

Other examples can be seen in the attraction of lightweight nonconductive objects like pieces of tissue paper or small pieces of Styrofoam to an object that has static electric charges.

Summary

Electrostatic induction is where electrical charges are redistributed in a material by bringing an electrically charged object near it. This is a way of creating static electricity, and it is most effective with conducting materials. Unfortunately, once the electrically charged object is removed, the conductor loses its charge. This can be solved by temporarily grounding the conductor. Certain nonconducting materials can also be given a static electric charge by electrostatic induction.


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