Electricity and Magnetism—introduction to electrostatics

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When teaching, I like to begin lectures with a funny story. In the electronic format, I have the advantage of being able to hyperlink to one of my favorite skits by Rowan Atkinson playing Mr. Bean, which is a perfect lead into a discusssion of electrostatics. Since I am pointing to various sites on the Internet, to return to this site after you have been there, simply click on th back arrow on the upper lefthand portion of your browser tool bar.

I have been asked to use more videos, which are part of the strength of online blogs. For me the easiest thing to do, is to find videos that illustrate various point that I want to make. This organizing of material that others have created is the best use of my time. The whole Internet contains my laboratory assistants. They have done a lot of the work of organizing demonstrations of various physical laws. I can add value to their work by organizing the material, and selecting what I consider the best demos.

Preliminaries

Benjamin Franklin Drawing Electricity from the Sky, a paintng by Benjamin West c. 1816, is now displayed at the Philadelphia Museum of Art. The image is a public domin work from Wiki Commons.

In the mid 1700s electricity and magnetism was the subject of a lot of parlour tricks. People experimenting with static electricity, such as our own Benjamin Franklin , who found that there are two types of charges that various materials can acquire by being rubbed together (triboelectricity).

Benjamin Franklin established the convention of positive and negative charges: a rubber rod rubbed with cat's fur acquires a negative charge; and a glass rod rubbed with silk, a positive charge. The positive and negative aspect of the charge numbering system fit the observations perfectly. Two different materials acquire equal and opposite charges when rubbed together. When combined together, these charges electrically neutralize each other. The conclusion is that uncharged matter actually contains positive and negative charges in equal numbers, so that normally the total effect of charge adds up to zero.

A preface to electrical demonstrations should be made at this point. There are two types of material regarding electricity: conductors, which transmit electric charges from one place to another; and insulators, which do not freely transmit charges. Metals, usually good conductors of electricity, readily pass electric currents. Salts melted or dissolved in liquids, such as water, also are conductors of electricity. Plastic, glass and rubber are examples of insulators.

The following video demonstrates the effects of the two types of charge: Electrostatics: Types of Charge. Experimentally observations repeatedly demonstrates that like charged objects repel each other and unlike charged objects attract. Note that when you put an electric charge on an insulator, the charge tends to remain fixed onto the area that was initially charged.

A direct application of like charges repelling is the instrument called the electroscope, which by responding to the presence of charged objects acts as a detector of static electricity. It you want to see how the electroscope works, and are interested in making one, then watch the following video: How to make an electroscope (DIY).

An important effect of electrostatics is the tendency of charge to reside on the outside surface of a closed hollow conductor. Lightning being a discharge of electricity, the safest place to to be during a thunderstorm is within a hollow conductor, such as an all metal automobile. The shielding effect of a hollow conductor is called a Faraday Cage. This effect is illustrated in the following video:

Another video demonstrates the nature of attraction between charged objects:

A video entitled, Introduction to Electrostatics . This video demonstrates the effects of the two types of charges that exist in nature: like charged objects (+ and + or – and -) repel each other, and unlike charged objects (- and +) attract.

There is a machine that physicists use to separate charges at high voltages, which is called a Van de Graaff generator. Large models have been used in physics departments to do nuclear physics experiments. Instructions for building a small Van de Graff generator out of an aluminum can, a small motor and other parts readily available around the house are given by the following video: How to Make/Build a Van de Graaff Generator Part 1 (Homemade/DIY) .

Prof. Water Levin of MIT lectures on Electric Charges and Forces as well as Coulomb's law in this video.

Incidentally, charge is another conserved quantity along side others, such as energy, momentum and angular momentum.

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