What is static electricity

All matter is made of atoms. An atom consists of a heavy, positively charged nucleus surrounded by one or more negatively charged electrons. It its normal state, an atom has as many negative charged electrons as it does positive charged protons. Electricity, and electrically charged objects, have their origins in the charges of these electrons and protons of atoms.

When an object becomes charged, it is because it has gained or lost electrons from the outside of some of its atoms, leaving it with a net negative or positive charge. (Note: Atoms cannot gain or lose protons. Only electrons can be exchanged between atoms.)

One way to charge some objects is by rubbing them. The friction causes the electrons to jump from one object to another. If a piece of rubber is rubbed vigorously with a felt cloth, electrons will jump from the cloth to the rubber. The rubber, now with more electrons, has a net negative charge. The felt cloth, missing some of its electrons, is now positive.

A similar process happens between a silk cloth and glass, but the charges are reversed—the glass becomes positive and the silk negative.

From these concepts we can see that there are two types of charges: negative charges (originating from the electrons in atoms) and positive charges (originating from the protons in atoms). We can also see that these charges are conserved—they don’t appear or disappear at random; they are not created or destroyed in normal reactions. When charging happens, it happens because electrons are transferred—if one object becomes negatively charged, there is an equal positive charge created at the same time somewhere else.

Conductors and Insulators

In some materials (for example plastics, glass, rubber, wood) the electrons in the material are bound very tightly to the nuclei. These types of materials are referred to as electrical insulators. They do not allow electrons to flow through them easily.

Other materials (all metals, salty water, pencil lead) have electrons that are very loosely held to their nuclei. These types of materials are called electrical conductors. They allow electrons to easily flow through them.

Seemingly in contrast to the definitions of insulators and conductors, insulators are the materials that can be charged by friction. The charges in an insulator will not redistribute themselves if there is an unequal charge on the surface of the material. In a conductor, because the electrons flow so freely, rubbing will not cause the conductor to become charged—the electrons move too fast for charging to take place.

Like charges attract. Unlike charges repel.

Experiments done on charged objects have produced some interesting results. If two oppositely charged objects are brought near to each other, they will attract. Two objects of like charge will repel.

Charging by Conduction and by Induction

Insulating materials can be charged by friction. There are two ways to charge conducting materials: conduction and induction.

To charge a metal by conduction, the metal to be charged must not be connected to the ground (there must not be a path to the ground for the charges to follow). Simple touch the metal to be charged with another charged conductor. The two are now equally charged, but ½ the original charge.

To charge a metal by induction, bring a charged object near it but do not touch the two together. The charges in the metal will rearrange themselves because of the forces of attraction and repulsion. However, if the original charged is removed, the metal will once again become uncharged as the charges go back to their original positions.

To permanently charge a metal by induction, the metal to be charged has to first be connected to the ground. Then when the charges are rearranged by induction, electrons will flow in or out of the ground. The connection to the ground is then severed before the original charged object is removed. When the original charged object is removed, the metal remains charged, but with the opposite charge of the original charged object.

The Electroscope

An electroscope, shown below, is a device that can be used for detecting the presence of charge. If a charged object is brought near or touches the metal sphere, than the leaves of the electroscope are charged by induction or conduction. When the leaves receive a charge, because like charges repel, the leaves will spread apart.

charging by induction

charging by conduction