Electricity

I

Introduction

Electricity, one of the basic forms of energy. Electricity is associated with electric charge, a property of certain elementary particles such as electrons and protons, two of the basic particles that make up the atoms of all ordinary matter. Electric charges can be stationary, as in static electricity, or moving, as in an electric current.

Electrical activity takes place constantly everywhere in the universe. Electrical forces hold molecules together. The nervous systems of animals work by means of weak electric signals transmitted between neurons (nerve cells). Electricity is generated, transmitted, and converted into heat, light, motion, and other forms of energy through natural processes, as well as by devices built by people.

Electricity is an extremely versatile form of energy. It can be generated in many ways and from many different sources. It can be sent almost instantaneously over long distances. Electricity can also be converted efficiently into other forms of energy, and it can be stored. Because of this versatility, electricity plays a part in nearly every aspect of modern technology. Electricity provides light, heat, and mechanical power. It makes telephones, computers, televisions, and countless other necessities and luxuries possible.

II

Electric Charge

Electricity consists of charges carried by electrons, protons, and other particles. Electric charge comes in two forms: positive and negative. Electrons and protons both carry exactly the same amount of electric charge, but the positive charge of the proton is exactly opposite the negative charge of the electron. If an object has more protons than electrons, it is said to be positively charged; if it has more electrons than protons, it is said to be negatively charged. If an object contains as many protons as electrons, the charges will cancel each other and the object is said to be uncharged, or electrically neutral.

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Electricity occurs in two forms: static electricity and electric current. Static electricity consists of electric charges that stay in one place. An electric current is a flow of electric charges between objects or locations.

III

Static Electricity

Static electricity can be produced by rubbing together two objects made of different materials. Electrons move from the surface of one object to the surface of the other if the second material holds onto its electrons more strongly than the first does. The object that gains electrons becomes negatively charged, since it now has more electrons than protons. The object that gives up electrons becomes positively charged. For example, if a nylon comb is run through clean, dry hair, some of the electrons on the hair are transferred to the comb. The comb becomes negatively charged and the hair becomes positively charged. The following materials are named in decreasing order of their ability to hold electrons: rubber, silk, glass, flannel, and fur (or hair). If any two of these materials are rubbed together, the material earlier in the list becomes negative, and the material later in the list becomes positive. The materials should be clean and dry.

A

Charging by Contact

Objects become electrically charged in either of two ways: by contact or by induction.

A charged object transfers electric charge to an object with lesser charge if the two touch. When this happens, a charge flows from the first to the second object for a brief time. Charges in motion form an electric current. When charge flows between objects in contact, the amount of charge that an object receives depends on its ability to store charge. The ability to store charge is called capacitance and is measured in units called farads.

Charging by contact can be demonstrated by touching an uncharged electroscope with a charged comb. An electroscope is a device that contains two strips of metal foil, called leaves, that hang from one end of a metal rod. A metal ball is at the other end of the rod. When the charged comb touches the ball, some of the charges on the comb flow to the leaves, which separate because they now hold like charges and repel each other. If the comb is removed, the leaves remain apart because they retain their charges. The electroscope has thus been charged by contact with the comb.

This flow of charge between objects with different amounts of charge will occur whenever possible. However, it requires a pathway for the electric charge to move along. Some materials, called conductors, allow an electric current to flow through them easily. Other materials, called insulators, strongly resist the passage of an electric current.

Under normal conditions, air is an insulator. However, if an object gains a large enough charge of static electricity, part of the charge may jump, or discharge, through the air to another object without touching it directly. When the charge is large enough, the air becomes a conductor. Lightning is an example of a discharge.

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