Electrical Units

I

Introduction

Electrical Units, units used to express quantitative measurements of all types of electrostatic and electromagnetic phenomena and of the electrical characteristics of components of electrical circuits. The basic electrical units are part of the centimeter-gram-second system, but because, in most cases, these units are either too large or too small for convenient measurement, a number of practical units have been adopted for use in engineering.

II

Electrostatic Units

The elemental unit of electricity is the absolute charge on a single electron or proton. The symbol for this unit is e. The CGS unit of electrical charge is the electrostatic unit (esu), which is defined as the quantity of electricity that when concentrated at a point in a vacuum will repel a like charge 1 centimeter away with a force of 1 dyne. The esu equals the aggregate charge carried by 2,082,000,000 electrons or protons.

The basic unit of electrical current, or flow, is the statampere, which is defined as a current of 1 electrostatic unit per second. The statvolt, the basic unit of electromotive force, or potential difference, is the difference in potential that exists between two points when 1 erg of work is required to force 1 electrostatic unit of electricity between those two points.

III

Electromagnetic Units

Besides the electrostatic units of charge, current, and potential difference, a parallel group of basic electromagnetic units exists. The basic magnetic unit, comparable to the elemental unit of electricity, is the unit magnetic pole, defined as a point magnetic pole that in a vacuum will act on a similar pole 1 centimeter away with a force of 1 dyne. The unit used to measure the strength of magnetic fields is the oersted. A field that acts on a unit magnetic pole with a force of 1 dyne has a strength of 1 oersted. The electromagnetic unit of electric current is called the abampere. If a current of 1 abampere flows in a wire 1 centimeter long, the wire is pushed sidewise with a force of 1 dyne by a magnetic field of 1 oersted acting at right angles to the wire. The abcoulomb is the quantity of electricity passing any point in a circuit in 1 second when a current of 1 abampere is flowing in the circuit. The abvolt, the electromagnetic unit of potential difference, is the potential difference between two points when 1 erg of work is necessary to move 1 abcoulomb of electricity from one point to the other. See also Potential Energy.

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The mathematical relationships between the electrostatic and electromagnetic units are as follows: 1 esu equals 3.3356 × 10^-11 abcoulombs; 1 statampere equals 3.3356 × 10^-11 abamperes; and one statvolt equals 29,979,245,800 abvolts. This last figure is exactly equal to the velocity of light through a vacuum, which is expressed in centimeters per second, as predicted by the electromagnetic-wave theory developed by British physicist James Clerk Maxwell. See Electromagnetic Radiation.

IV

International System Units

The International System of Units is a system of units that are practical to use in the laboratory. They are commonly referred to as SI units, from the initials of the French words Système International. The SI unit of electrical current is the ampere (amp), which is defined as 0.1 abamperes. The SI unit of electrical quantity is the coulomb, the amount of electricity passing a given point in a circuit in 1 second when a current of 1 ampere is flowing. The volt (V) is the SI unit of potential difference. It is equal to 100 million abvolts and can be defined as the potential difference existing between two points when 1 joule (10 million ergs) of work is required to move 1 coulomb of electricity from one of the points to the other. The SI unit of electrical work is the watt. It represents the generation or use of electrical energy at the rate of 1 joule per second. The kilowatt is equal to 1000 watts.

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