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Systems of Units Help

By — McGraw-Hill Professional
Updated on Aug 29, 2011

Introduction to System of Units

Units are devices that scientists use to indicate, estimate, and calculate aspects of the world and the universe. Numbers by themselves are abstract. Try to envision the number 5 in your mind. You think of a set or object: five objects, five dots, a line five meters long, a five-pointed star, or a pentagon. However, these are sets or objects, not the actual number. It is still more difficult to directly envision the square root of two (2 1/2 ), pi (π), or the natural logarithm base ( e ), which aren’t whole numbers.

Most people think of numbers as points on a line that are certain distances from the origin , or zero point. Displacement might be 2 1/2 units or π meters. You might think of a specific length of time, such as e seconds. Maybe you think of mass in kilograms or even something more exotic, such as the intensity of an electric current in amperes , or the brilliance of a light bulb in candelas .

There are various schemes, or systems, of physical units in use throughout the world. The meter-kilogram-second (mks) system , also called the metric system or the International System , is favored by most physicists. The centimeter-gram-second (cgs) system is used less often, and the foot-pound-second (fps) system , also called the English system , is used rarely by scientists but is popular among nonscientists. Each system has several fundamental, or base , units from which all the others are derived.

The International System (si)

The International System is often abbreviated SI, which stands for Système International in French. This scheme in its earlier form, mks, has existed since the 1800s, but more recently it has been defined in a rigorous fashion by the General Conference on Weights and Measures.

The base units in SI quantify displacement, mass, time, temperature, electric current, brightness of light , and amount of matter (in terms of the number of atoms or molecules in a sample). Respectively, the units in SI are known as the meter , the kilogram , the second , the kelvin (or degree kelvin ), the ampere , the candela , and the mole . We’ll define these in detail shortly.

The Cgs System

In the centimeter-gram-second (cgs) system, the base units are the centimeter (exactly 0.01 meter), the gram (exactly 0.001 kilogram), the second , the degree Celsius (approximately the number of Kelvins minus 273), the ampere , the candela , and the mole . The second, the ampere, the candela, and the mole are the same in cgs as they are in SI.

The English System

In the English or foot-pound-second (fps) system, the base units are the foot (approximately 30.5 centimeters), the pound (equivalent to about 2.2 kilograms in the gravitational field at the Earth’s surface), the second , the degree Fahrenheit (where water freezes at 32 degrees and boils at 212 degrees at standard sea-level atmospheric pressure), the ampere , the candela , and the mole . The second, the ampere, the candela, and the mole are the same in fps as they are in SI.

Practice problems of these concepts can be found at: Units And Constants Practice Test

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