Sunday, February 25, 2018

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The International System of Units

The very purpose of physics is to quantitatively describe and explain the physical world in as few terms as possible. This principle extends to units of measurement as well, which is why we usually find different units used in science actually defined in terms of more fundamental units. The watt, for example, is one joule of energy transferred per second of time. The joule, in turn, is defined in terms of three base units, the kilogram, the meter, and the second:Joule Formula

Within the metric system of measurements, an international standard exists for which units are considered fundamental and which are considered “derived” from the fundamental units. The modern standard is called SI, which stands for Systeme International. This standard recognizes seven fundamental, or base units, from which all others are derived1:

 

Physical quantity

SI unit

SI symbol

Length

meter

m

Mass

kilogram

kg

Time

second

s

Electric Current

ampere

A

Temperature

kelvin

K

Amount of substance

mole

mol

Luminous Intensity

candela

cd

 

An older standard existed for base units, in which the centimeter, gram, and second comprised the first three base units. This standard is referred to as the cgs system, in contrast to the SI system2. You will still encounter some derived cgs units used in instrumentation, including the poise and the stokes (both used to express fluid viscosity). Then of course we have the British engineering system which uses such wonderful3 units as feet, pounds, and (thankfully) seconds. Despite the fact that the majority of the world uses the metric (SI) system for weights and measures, the British system is sometimes referred to as the Customary system.

 

1 The only exception to this rule being units of measurement for angles, over which there has not yet been full agreement whether the unit of the radian (and its solid counterpart, the steradian) is a base unit or a derived unit.

2 The older name for the SI system was “MKS,” representing meters, kilograms, and seconds.

3 I’m noting my sarcasm here, just in case you are immune to my odd sense of humor.

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