Units play an important role in our daily lives. In effect, everything we see and feel and everything we buy and sell is measured and compared by means of units. Some of these units have become so familiar that we often take them for granted, seldom stopping to think how they started, or why they were given the sizes they have.
Centuries ago the foot was defined as the length of 36 barleycorns strung end to end, and the yard was the distance from the tip of King Edgar's nose to end of his outstretched hand.
Since then we have come a long way in defining our units of measure more precisely. Most units are now based upon the physical laws of nature, which are both invariable and reproducible. Thus the meter and yard are measured in terms of the speed of light, and time by the duration of atomic vibrations. This improvement in our standards of measure has gone hand in hand with the advances in technology, and the one could not have been achieved without the other.
Getting used to SI
The official introduction of the International System of Untis, and its adoption by most countries of the world, did not, however, eliminate the systems that were previously employed. Just like well-established habits, units become a part of ourselves, which we can not readily let go. It iis not easy to switch overnight from yards to meters and from ounces to grams. And this is quite natural, because long familiarity with a unit gives us an idea of its magnitude and how it relates to the physical world.
Nevertheless, the growing importance of SI (particularly in the electrical and mechanical fields) makes it necessary to know the essentials of this measurement system. Consequently, one must be able to convert from one system to another in a simple, unambiguous way.
The SI possesses a number of remarkable features shared by no other system of units:
1. It is a decimal system.
2. It employs many units commonly used in industry and commerce; for example, volt, ampere, kilogram, and watt.
3. It is a coherent system that expresses with startling simplicity some of the most basic relationships in electricity, mechanics, and heat.
4. It can be used by the research scientist, the technician, the practicing engineer, and by the layman, thereby blending the theoritical and the practical worlds.
Base and derived units of the SI
The foundation of the International System of Units rests upon the seven base units listed in Table 1A.
From these base units we derive other units to express quantities such as area, power, force, magnetic flux, and so on. There is really no limit to the number of units we can derive, but some occur so frequently that they have been given special names. Thus, instead of saying that the unit of pressure is the newton per square meter, we use a less cumbersome name, the pascal. Some of the derived units that have special names are listed in Table 1B.
In this section, it will be explained some of these quantities;
Coulomb : is the quantity of electricity transported in 1 second by a current of 1 ampere.
Farad : is the capacitance of a capacitor between the plates of which there appears a difference of potential of 1 volt when it is charged by a quantity of electricity equal to 1 coulomb
Henry : is the inductance of a closed circuit which an electromotive force of 1 volt is produced when the electric current in the circuit varies uniformly at a rate of 1 ampere per second (Hence 1 henry = 1 volt second per ampere)
Hertz : is the frequency of a periodic phenomenon of which the period is 1 second.
Weber : is the magnetic flux that, linking a circuit of one turn, produces in it an electromotive force of 1 volt as it is reduced to zero at a uniform rate in 1 second (Hence 1 weber = 1 volt second)
Steradian : is the unit of measure of a solid angle with its vertex at the center of a sphere and enclosing an area of the spherical surface equal to that of a square with sides equal in length to the radius.
And also, in addition to them;
And also, in addition to them;
Resistivity(ohm meter) => Ωxm
Magnetic field strength (ampere per meter) => A / m
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