System of Units
A complete sets of units, both fundamental and derived, for all physical quantities is called a system of units. There are now mainly three systems
in common use, each of which derives its name from the initial letters of the fundamental units upon which it is based. These are:
(1 C.G.S. system, having centimetre, gram and second a fundamental units .
(2) M.K.S system, in which fundamental units are metre, kilogram and second.
(3) F.P.S system, based on foot,pound and second as fundamental units.
A fourth system of unit, namely S.I., (systeme internationale) is introduced in 1960, is described in the appendix.
The first two namely C.G.S. and M.K.S. systems are also called metric system
and obviously metric standards of length and mass are used in them.
The F.P.S on the other hand based on the British standards.
The metric system has a definite advantage over the F.P.S. system calculations
are made easier , since the various multiples and submultiples of unit are
related by simple power of 10 .
On the other hand the F.P.S system has however , no such advantage .The
relation between the various subdivisions of a unit are rather obscure and inconvenient , such as 1 foot =12 inches ,1 yard =3 feet, 1 mile=1760 yds.
Even an inch is divided into 8ths, 16ths, 64ths to measure very small lengths.
Units of volumes of liquids.
The unit of volume of liquids, through derived ones, should be mentioned
especially, since these have different names. Thus , in metric system unit of volume is litre which is defined as the volume of 1 kg of water, at normal atmospheric pressur and the temperature of its maximum density (at 4°C).
Standard.
A question still remains weather there is a unique standard for a particular fundamental quantity . Unfortunately, the answer is ‘no’. World is still divided into two major portions using two different standards for each of length and mass.
In metric system the standard of length is taken as the distance between two fine lines engraved on a platinum-irridium bar kept at International Bureau of Weight and Measures in
Defination of Metre: A metre is defined as 1,553,164.1 times the waves length of the red line in the spectrum of cadmium.
Defination of Kilogram : International Proto-type Kilogram is the standard of mass in the metric system and is defined as the mass of a platinum-irridium cylinder kept together with Proto-type metre at Serves near
Defination of second : The standard of time called mean solar second (second) is the same in both systems and is defined as 1/86400 of the average length of a solar day (mean solar day) throughout the year. This definition of a second , although
Satisfactory for our daily routine, fails to satisfy modern scientist since the mean solar day varies from year to year and century to century.
Hence, in 1946, International Bureau of Weights and Measures defined the second as the time in which a cesium atom(which has only one isotope) undergoes 9,192,631,770 internal vibrations.
There are also some bigger units of length they are namely
1. Kilometer(km) 2. Light year 3. Astronomical unit(AU)
1 kilomatre is equal to 1000 metre
i.e 1 kg = 1000 m or(103m)
2. 1 Light year is the distance traveled by light in Vaccum in one year.
1 Light year = Speed of light x 1 year
= 3x103m/s x 1 year
= 3x103m/s x (365x24x60x60)s
=9.46x1012 km
3. Astronomical unit(AU) : It is defined as the distance of the earth from the
sun.
1 A.U =1.499x 10 metre.
Smaller units of length . There are some smaller units of length they are
namely, (SI unit of length is metre)
1 metre = 100 cm =1/100m
1 metre = 1000mm=1/1000m
1 metre = 1000000 micron or 1 micron =10-6 m
1 Nanometre =10-9m
No comments:
Post a Comment