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Features of SI System

(i)    It is a non-gravitational system or units. It clearly distinguishes between the units of mass and weight (force) which are kilogram and Newton respectively.

(ii)    It is rationalized system of units, applicable to both, magnetism and electricity.

(iii)    The base unit of length an mass are relatively large in comparison to those of  c g s system and hence more convenient for use.

(iv)    All the units of the system can be derived from the base and supplementary units.

(v)    For any quantity there is one and only one SI unit. For example joule is the unit of energy of all forms such as mechanical, heat, chemical electrical and nuclear. However kwh will also continue to be used as unit of electrical energy.

(vi)    The decimal relationship between units of same quantity makes it possible to express any small or large quantity as a power of 10.

(vii)    It is a coherent system of units, i.e. product or quotient of any two base quantities results in a unit resultant quantity. For example unit length divided by unit time gives unit velocity.

Merits :
(i)    SI system being expanded form of RMKSA system possesses all the advantages had by RMKSA system in the field of electrical engineering and electronics.

(ii)    Being an absolute system, it avoids the use of factor ‘g’ i.e. acceleration due to gravity in several expressions in physics and engineering which had been a nuisance in all numerical in physics and engineering.

(iii)    Units for various quantities are related through series of simple and basic relationships.

(iv)    Joule is the only sole unit of energy of all forms and watt is the sole unit of pore hence a lot of labour is saved in calculations.

(v)    Various redundant units for a given quantity are eliminated.

(vi)    Metre and kilogram, the base units of length and mass in SI system are relatively large in comparison to those in c.g.s. system, which are quite practicable for use in engineering and technology.

(vii)    It is a coherent system of units and involves only decimal coefficients. Hence is is very convenient and quick system for calculations.

(viii)    In  electricity, all practical units such as ampere, volt, ohm, henry, farad, coulomb, joule and war employed in industry and laboratories all over the world for well over a  century have become absolute in their own right in the SI system, without the used for any more practical units.

Demerits :
(i)The base unit kilogram (kg) includes a prefix, which creates an ambiguity in the use of multipliers with gram.

(ii) The non-SI time units ‘minute’ and ‘hour’ will still continue to be used unit the clocks and watches are all changed to kilo seconds and mega seconds etc.

(iii) There are difficulties with regard to developing new SI units for apparent and reactive energy while joule is the accepted unit for active energy in SI system.

(iv) SI units for emery, power and pressure (i.e. joule, watt and paschal) are too small to be expressed in science and technology, and, therefore, in such cases the use of larger units, such as MJ ,KW, KPa will have to be made.

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