SI, the international system of units are divided into three classes :
1. Base units
From the scientific point of view division of SI units into these classes is to a certain extent arbitrary, because it is not essential to the physics of the subject. Nevertheless the General Conference, considering the advantages of a single, practical, world-wide system for international relations, for teaching and for scientific work, decided to base the international system on a choice of six well-defined units given in Table 1 below :
The second class of SI units contains derived units, i.e., units which can be formed by combining base units according to the algebraic relations linking the corresponding quantities. Several of these algebraic expressions in terms of base units can be replaced by special names and symbols can themselves be used to form other derived units.
Derived units may, therefore, be classified under three headings. Some of them are given in Tables 2, 3 and 4.
The SI units assigned to third class called “Supplementary units” may be regarded either as base units or as derived units. Refer Table 5 and Table 6.
(iii) Second. The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
(iv) Ampere. One ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section and placed one metre apart in vacuum, would produce between these conductors a force equal to 2 × 10–7 newton per metre of length.
(v) Kelvin. The kelvin is the fraction
1
273.16
of thermodynamic temperature of the triple point of water.
(vi) Candela. The candela is the luminous intensity, in the perpendicular direction, of a
surface of a
1
600,000
square metre of a black body at a temperature of freezing platinum under a pressure of 101, 325 newton per square metre.
(vii) Mole. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kg, carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles or specified groups of such particles.
(viii) Radian. The radian is the plane angle between two radii of a circle that cut off on the circle an arc equal in length to the radius.
(ix) Steradian. The steradian is the solid angle which having its vertex in the centre of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.
1. Base units
2. Derived units
3. Supplementary units.
From the scientific point of view division of SI units into these classes is to a certain extent arbitrary, because it is not essential to the physics of the subject. Nevertheless the General Conference, considering the advantages of a single, practical, world-wide system for international relations, for teaching and for scientific work, decided to base the international system on a choice of six well-defined units given in Table 1 below :
Quantity Name Symbol
length metre m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
luminous intensity candela cd
amount of substance mole mol
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
luminous intensity candela cd
amount of substance mole mol
Derived units may, therefore, be classified under three headings. Some of them are given in Tables 2, 3 and 4.
The SI units assigned to third class called “Supplementary units” may be regarded either as base units or as derived units. Refer Table 5 and Table 6.
DEFINITIONS
The SI seven base units and two supplementary units are defined below :
(i) Metre. The metre is the length equal to 1,650, 76373 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton- 86 atom.
(ii) Kilogram. One kilogram is equal to the mass of the international prototype of the kilogram.(iii) Second. The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
(iv) Ampere. One ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section and placed one metre apart in vacuum, would produce between these conductors a force equal to 2 × 10–7 newton per metre of length.
(v) Kelvin. The kelvin is the fraction
1
273.16
of thermodynamic temperature of the triple point of water.
(vi) Candela. The candela is the luminous intensity, in the perpendicular direction, of a
surface of a
1
600,000
square metre of a black body at a temperature of freezing platinum under a pressure of 101, 325 newton per square metre.
(vii) Mole. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kg, carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles or specified groups of such particles.
(viii) Radian. The radian is the plane angle between two radii of a circle that cut off on the circle an arc equal in length to the radius.
(ix) Steradian. The steradian is the solid angle which having its vertex in the centre of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.
Some other Definitions
Newton. The newton (N) is a derived unit of force and is defined as the unit of force which
when acting on a mass of 1 kilogram gives it an acceleration of one metre per second. Since
acceleration due to gravity equals 9.81 m/s2, one kilogram force equals 9.81 newtons.
Joule. The joule (J) is a derived unit of energy, work or quantity of heat and is defined as
the work done when a force of one newton acts so as to cause a displacement of one metre. Energy
is defined as the capacity to do work. A unit of energy in nuclear physics is the electron volt (eV)
which is defined as the energy gained by an electron in rising through a potential difference of one
volt.
1 eV = 1.6021 × 10–19 J.
Watt. The watt (W) is a unit of power (i.e., rate of doing work)
Power in watts = work (or energy) in joules/time in seconds
Thus 1 watt equals 1 Joule/sec.
1 kilo watt-hour (kWh) = 1000 watt-hours = 36000000 joules.
Coulomb. The coulomb (C) is the derived unit of charge. It is defined as the quantity of
electricity passing a given point in a circuit when a current of 1 A is maintained for 1 second.
Q = i.t
where Q = charge in coulombs,
i = current in amperes, and
t = time in seconds.
l coulomb represents 6.24 × 1018 electrons.
Ohm. The ohm (Ω) is the unit of electric resistance and is defined as the resistance in
which a constant current of 1 A generates heat at the rate of 1 watt.
Siemen. The siemen is a unit of electric conductance (i.e., reciprocal of resistance). If a
circuit has a resistance of 5 ohms, its conductance is 0.2 siemens. A more commonly used name
for siemen is mho
Volt. The volt is a unit of potential difference and electromotive force. It is defined as the
difference of potential across a resistance of 1 ohm carrying a current of 1 ampere.
Hertz. The hertz (Hz) is a unit of frequency. 1 Hz = 1 cycle per second.
Horse-power. It is a practical unit of mechanical output. BHP (British horse power or
brake horse power) equals 746 watts. The metric horse power equals 735.5 watts. To avoid confusion
between BHP and metric horse power, the mechanical output of machines in SI units, is expressed
in watts or kilowatts.
SALIENT FEATURES OF SI UNITS
The salient features of SI units are as follows :
1. 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.
2. It is a rationalised system of units, applicable to both, magnetism and electricity.
3. It is a non-gravitational system of units. It clearly distinguishes between the units of
mass and weight (force) which are kilogram and newton respectively.
4. All the units of the system can be derived from the base and supplementary units.
5. The decimal relationship between units of same quantity makes possible to express any
small or large quantity as a power of 10.
6. 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.
Advantages of SI Units :
1. Units for many different quantities are related through a series of simple and basic
relationship.
2. 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
numericals in physics and engineering.
3. Being a rationalised system, it ensures all the advantages of rationalised MKSA system
in the fields of electricity, magnetism, electrical engineering and electronics.
4. Joule is the only sole unit of energy of all forms and watt is the sole unit of power hence
a lot of labour is saved in calculations.
5. It is a coherent system of units and involves only decimal co-efficients. Hence it is very
convenient and quick system for calculations.
6. In electricity, all the practical units like volt, ohm, ampere, henry, farad, coulomb, joule
and watt accepted 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 need for any more
practical units.
Disadvantages :
1. The non-SI time units ‘minute’ and ‘hour’ will still continue to be used until the clocks
and watches are all changed to kilo seconds and mega seconds etc.
2. The base unit kilogram (kg) includes a prefix, which creates an ambiguity in the use of
multipliers with gram.
3. SI units for energy, power and pressure (i.e., joule, watt and pascal) 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.
4. There are difficulties with regard to developing new SI units for apparent and reactive
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