DEFINATION OF BASE UNITS

1.METER: In a1960 a meter was defined to be the equal to 1650763.73 wavelength of orange red light( of wavelength 6.0578021*10^-7m) emitted in vacuum by krypton-86 gas due to transition from 5d^5 to 2p^10 levels. In 1963 it was redefined in terms of speed of light in vacuum as the length of path traveled by light during a time interval of  1/2,99,792,458 of a second.

KILOGRAM: A kilogram is equal to the mass of a cylinder of platinum – iridium kept at all international bureau of weights and measures at serves, near Paris, France.

3.SECOND: A second is defined as the duration of 9,192,631,770 periods of radiation corresponding to transition between two hyperfine levels of the ground state of cesium-133.

Table 1.4: Some derived SI unit with special names

S.NO	PHYSICAL QUANTITIES	UNIT	SYMBOL OF UNIT
1	FORCE	NEWTON	N
2	ENERGY	JOULE	J
3	POWER	WATT	W
4	PRESSURE	NEWTON	N/M^2
/(METER)^2
5	FREQUENCY	HERTZ	HZ
6	SPECIFIC HEAT	JOULE/KG/	J/KG/K,JKG^-1 K^-1
KELVIN
7	LUMINOUS FLUX	LUMEN	LM
8	ILLIUMINANCE	LUX	LX
9	ELECTRIC CHARGE	COULOMB	C
10	ELECTRIC CURRENT	AMPERE	A
11	ELECTRIC POTENTIAL	VOLT	V
12	ELECTRIC INTENCITY	VOLT/METER	V/M OR NC^-1
OR NEWTON/
13	ELECTRIC CAPACITY,	FARAD	F
CAPACITANCE
14	ELECTRIC RESISTANCE	VOLT/AMPERE	V/M OR OHM
,OHM
15	PERMITTIVITY	FARAD/METER	F/M
16	RESISTIVITY	OHM-METER	OHM-M
17	INDUCTANCE	HENRY	H
18	MAGNETIC FLUX	WEBER	WB
19	MAGNETIC FLUX	WEBER/M^2	WB/M^2 OR T
DENSITY
20	MAGNETIC FIELD	AMPERE/	A/M
INTENCITY

AMPERE: An ampere is defined as that constant current which, when flowing in two infinitely long straight parallel conductors of negligible cross section and placed one meter apart in vacuum, would produce between them a force equal to 2*10^-7 Newton per meter of length of each conductor. We will learn more about it in electricity and magnetism.

KELVIN:A Kelvin is defined as the fraction of 1/273.16 of the thermodynamic temperature of the triple point of water. We will learn more about it in heat unit.

CANDELA: The luminous Intensity in a given direction of a source that emits monochromatic radiation of frequency 5.40*10^14 HZ having a radiation intensity in that direction of 1/683w per steradian.

7.MOLE:  A mole is the amount of substance which contains as many elementary entities(molecules) as there are atoms in 12*10^-3 kg of carbon-12. This units is very often used in chemistry.

ADVANTAGE OF SI UNITS

SI has following advantage over other system of units:

1)   It is a rational system of units. SI makes use of only one unit for one physical quantity. For example, for all types of energies example mechanical, heat, electrical etc.The units is joule.

2)   SI is a coherent system of units. In SI, all the derived units can be obtained by dividing and multiplying the base and supplementary units without introducing numerical factors.

• 3)   SI is a metric system. Like CGS and MKS system, the multiples and submultiples can be expressed as power of 10.