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Class 11 Physics Chapter 2 - Units and Measurement
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Explanation:
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Answer:
Physical quantities:
Quantities that can be measured also, in term of which laws of Physics can be described are called physical quantities. Example: mass, length etc.
Unit:
Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard known as unit.
Fundamental (or Base) units & Derived units:
The units for the base or fundamental quantities are called base or fundamental units.
There are 7 fundamental quantities and they are:
• Length
• Mass
• Time
• Electric Current
• Thermo Temperature
• Amount of Substance
• Luminous Intensity
The units of all other physical quantities can be expressed as combinations of the base units. Such units obtained for the derived quantities are called derived units.
System of Units:
A complete set of these units, both the base units and derived units, is known as the system of units.
The international system of units:
Three common systems of units frequently used in Physic are:
• CGS system (In CGS system they were centimetre, gram and second respectively)
• FPS (or British) system (In FPS system they were foot, pound and second respectively)
• MKS system (In MKS system they were metre, kilogram and second respectively)
SI System of Units:
The system of units which is at present internationally accepted for measurement is the Système Internationale d’ Unites (French for International System of Units), abbreviated as SI.
The SI, with standard scheme of symbols, units and abbreviations, was developed and recommended by General Conference on Weights and Measures in 1971 for international usage in scientific, technical, industrial and commercial work.
SI units used decimal system, conversions within the system are quite simple and convenient.
SI units of 7 Fundamental & 2 Supplementary Physical Quantities (and their definitions):
SI units of 7 Fundamental Physical Quantities
Base quantity
Name (& Symbol) of SI Units
Definition
Length
metre (m)
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. (1983).
Mass
kilogram (kg)
The kilogram is equal to the mass of the international prototype of the kilogram (a platinum-iridium alloy cylinder) kept at international Bureau of Weights and Measures, at Sevres, near Paris, France. (1889)
Time
second (s)
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. (1967)
Electric current
ampere (a)
The ampere is that constant current which, if maintained in current two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10–7 newton per metre of length. (1948)
Thermo dynamic Temperature
kelvin (k)
The kelvin, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. (1967)
Amount of substance
mole (mol)
The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon - 12. (1971)
Luminous intensity
candela (cd)
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. (1979)
SI units of 2 Supplementary Physical Quantities
Plane angle
Radian (rad)
Solid angle
Sterdian (sr)
Plane angle (dθ): as the ratio of length of arc ds to the radius r.
Solid angle (dΩ): as the ratio of the intercepted area dA of the spherical surface, described about the apex O as the centre, to the square of its radius r.

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