Chapter: - 2
Units
Scope: -
SI units, Fundamental & Derived Units correct symbols and conventions
Accuracy and errors in measurement, least count of measuring instruments and implications for
errors in experimental measurements and calculations.
Definitions:-
1) Physics:- The branch of science which deals with study of natural phenomenon (facts or circumstances or occurrence observed in nature)
2) Measurement: - The comparison of a thing with a standard.
3) Physical Quantities: - The quantities that are measurable is called a Physical Quantity.
OR Anything that can be measured is called a Physical Quantity
4) Fundamental Physical Quantities:- A Physical quantity which does not depend on other quantity
and cannot be simplified further is called a fundamental
physical quantity.
Ex. Length, mass, time, temperature, etc
5) Derived Physical Quantities: - Physical quantities that can be obtained and expressed in terms of
Fundamental quantities are called derived quantity.
Ex. Velocity, acceleration, displacement etc
6) Unit:- The standard used for the measurement of Physical quantities is called a Unit
7) Fundamental Unit :- The units used in the measurement of fundamental quantities are called as
Fundamental units.
Ex. Metre, seconds, kilogram, degree Celsius etc
8) Length :- The length of a path travelled by Light in vacuum during a time interval of
1/299,792,458 of a second.
i) Planck Length: Planck’s length is the fundamental quantum of length equal to
10-35 m
ii) Planck time :- The smallest time interval possible is 10-43 sec is called the
Planck time
iii) Fermi ( F) :- One Fermi or femtometre is equal to 10-15 m
iv) Angstrom :- One angstrom is equal to 10-10 m
v) Light year :- The distance travelled by light in vacuum in one year is called
One light year 1 light year = 9.46 x 1015 m
9) Mass:- One kilogram is defined as the mass of a platinum –iridium kept at the international
Bureau of weights and Measures.
10) Time: - One second is defined as 9,192,631,770 periods of the radiation from cesium -133 atoms.
11) Ampere:- The ampere is the constant current which flowing in two infinitely long straight parallel conductors of negligible circular cross section placed in vacuum one metre apart ,produces between them a force of 2 x 10 -7 per metre of their length.
Characteristics of Unit:-
1) Its Value must not vary with place and time
2) It should be capable of being reproduced easily
3) It must be well defined
4) It should be of proper size
System of Units:-
1) C.G.S System or Gaussian System:- Centimetre ,Gram , Second are the fundamental units
of length, mass, time respectively.
2) F.P.S System :- Foot , Pound , Second are the fundamental units of length , mass , time
Respectively.
3) M.K.S System: - Metre, Kilogram, Second are the fundamental units of length, mass, time respectively.
4) S.I Units: - There are seven fundamental units and two supplementary units.
Fundamental Quantities and their Units.
Sr.No / Physical Quantity / Unit / Symbol Of unit1 / Length / metre / m
2 / Mass / kilogram / kg
3 / Time / second / s
4 / Current / ampere / A
5 / Temperature / kelvin / K
6 / Luminous Intensity / candela / cd
7 / Amount of Substance / mole / mol
Supplementary Quantities and their Units.
Sr.No / Physical Quantity / Unit / Symbol Of unit1 / Plane angle / radian / rad
2 / Solid angle / steradian / Sr
Prefixes Used in S.I Units
Sr.No / Power / Prefix / Abbreviation1 / 10-24 / Yocto / Y
2 / 10-21 / Zepto
/ Z
3 / 10-18 / atto / a
4 / 10-15 / femto / f
5 / 10-12 / pico / P
6 / 10-9 / nano / n
7 / 10-6 / micro / µ
8 / 10-3 / milli / M
9 / 10-2 / centi / c
10 / 10-1 / deci / d
11 / 101 / deka / da
12 / 103 / kilo / k
13 / 106 / mega / M
14 / 109 / giga / G
15 / 1012 / tera / T
16 / 1015 / peta / P
17 / 1018 / exa / E
18 / 1021 / zetta / Z
19 / 1024 / yotta / Y
Rules for Using Symbols in S.I units
1) The full name of the units do not begin with a capital letter ex newton
2) The symbols of units named after scientists have initial capital letters.
Ex. J for joule
3) Symbols do not have plural forms .Ex. 10 kg not 10 kgs.
4) No full stop or comma to put after the symbol.
5) Multiplication of units is shown by leaving a space or a raised dot. Ex. Nm not N x m
6) A space must be left between a number and a unit.
Numerical for Practice
1) The time taken by light to travel from sun to earth is approximately
a) 8 light years b) 8 seconds
c) 8 minutes d) 8 hours
2) S.I unit of universal gas constant is
a) J mol-1 b) J k -1mol-1
c) J mol k-1 d) N k-1 mol-1
3) One Planck length is equal to
a) 10-12 m b) 10-25 m
c) 10-35 m d) 10-40 m
4) One par second is equal to
a) 10-6 s b) 10-6 m
c) 3.08 x 1016 m d) 3.08 x 10-16 m