Physical Quantities and UnitsPhysicsClass 1
In this post, the important physical quantities and Units are Explained. This topic is Useful for the school/college students and aspirants of Competitive Exams like SSC,RRB,State PSC's etc.
Phyisical Quantities and UnitsPhysics
Physical/SI Unit
Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit.
The result of a measurement of a physical quantity is expressed by a number (or numerical measure) accompanied by a unit.
Although the number of physical quantities appears to be very large, we need only a limited number of units for expressing all the physical quantities, since they are interrelated with one another.
Physical Quantities
All the quantities in terms of which laws of physics are described and whose measurement is necessary are called.
What is a Unit?
A unit is defined as a standard or fixed quantity of one kind, used to measure other quantities of the same kind.
The unit of a physical quantity is the reference standard used to measure it.
Properties of units.
• It should be well defined.
• It should be of suitable size such as neither too large nor too small in comparison to the quantity to be measured.
• It should be easily reproducible at all places.
• It should not change with time and from place to place.
• It should not change with change in its physical conditions such as pressure, temperature and so on.
• It should be easily accessible.
Types of Units
1. Fundamental Units:
The units for the fundamental or base quantities are called fundamental or base units.
The quantities that do not depend on any other physical quantities for their measurement are known as fundamental quantities.
In physics, there are seven fundamental quantities of fundamental units.

Basic Unit





Time

Second (s)


Temperature

Kelvin (K)


Electric Current

Ampere (A)


Luminous Intensity

Candela (cd)


Length

Meter (m)


Amount of Substance

Mole (mol)

2. Derived Units:
Units which are derived from basic units and bear a constant relationship with the fundamental units are called derived units.
The units of all other physical quantities can be expressed as combinations of the base units.
Except for 7 fundamental quantities, all other quantities are derived quantities.
Example: Volume, velocity, force, speed, area, density, power, etc.
The International System of Units
In earlier time scientists of different countries were using different systems of units for measurement.
Three such systems, the CGS, the FPS (or British)system and the MKS system were in use extensively till recently.
The base units for length, mass and time in these systems were as follows:
• F.P.S system is the British system in which the basic unit of length, mass and time are foot, pound and second respectively.
• C.G.S system is the Metric system in which the basic unit of length, mass and time are centimeter, gram and second respectively.
• M.K.S system is the Metric system in which the basic unit of length, mass and time are meter, kilogram and second respectively.
Length, mass and time are the fundamental units in all the systems i.e. F.P.S, C.G.S, M.K.S and SI systems.
Derived Units of S.I system
Quantity

Unit

Symbol

Area

square
metre

m2

Volume

cubic
metre

m3

Density

kilogram
per cubic metre

kg/m3

Velocity

metre
per second

m/s

Acceleration

metre
per second per second

m/s2

Force

Newton

N
(kg.m/s2)

Pressure, Stress

Newton per square metre

N/m2

Moment of force

Newton metre

N.m

Work, Energy, Heat

Joule

J (N.m)

Power

Watt

W (J/s)

Calorific value

kilojoule per kilogram

kJ/kg

Specific fuel consumption

kilograms per kilowatt hour

kg/kWh

Decimal multiples and parts of units
Prefixes

Symbol

Decimal Power

Exa

E

10 ^{18}

Peta

P

10^{15 }

Tera

T

10^{12}

Giga

G

10^{9}

Mega

M

10^{6 }

Kilo

k

10^{3 }

Hector

h

10^{2 }

Deca

da

10^{1 }

metre

m

10^{0} = 1

deci

d

10^{1 }

centi

c

10^{2}

milli

m

10^{3}

micro

μ

10^{6}

nano

n

10^{9}

pico

p

10^{12}

femto

f

10^{15}

atto

a

10^{18}

Units and the British Units
Quantity

SI Unit → British Unit

British Unit → SI Unit

Length

1 m = 3.281 ft
1 km = 0.621 mile

1 ft = 0.3048 m
1 mile = 1.609 km

Mass

1 kg = 2.205 lb

1 lb = 0.454 kg

Force

1 N = 0.225 lbf

1 lbf = 4.448 N

Energy, work

1 J = 0.239 calorie
1 kJ = 0.9478 Btu
1 kJ = 0.526 CHU

1 calorie = 4.186 J
1 Btu = 1.05506 kJ
1 CHU = 1.9 kJ

Power

1 kW = 1.34 hp

1 hp = 0.7457 kW

Temperature units
Scale

Freezing point

Boiling point

Centigrade (°C)

0°C

100°C

Faranheit (°F)

32°F

212°F

Kelvin (K)

273 K

373 K

Reaumer (°R)

0°R

80°R

Electrical Quantities
Electric potential

Volt (V)

Electromotive force

Volt (V)

Electric current

Ampere (A)

Electric Resistance

Ohm (Ω)

Specific Resistance

Ohm metre (Ωm)

Conductance (Ω1)

Siemens (S)

Some important conversion factors
Length
• 1 inch = 25.4 mm
• 1 metre = 39.37 inch
• 1 foot = 0.305 m
• 1 yard = 0.914 m
• 1 nautical mile = 1852
m
• 1 geographical mile =
7420 m
• 1 light year = 9.46 ×
10^{15} m
• 1 parsec = 3.084 × 10^{16}
m
• 1 fermi = 10^{15}
m
• 1 angstrom (Å) = 10^{10} m
• 1 astronomical unit (A.U.) = 1.496 × 10^{11} m
• 1 amu = 1.66 × 10^{23}
kg
Pressure
• 1 Pa = 1 N/m^{2 }
• 1 bar = 10^{5} Pa
• 1 atm = 101.325 kPa = 1 kgf/cm^{2 }=
735.6 mm of Hg
• 1 Torr = 133.32 Pa
Area
• 1 cm2 = 100 mm^{2}
• 1 Hectare = 2.47
acres
Weight
• 1 kgf = 1000 mgf
• 1 Ton = 1000 kgf
• 1 Ounce = 28.35 gf
• 1 Pound = 0.454 kgf
• 1 Newton = 105 dynes
• 1 kg wt. = 9.81 N
• 1 gm weight = 980
Dynes
Volume
• 1 cm3 = 1000 mm^{3 }
• 1 litre = 1000 cm^{3}
• 1 Gallon = 4.54 litre
Work/Energy
• 1 kgf.m = 9.81 J = 9.81 Ws
• 1 J = 1 N.m = 1 Ws
• 1 kWh = 3.6 × 106 J
•
1 calorie(cal) = 4.184 J
Power
• 1 W = 1 J/s
• 1 HP (British) = 746 watts
• 1 HP (Metric) = 735.5 watts
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