Physical Quantities and Units-Physics-Class 1

Physical Quantities and Units-Physics-Class 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 Units-Physics

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 Quantity	Basic Unit
Mass	Kilogram (kg)
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	1015
Tera	T	1012
Giga	G	109
Mega	M	106
Kilo	k	103
Hector	h	102
Deca	da	101
metre	m	100 = 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 (Ω-1)	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 × 10¹⁵ m

• 1 parsec = 3.084 × 10¹⁶ m

• 1 fermi = 10^-15 m

• 1 angstrom (Å) = 10^-10 m

• 1 astronomical unit (A.U.) = 1.496 × 10¹¹ m

• 1 amu = 1.66 × 10^-23 kg

Pressure

• 1 Pa = 1 N/m²

• 1 bar = 10⁵ Pa

• 1 atm = 101.325 kPa = 1 kgf/cm² = 735.6 mm of Hg

• 1 Torr = 133.32 Pa

Area

• 1 cm2 = 100 mm²

• 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 mm³

• 1 litre = 1000 cm³

• 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