# 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.

### 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)

### 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