Physical Quantities
The physical phenomena which can be measured are called Physical Quantities.
Length, Time, Speed, and Magnetic Field are Physical Quantities as they can be measured.
Smell, Taste are not Physical Quantities as they can not be measured in a general sense.
There are two types of physical quantities: Fundamental Quantities and Derived Quantities.
Fundamental Quantities:
The quantities that are independent of other quantities are called fundamental quantities.
There are seven fundamental quantities. They are Length, Mass, Time, Temperature, Electric Current, Luminous Intensity, and Amount of Substance.
Derived Quantities:
The quantities that are dependent on other quantities are called derived quantities.
Area = Length Length and Speed = . Here, area depends on length and speed depends on length and time hence they are derived quantities.
Other derived quantities are Volume, Force, Power, Intensity of sound, Magnetic Flux, etc.
Units
Unit is the standard reference in which physical quantity is expressed.
Length is expressed in meters and speed is expressed in meters/second. The meter and meter/second are the units.
System of Units
The standard references can vary based on which unit Mass, Length, Time, and other fundamental quantities are measured. On the basis of that, there are four systems of units. They are CGS, FPS, MKS, and SI systems.
Quantity CGS System FPS System MKS System 1. Length Centimeter Foot Meter 2. Mass Gram Pound Kilogram 3. Time Second Second Second SI system is an extension of the MKS system in which units of all fundamental quantities are included.
Quantity SI Unit Symbol 1. Length meter m 2. Mass Kilogram kg 3. Time Second s 4. Electric Current Ampere A 5. Temperature Kelvin K 6. Luminous Intensity Candela Cd 7. Amount of Substance Mole mol Types of Units
There are three types of units. Namely, Fundamental units, Derived Units, and Supplementary Units.
Fundamental Units: The units of fundamental quantities which are independent of other units are called fundamental units. There are seven fundamental units. eg. m,kg, s, K, Cd, Mol, A.
Supplementary Units: The units of plane angle and solid angle which are two purely geometric angles are called supplementary units.
Quantity Unit Symbol 1. Plane Angle Radian Rad 2. Solid Angle Steradian Sr Derived Units: The units of derived quantities which are dependent on fundamental units and supplementary units are called derived units. For eg. m/s, kgms, m/s etc.
To cover all the scientific measurements in terms of SI unit, the prefixes are used for the powers of ten.
Powers of Ten Prefixes Symbol Powers of Ten Prefixes symbol Yotta Y Yocto y Zetta Z zepto z Exa E Atto a Peta P femto f Tera T Pico p Giga G nano n Mega M micro Kilo K Milli m Hecto h centi c deca da deci d The size of the nucleus is in order of m and the size of an atom is in order of
Common Non-SI Units
SI units are commonly used worldwide and cover all scientific measurements. Due to historical, political, and situational significance, there are common Non-SI units in use.
Length
Common Non-SI unit Symbol Value in SI unit Significance 1 Astronomical Unit 1 AU 1.496 10 m The distance from Earth to the Sun 1 Light Year 1 Light Year 9.46 10 m The distance covered by light in one year 1 Parallactic Second 1 Parsec 3.084 10 m/(3.26 Light Year) the distance at which the length of one astronomical unit subtends an angle of one second of an arc. 1 Angstrom 1 1 10m named after 19th-century Swedish physicist Anders Jonas Ångström 1 Fermi 1 Fermi 1 10m The smallest unit of distance used in nuclear physics/ named after Enrico Fermi 1 X-ray Unit xu 1 10m used to quote the wavelength of X-rays and gamma rays. 1 Inch 1 " 0.0254 m Imperial System 1 Foot 1 ft/ 1' 0.3048 m Imperial System 1 Yard 1 yd 0.9144 m Imperial System 1 mile 1 mile 1609.344 m Imperial System 1 Nautical mile 1 Nm 1852 m Used in air, marine, and space navigation Mass
Common Non-SI unit Symbol Value in SI unit Significance 1 Pound 1 lb 0.4536 kg Imperial System 1 Slug 1 slug 14.59 kg British system based on standard gravity 1 Quintal 1 Q 100 kg historical unit of mass 1 Metric tonne 1 t 1000 kg historical unit of mass 1 amu 1 amu 1.66 10 kg precisely 1/12 the mass of an atom of carbon-12 Time
Common Non-SI unit Symbol Value in SI unit Significance 1 Minute 1 min 60 s Convenience 1 Hour 1 hr = 60 min = 3600 s Convenience 1 Day 1 day = 24 hr = 86,400 s Convenience 1 year 1 yr 1= 365.25 day = 3.156 s Convenience 1 shake 1 Shake 1 10 s Used in nuclear physics Other Conversion
Quantities Non SI values Value in SI unit Pressure 1 Pascal 1 Nm Pressure 1 atm = 760 mmHg 1.01 Nm Pressure 1 bar= 1 atm 1.01 Nm Pressure 1 torr= 1 mmHg 133.322 Nm Volume 1 liter = 1 cm Energy 1 eV 1.6 J Energy 1 erg 1 J Energy 1 KwHr 3.6 J Power 1 Hp 746 W Power of lens 1 D 1 m Plane Angle 1 Degree
Measurement
- The comparison of the amount of physical quantity in terms of standard reference of the same quantity is called measurement.
The standard reference amount is a unit (u).
The scale of the amount with reference to the unit is the numerical value (n).
Hence,
Measured Quantity(Q) = Numerical value (n) unit (u)
Eg. 10 Kg where, numerical value = 10 and unit = 1 Kg.
For a given measurement, the measured quantity is constant. i.e. Q = nu = constant.