Physical Quantities - Types of Physical Quantities - SI Units - SI Prefixes - Fundamental Physical Quantities - Derived Physical Quantities

History of Physical Quantities

We have been seeing the nucleus to the earth’s age from small dimensions of sub nuclear atoms to the vast distance to the edges of the universe, from the fore exerting between the earth and sun. it was impossible to understand the nature more deeply without giving numerical values for physical quantities and equations for physical principles. To realize these massive ranges, we must also have accepted units in which to precise them and we found that all physical quantities can be expressed as combinations of only four fundamental physical quantities: length, mass, time, and electric current.

Physical Quantities

We describe physical quantities by stating how it is measured or by declaring how it is calculated from other measurements for example, we state distance and time by requiring methods for measuring them, whereas we define average speed by declaring that it is calculated as distance traveled divided by time of travel.

Measurements of physical quantities are stated as units. Units are standard values for example: the acceleration of any object is expressed in meters, kilometers etc. and without standard units it will be impossible to express and compare measured values. There are two system of units which are used in the world named as SI Units and English Units. SI Units are known as the metric system whereas English Units are known as imperial or customary system. English units were used in those nations which were ruled by the British Empire and these are still mostly used in United States only. The other countries now use SI Units as the standard system because this system is also agreed by the scientists and mathematicians that this is the standard system.

Types of Physical Quantities

SI units are divided into its two types: Fundamental Units and Derived Units. The physical quantities that can be defined only in terms of the procedure used to measure them are called the fundamental quantities and the units in which these are measured are called fundamental units while the other physical quantities like force, electric charge etc which can be expressed as algebraic combinations of length, mass, time, current for example: speed is equal to length divided by time; so these types of quantities are called derived quantities and the units of these quantities are known as derived units.

SI Units

The SI Units are as follows:

Basic Units

Quantity Unit Symbol Length Meter M Mass Kilogram Kg Time Second S Electric current Ampere A Temperature Kelvin K Quantity of substance Mole Mol Luminosity Candle Cd

Derived units

Quantity Unit Symbol Equals Plane angle Radian rad Solid angle steradian sr Frequency hertz Hz 1 s-1 Force newton N 1 kg.m.s-2 Pressure pascal Pa 1 N.m-2 Energy Joule J 1 N.m Power Watt W 1 J.s-1 Temperature celsius oC 1 K Charge coulomb C 1 A.s Potential Volt V 1 W.A-1 Resistance Ohm Ω 1 V.A-1 Conductance siemens S 1 A.V-1 Capacitance farad F 1 C.V-1 Inductance henry H 1 V.s.A-1 Magnetic flux weber Wb 1 J.A-1 Magnetic flux density Tesla T 1 Wb.m-2 Luminous flux lumen lm 1 cd.sr Illuminance Lux lx 1 lm.m-2 Convergence dioptry dioptry 1 m-1 Activity becquerel Bq 1 s-1 Absorbed dose gray Gy 1 J.kg-1 Dose equivalent Sievert Sv 1 J.kg-1 Katalytic activity katal Kat 1 mol.s-1

SI Units prefixes The SI Units Prefixes are as follows:

Prefix Symbol Value Example Yotta Y 1024         Zetta Z 1021         Exa E 1018 Exameter Em 1018m   Peta P 1015 Petasecond Ps 1015 s   Tera T 1012 Terawatt TW 1012W   Giga G 109 gigahertz GHz 109 Hz A microwave frequency Mega M 106 megacurie MCi 106 Ci high radioactivity Kilo K 103 kilometer km 103 m about 6/10 mile Hecto H 100 hectoliter hL 102 L 26 gallons Deca Da 10 dekagram dag 101 g teaspoon of butter ------- ------- 100=1         Deci D 0.1 deciliter dL 10−1 L less than half a soda Centi C 0.01 centimeter cm 10−2 m fingertip thickness Milli M 10-3 millimeter mm 10−3 m flea at its shoulders Micro µ 10-6 micrometer µm 10−6 m detail in microscope Nano N 10-9 nanogram ng 10−9 g small speck of dust Pico P 10-12 picofarad pF 10−12 F small capacitor in radio Femto F 10-15 femtometer fm 10−15 m size of a proton Atto A 10-18 attosecond as 10−18 s time light crosses an atom Zepto Z 10-21         Yocto Y 10-24

Binary prefixes

Prefix Symbol Factor Value Kilo KB 210 1024 Mega MB 220 1 048 576 Giga GB 230 1 073 741 824 Tera TB 240 1 099 511 627 776 Peta PB 250 1 125 899 906 842 624 Exa EB 260 1 152 921 504 606 846 976 Zetta ZB 270 1 180 591 620 717 411 303 424 Yotta YB 280 1 208 925 819 614 629 174 706 176

Accepted Non-SI units

Units Quantities Symbol Equals Degree of arc plane angle o (π/180) rad Minute of arc plane angle ' (1/60) o Second of arc plane angle " (1/60)' Minute Time Min 60 s Hour Time H 60 min Day Time D 24 h Liter Volume L, l 0.001 m3 Gram Mass G 0.001 kg Ton Mass T 1000 kg Bit Information Bit - Baud rate info flux Baud 1 bit.s-1 Neper Ratio Np log(A/B) Bel Ratio B 0.5 Np

Non-SI Units

Units Quantities Symbol Equals Electronvolt energy eV 1.60217733(49).10-19 J Astronomical unit length au, AU, ua 1.49597870(30).10+11 m Atomic mass unit mass u 1.6605402(10).10-27 kg

Units deprecated by the SI

Units Quantities Symbol Equals Nautical mile length mile 1852 m Knot velocity knot 1 mile.h-1 Are area are 100 m2 Hectar area ha 100 are Bar pressure bar 100000 Pa Calory energy cal 4.1868 J Ångström length Å 10-10 m Barn area b 10-28 m2 Curie Radioactivity b 3.7*10+10 Bq Röntgen Radiation dose R 0.000258 Ci.kg-1 Rad Radiation dose rad 0.01 Gy Rem Equivalent dose rem 0.01 Sv

 

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