S.I. Units of Physical Quantity

S.I. Units of Physical Quantity

Unit-The chosen standard used for measuring a physical quantity is called unit.
Unit should be:

  • Well defined
  • Easy to reproduce
  • Easy to compare
  • Internationally accepted
  • Independent of changes in physical conditions

Units are of two types:

  1. Fundamental Unit :
    • Are expressed in terms of standard units.
    • There are seven (7) basic physical quantities.
    • Examples- mass, length, time, electric current, temperature, luminous intensity and amount of substance.
    • All other quantities are expressed in terms of these quantities.
  2. Derived Unit :
    • When a physical quantity expresses itself in terms of two or more fundamental physical quantities.
    • It is derived by multiplying or dividing one basic physical quantity with another basic physical quantity.
    • Examples- density, volume, force, power, velocity, acceleration etc.

System of Units –Units depends on choice .each choice of units leads to a new system of units. The internationally accepted systems are
i) CGS System          ii) MKS System        iii) FPS System    iv) SI Units.
In SI Units(International System of Units)- There are seven fundamental units and 2 supplementary units.All the units which are defined/ expressed in terms of fundamental units are called derived units.such as force work etc.
Here list of most often used physical quantities their unit and symbols.

Quantity SI Symbol
Length Meter M
Mass Kilogram Kg
Time Second S
Work and Energy Joule J
Electric Current Ampere A
Temperature Kelvin K
Intensity of flame Candela Cd
Angle Radian Rad
Solid angle Steredian Sr
Force Newton N
Area Square meter M2
Volume Cubic meter M3
Speed Meter per second Ms-1
Angle Velocity Radian per second Rad s-1
Frequency Hertz Hz
Moment of inertia Kilogram square meter Kgm2
Momentum Kilogram meter per second Kg ms-1
Impulse Newton second Ns
Angular Momentum Kilogram square meter per second Kgm2s-1
Pressure Pascal Pa
Power Watt W
Surface tension Newton per meter Nm-1
Viscosity Newton second per square m. N.s.M-2
Thermal Conductivity Watt per meter per degree celcius Wm-1c-1
Specific Heat capacity Joule per kilogram per Kelvin Jkg-1K-1
Electric charge Coulomb C
Potential Difference Volt V
Electric Resistance Ohm O
Electrical Capacity Farad F
Magnetic Induction Henry H
Magnetic Flux Weber Wb
Or photometric power Lumen Lm
Intensity of illumination Lux Lx
Wave length Angstrom A0
Astronomical distance Light year ly

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