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:
- 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.
- 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 |
