Physical and Fundamental quantity
A quantity that can be measured is called a physical quantity. Fundamental quantities are those which cannot be expressed or measured in terms of other physical quantities.
Physical quantities which have magnitude only and no direction are called scalar quantity. Physical quantities that have magnitude and direction both and which obey triangle law are called vector quantities.
System of units : fundamental or base units and derived units
Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit. These unit is classified in two types (1) base unit (2) derived unit.
- The units for the fundamental or base quantities are called fundamental or base units. The units of all other physical quantities can be expressed as combinations of the base units. Such units obtained for the derived quantities are called derived units. A complete set of these units, both the base units and derived units, is known as the 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.
SI Units is system of units which is at present internationally accepted for measurement, SI Units full form is Système Internationale d’ Unites (In French).
- In SI, there are seven base units, Length (metre, m), Mass (Kilogram, Kg), Time (Second, s), Electric current (ampere, A), Thermo dynamic Temperature (kelvin, K), Amount of substance (mole, mol), Luminous intensity (candela, cd). [METAL]
- Besides the seven base units, there are two more units that are defined as (1) Plane angle (radian, rad) and (2) Solid angle (steradian, sr), Both these are dimensionless quantities.
There are some examples of derived units
Energy (joule, J), Work (Joule, J), Power (watt, W), Pressure (pascal, Pa), Force (newton, N)
Electric power (watt, W), Electric charge (coulomb, C), Electric current (ampere, A), Resistance (R) and Resistivity (ρ)
- Work done (W) = Force (N) × Displacement [W=Fd]
- Power (P) = work (J)/ time (s) [P=W/t]
- Density (ρ) = mass/volume [ρ = m/V] kg/m3
- Electric charge (Q) = 1 ampere-second [Q=It], Unit – coulomb, Faraday, Ampere-Hour
- Electric current (I) = coulomb per second [I=Q/t] and [I=V/R] where V – voltage, R – resistance
- Force (F) = mass x acceleration [F=ma]
- Pressure (Pa) = Force (N)/ Area (m2)
Dimensionless quantity
- The ratio of the change in dimensions of an object to its original dimensions is called ‘deformation’. Strain is a dimensionless quantity, that is, it has no unit.
Unit Conversion
- Multiplying the centimeter unit by 0.39 will yield inches. [1 cm = 0.39 inch]
- 1 nanometer = 10-9 meter or 10-7 sentimeter.
- 1 micron = 10-6 meter or 10-4 sentimeter.
- 1 Angstrom = 10-10 meter or 10-8 sentimeter.
- Megawatt (MW) and Kilowatt (kW), Both terms used in the power business when describing generation or load consumption.
- The unit of force in C.G.S. system is ‘dyne’.
- 1 hp (Horsepower) = 746 watts
- Calorie, joule and erg are units of heat. Heat is a form of energy. When energy is transferred from one body to another due to temperature difference, then the energy transferred is called ‘heat’.
- Small units for measuring mass are milligram, microgram, nanogram, picogram and femtogram. 1 picogram = 10-12 meter
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