System of Units

Physics is also known as the science of measurement. The quantities which are measurable in physics is known as physical quantity. Example: length, mass, time, force, etc. Physical quantities are divided into two groups:

1. Fundamental physical quantity
   Certain physical quantities are selected independent of each other. These are called fundamental physical quantities. They are mass, length and time. The number of physical quantities depends upon the system of the unit.
2. Derived physical quantity
   These are physical quantities which can be expressed by a proper combination of fundamental quantities.

Let us consider the velocity of a body. It is defined as the rate of change of displacement per unit time. It can be represented by the combination of length and time. Hence, it is a derived physical quantity. Other examples are Work, Energy, Power, etc.

Units

Unit is the standard quantity used in the measurement. First, a standard is selected and the physical quantity is compared with the standard which is the process of measurement.

System of units

1. FPS System
   Length – Foot (F)
   Mass – Pound (P)
   Time – Second (S)

2. CGS system
   Length – centimeter (C)
   Mass – Gram (G)
   Time- Second (S)

3. MKS system
   Length – meter (M)
   Mass – Kilogram (K)
   Time – Second (S)

4. SI system
   Length – meter (M)
   Mass – Kilogram (K)
   Time – Second (S)
   Amount of system – Mole (mol)
   Temperature – Kelvin (K)
   Luminous intensity – Candela(C)
   Electric current – Ampere (A)

5. Supplementary units

   Plain angle – radian (rad)

   Solid angle – steradian (sr)

Definitions of fundamental quantities

1. Kilogram
   It is defined as the mass of the platinum-iridium cylinder kept at the International Bureau of Weights and Measurements in France.
2. Meter
   It is defined as the distance traveled by light in \(\frac{1}{299792458}\) seconds in the vacuum.
3. Second
   It is defined as the 9192631770 periods of electromagnetic radiations emitted during a transition between two lower levels of Cesium-133 atoms.
4. Mole
   It is the amount of substance that contains as much atoms/molecules as there are in 0.012 kg of C¹².
5. Kelvin
   It is \(\frac{1}{273.16}^\text{th}\)of a thermodynamic temperature of the triple point of water.
6. Candela
   It is defined as the luminous intensity in a certain direction emitted by a source of electromagnetic radiation of 540 x 10¹²Hz that has intensity of\(\frac{1}{683}\) watt per steradian in that direction.
7. Ampere
   It is that constant current which when maintained in 2 parallel conductors of negligible cross-section and infinite length separated in the vacuum by a distance of 1m produces a force of 2 x 10^-7N per meter on either way.
8. Radian
   It is the angle between two radii of a circle that cuts an arc equal to the length of the radius on the circumference.
9. Steradian
   It is the angle subtended at the center of a sphere by an area equal to the square of the radius.