**Units of Measurement**

Physics is about the study of forces and energy. In order to test and measure physical quantities we need to define some standard measures. It explains the law of nature in a special way. This includes a quantitative description, comparison and measurement of certain physical quantities.

To measure and compare a physical quantity we need to fix some standard unit of the quantity. For example, the weight of an Elephant is heavier than a buffalo. But how many times? If a person is taller than another person. But much tall? To answer such problems, we need to fix some unit. Thus, the physical quantities are described in terms of a unit of that quantity.

**What is Unit?**

To measure any quantity and to compare two quantities we need an internationally acquired standard called as Unit. The measurement of any physical quantity is expressed in terms of a number and a specific unit.

Measurement = Quantity x Unit.

**Fundamental Quantities: **The quantities which do not depend on other physics quantities of measurement are called Fundamental Quantities. There are seven fundamental quantities of measurement defined by the International System, rest physical quantities are known as Derived Quantities. These define quantities that cannot be expressed by any other quantity. Sometimes, the unit concerned is chosen for convenience. For example, electric current (Ampere) is chosen as a fundamental unit of Measurement instead of electric charge (Coulomb) since it is easier to measure electric current.

In this case, the charge becomes a derived unit, Charge (q) = Current (I) x Time (t).

1 Coulomb = 1 Ampere x 1 second.

**1. Mass – Kilogram (Kg): **It is the unit of mass. The mass of a cylinder made of platinum-iridium alloy placed in International Bureau of Weights and Measures is defined as 1 Kg.

**2. Time – Second (s): **It is the unit of time. Caesium – 133 atom releases electromagnetic radiation of several wavelengths. A particular radiation is selected corresponding to the transition involved between two hyperfine levels of ground state of Cs – 133. Each radiation has a time period. The duration between 9,192,631,770 time periods is defined as 1 second.

**3. Temperature – Kelvin (K): **We measure temperature in Kelvin. 1 Kelvin is defined as 1/ 273. 16^{th}part of thermodynamic temperature of triple point of water

**4. Electric Current – Ampere (A): **The standard unit of measuring current is Ampere. 1 Ampere is defined as the current flowing in same direction placed parallel to each other and attracting each other with a force of 2 x 10⁻⁷ N/ m. Where Newton is the unit of force.

**5. Luminous Intensity – ****Candela (cd): **It is the luminous intensity of a source that emits radiation of a constant frequency of 540 x 10¹² Hz with a radiant intensity of 1/ 683 Watt per steradian in any given direction.

**6. Length – Meter (m): **The unit of length is meter. 1 meter is defined as the distance travelled by vacuum in 1/ 299792458 Seconds.

**7. Amount of a substance – ****Mole (mol): **We measure amount of substance in moles. 1 mole contains as many atoms as in 0.012 kg of C – 12 atom.