# Base Units (Fundamental Units) : Physics notes

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There are certain physical quantities which are very fundamental in nature. If the units of these quantities are defined, the units of all other quantities can be derived from these. Such quantities are called basic or fundamental quantities. The units of these quantities are called base units of the system. In mechanics three base units are required. These are units of length, mass and time. In heat and thermodynamics, standard of two more quantities are required. These are temperature and amount of substance. In electricity and magnetism a standard for current is required in addition to the three base units of mechanics. In light a standard for luminous intensity is required. Thus in all, in any self-constant and well developed system of measurements, seven base units are required.

### Properties of Base Units

Any standard unit should have the following two properties:

(a) Invariability : The standard unit must be invariable. Thus, defining distance between the tip of the middle finger and the elbow as a unit of length is not invariable.

(b) Availability: The standard unit should be easily made available for comparing with other quantities.

### Seven bases units of the SI system

By international agreement the seven bases units of the SI system are:

(i) The metre (m) – standard of length
(ii) the kilogram (kg) – standard of mass
(iii) the second (s) – standard of time
(iv) the ampere (A) – standard of electric current
(v) the kelvin (K) – standard of temperature
(vi) the candela (cd) – standard of luminous intensity
(vii) the mole (mol) – standard of amount of substance

The metre (m): This is defined as 1650763.73 times the wavelength, in vacuum of the orange light emitted by  in transition from 2p10­  to 5d5.

The kilogram (kg): This is defined as the mass of a platinum-iridium cylinder kept at Sevres.

The second (s): This is the time taken by 9192631770 cycles of the radiation from the hyperfine transition in cesium – 133 when unperturbed by external fields.

The ampere (A): This is defined as the constant current which, if maintained in each of two infinitely long, straight, parallel wires of negligible cross-section placed 1 m apart, in vacuum, produces between the wires a force of 2×107 newton per meter length of the wires.

The Kelvin (K): In SI units, temperatures are measured on the thermodynamic scale with absolute zero as zero and the triple point of water (i.e., the temperature at which ice, water and water vapour are in equilibrium) as the upper fixed point. The interval is divided into 273.15 divisions and each division is taken as unit temperature. This unit is called the Kelvin.

The candela (cd): This is defined as the luminous intensity in the perpendicular direction of a surface of 1/600000 square metre of a full radiator at the temperature of freezing platinum under a pressure of 101325 newtons per square meter.

The mole (mol): The mole is the amount of any substance which contains as many elementary entities as there are atoms in 0.012 kg of the carbon isotope 126C.