# Units and Dimensions in Physics

By HWA | Publish On: June 6, 2011 | Posted In:

The Systeme Internationale (SI) of Units

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The topic of Units and Dimensions is very complicated. When talking about units and dimensions, there is a lot of difference them. A dimension is a measure of a physical variable, while a unit is a way to assign a number or measurement to that dimension. For example, length is a dimension, but it is measured in units of feet (ft) or meters (m).
There are three primary unit systems in use today.

1. The International System of Units (SI units)
2. The English Engineering System of Units (commonly called English units)
3. The British Gravitational System of Units (BG)

The latter two are similar, except for the choice of primary mass unit and use of the degree symbol. SI units are widely used around the world as standard. Let’s discuss about SI units.
The International System of Units (SI units)
In 1960, the International Bureau of Weights and Measures established the international system of units, commonly known as SI units to unify communication amongst world scientists. The International system contains three classes of units:

• Fundamental Units
• Supplementary Units
• Derived Units

Fundamental Units in SI – There are seven fundamental units

 Physical Quantity Unit Symbol Length Meter m Mass Kilogram Kg Time Second s Electric Current Ampere A Thermodynamic temperature Kelvin K Luminous intensity Candela Cd Amount of substance Mole mol

Supplementary Units
Two supplementary units are at present defined, the radian (rad) and the steredian (Sr) which are the units for plane angle and solid angle respectively. These are not physical quantities.
Derived Units
These can be formed by combining fundamental units. For example, unit of force is Kg/ms2 which is called newton, the unit of work is newton metre which is known as joule.

Definition of Base Units

Meter: It is SI unit of length. As adopted in 1983 by GCWM. The meter is defined as the distance travelled by light in free space in 1/29972.458 second. Earlier, meter was defined as distance containing 1650763.7 wavelengths in vacuum of orange red light emitted by Krypton 86 in vacuum.

Kilogram: It is SI Unit of mass. Originally kilogram was defined as mass of 1000 cc (1 liter) of water at 4°C. At present the standard kg is the mass of a platinum iridium cylinder kept in a special vault at International Bureau of Weights and Measures in Severs, France.

Second: At present the second is defined as the time interval of 9192631770 vibrations or radiations (corresponding to the transition between two specific hyperfine levels in cesium Cs-133).

Ampere: If equal currents are maintained in two parallel wires placed in vacuum at a separation of 1 meter so that the force between them is 2 × 10-7 newton/meter of the wires the current in any of the wires is called 1 ampere (A).

Kelvin: The fraction 1/273.16 or thermodynamic temperature of Triple point of water is called 1K.

Candela: The luminous intensity of a black body of surface area 1/600000 m2 placed at the temperature of freezing platinum at a pressure of 101,325 N/m2 in a direction perpendicular to its surface is called 1 candela (Cd).

Mole: The amount of a substance that contains as many elementary entities (molecules or atoms in case of monoatomic substance) as there are number of atoms in 0.012 kgof carbon -12 (C-12) is called a mole. This number is called Avogadro number having value 6.022045 × 1023.

Radian: It was adopted as the unit of solid angle with its apex at the center of a sphere that cuts an area on the surface of the sphere equal to the radius of the sphere. Solid angle in steradian = area cut from the surface of sphere/radius2.

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