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Physics > Units and Measurement > The International System of Units
Units and Measurement

The International System of Units

The International System of Units (SI system) is a set of seven physical quantities that form the basic units to measure all other physical quantities. How is the length of 1m same everywhere in the world? How long is a second? And why do we use certain SI units to measure specific quantities? Let’s find out the answers.

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The International System of Units (SI Units)

A unit is a well defined physical quantity against which we compare other quantities during the process of measurement. The International System of Units, also known as the SI system specifies seven such quantities or fundamental units. The seven quantities and their respective units in the SI units system are as follows:


The International System of Units

In the SI units system, kilograms or kg is the unit of mass. One kilogram is the mass of a Platinum-Iridium cylinder which is 90% Platinum and 10% Iridium. Also, it has its height equal to the diameter and both are equal to 39 mm. This cylinder is the International Prototype Kilogram.


The unit of length in the SI system is the meter or ‘m’. One meter is defined as the length travelled by light in vacuum in  \( \frac{1}{299792458} \) th of a second. This number was chosen to make the distance travelled by light in one second equal to the known value of 299792458 m.


Second is the unit of time in SI system. The time taken by 9,192,631,770 cycles of the radiation emitted by a Cesium-133 atom during its transition between two specified states is a second. It is also equal to the time taken by light to cover 299792458 m in the vacuum.

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In the SI units system, the unit of temperature is Kelvin or K. 1K is the \( \frac{1}{273.16} \) th of the temperature of the triple point of water. The triple point is the point at which the three states of a substance  (solid, liquid and gas) co-exist.


The International System of Units

The SI unit of current is Ampere or A. The ampere is defined as that constant current which, if maintained in two straight parallel conductors of infinite length, a negligible area of cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 N/m.

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Luminous Intensity

The unit of luminous intensity in the SI system is Candela or cd. The luminous power emitted per unit solid angle by a point source of light in a given direction is equal to 1 candela. The luminous intensity of a common candle is almost 1 cd.

Amount of substance

In SI the mole is the unit of the amount of a substance. One mole is the amount of a substance containing as many elementary entities as there are atoms in exactly 0.012 kilogram (or 12 grams) of carbon-12. It is equal to the Avogadro number, \(N_A\)  = 6.022 141 79×1023 mol-1

Solved Examples for You

1. Which of the following does not have a fundamental unit:

  1. Mass
  2. Time
  3. Force
  4. Electric current

Ans. c. Force

2. The units which can neither be derived from one another nor resolved into anything more basic are called

  1. Fundamental Unit
  2. Scale
  3. Derived Unit
  4. Standard Unit

Ans. a. Fundamental Unit.

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