International System of Units (SI) (Meaning and Explanation)

We explain what the International System of Units is, how it was created and what it is for. Also, its basic and derived units.

international system of units kilogram — The International System of Units is the most used throughout the world.

What is the International System of Units?

It is known as the International System of Units (abbreviated SI) when system of measurement units used practically throughout the world. It is used in the construction of the most numerous measuring instruments for both specialized and everyday consumption.

A system of units is a scientific pattern that allows things to be related based on a set of imaginary units. That is, it is a system to record reality: weighing, measuring, timing, etc., based on a set of units that are always equal to themselves and that can be applied anywhere in the world with equal value. .

The International System of Units It is the most accepted of all measurement systems (although not the only one, since in some countries they still use the Anglo-Saxon system) and the only one that tends towards a certain universalization today.

From time to time the SI is reviewed and refined, to ensure that it is the best system of units available, or to adapt it to recent scientific discoveries. In fact, in 2018 the redefinition of four of its basic units was voted in Versailles, France, to adjust them to fundamental parameters constant in nature.

History of the International System of Units

The YES It was created in 1960, during the 11th General Conference on Weights and Measuresfounded in 1875 to make decisions regarding what was then the French metric system. This is the body currently in charge of revising the International System of Measurements and is based at the International Bureau of Weights and Measures in Paris.

What is the SI for?

The SI, put very simply, is the system that allows us to measure. Or better, the one ensures that our measurements, made here or in any other region of the world, are always equivalent and they mean the same thing.

That is to say: how do you know that a meter of distance is, in fact, a meter? How do you know that a meter here is exactly the same as a meter in China, Greenland or South Africa? Well, that is precisely what this system takes care of.

For this reason, it establishes the necessary guidelines so that, so to speak, a kilogram is always a kilogram, regardless of the place or even the type of instrument used to measure it.

SI Basic Units

international system of basic units — Each unit allows measuring a different physical quantity.

The SI comprises a set of seven basic units, each linked to one of the main physical quantities, and which are:

Meter (m). The basic unit of length, scientifically defined as the path traveled by light in a vacuum in a time interval of 1/299,792,458 seconds.
Kilogram (kg). The basic unit of mass, scientifically defined from a kilogram prototype composed of an alloy of 90% platinum and 10% iridium, cylindrical in shape, 39 millimeters high, 39 millimeters in diameter and an approximate density of 21,500 kg/kg. m³. However, in more recent versions it is proposed to redefine the kilogram based on a value related to Planck's constant (h).
Second(s). The basic unit of time, scientifically defined as the duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of a cesium-133 atom.
Ampere (A). The basic unit of electric current, which pays tribute to the French physicist André-Marie Ampère (1775-1836), and scientifically defined as the intensity of a constant current that, maintained in two parallel rectilinear conductors of infinite length, negligible circular section and located one meter apart in a vacuum, produce a force between them equal to 2 x 10^-7 Newton per meter of length. It has recently been proposed to vary its definition taking into account some value of the fundamental electric charge (and).
Kelvin (K). The basic unit of temperature and thermodynamics, which pays tribute to its creator, the British physicist William Thomson (1824-1907), also known as Lord Kelvin. It is defined as the fraction 1/273.16 of the temperature that water has at its triple point (that is, where its three states coexist in harmony: solid, liquid and gaseous). It has recently been proposed to redefine Kelvin taking into account a value of the Boltzmann constant (k).
Mol (mol). The basic unit for measuring the amount of a substance within a mixture or solution, scientifically defined as the amount of substance in a system that contains as many elementary units as there are atoms in 0.012 kg of carbon-12. Thus, when this unit is used, it must be specified whether we are talking about atoms, molecules, ions, electrons, etc. Recently it has been proposed to redefine this unit using some value of Avogadro's constant (N_TO).
Candela (cd). This is the basic unit of luminous intensity, scientifically defined as that possessed, in a given direction, by a source that emits monochromatic radiation of 540 x 10¹² hertz of frequency, and whose energy intensity in said direction is 1/683 watts per steradian.

SI Derived Units

As its name indicates, The units derived from the SI are derived from the basic units, through combinations and relationships between them, in order to be able to express physical magnitudes mathematically.

We should not confuse these units with the multiples and submultiples of the basic units, such as kilometers or nanometers (multiple and submultiple of the meter, respectively).

There are many derived units, but we can mention the main ones below:

Cubic meter (m³). Derived unit constructed to measure the volume of a substance.
Kilogram per cubic meter (kg/m³). Derived unit constructed to measure the density of a body.
Newton (N). Paying tribute to the father of modern physics, the British Isaac Newton (1643-1727), it is the derived unit constructed to measure force, and expressed as kilograms per meter per second squared (kg.m/s²), from Newton's own equation for calculating force.
Joules/Joule (J). It takes its name from the English physicist James Prescott Joule (1818-1889), and is the SI-derived unit used to measure energy, work or heat. It can be defined as the amount of work required to move a charge of one coulomb through a voltage of one volt (volt per coulomb, VC), or as the amount of work required to produce one watt of power for one second ( watt per second, Ws).

There are many other derived units, most with special names that pay homage to their creators or important scholars of the phenomenon the unit serves to describe.

Advantages and limitations of SI

international system of kilometer units — The SI allows us to know that a unit is worth the same throughout the world.

Traditionally The weak points of the SI were its units of mass (kg) and force (N)which were constructed arbitrarily. But in the face of modern updates and tunings like those detailed above, this no longer presents a major problem.