international system of units (si)

Knowledge

2022

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

The International System of Units is the most widely used throughout the world.

What is the International System of Units?

It is known as the International System of Units (abbreviated SI) to the system of measurement units used practically all over the world. It is used in the construction of the most numerous instruments of measurement 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 be able to register the reality: weigh, to size, time, 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 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 currently tends towards a certain universalization.

From time to time the SI is revised and refined, to ensure that it is the best available system of units, 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 constant fundamental parameters in the nature.

History of the International System of Units

The SI was created in 1960, during the 11th General Conference on Weights and Measures, founded in 1875 to take decisions compared to what was then the French metric system. This is the body currently in charge of the review of the International System of Measures and is based at the International Office of Weights and Measures, in Paris.

In its creation, the SI considered only six basic units, to which others were later added, such as the mole in 1971. Its terms were harmonized between 2006 and 2009 with the collaboration of the organizations ISO (International Organization for Standardization) and CEI (International Electrotechnical Commission), originating the ISO / IEC 80000 standard.

What is the SI for?

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

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

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

SI base units

Each unit allows a different physical quantity to be measured.

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

  • Meter (m). The basic unit of length, scientifically defined as the path traveled by the light in vacuum in a time interval of 1 / 299,792,458 seconds.
  • Kilogram (kg). The basic unit of massscientifically defined from a kilogram prototype composed of a alloy 90% platinum and 10% iridium, cylindrical in shape, 39 millimeters high, 39 millimeters in diameter and a density approximately 21,500 kg / m3. However, in more recent versions it is proposed to redefine the kilogram from a value related to Planck's constant (h).
  • Second (s). The basic unit of weather, 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 atom of cesium-133.
  • Ampere (A). The basic unit of the electric current, which pays homage 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 from one of the another in a vacuum, produce a force between them equal to 2 x 10-7 Newtons 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 the temperature and the thermodynamics, which pays tribute to its creator, 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, in which its three states coexist in harmony: solid, liquid and gaseous). It has recently been proposed to redefine the Kelvin taking into account a value of Boltzmann's constant (k).
  • Mol (mol). The basic unit for measuring the amount of a substance within a mixture or dissolution, scientifically defined as the amount of substance of a system that contains as many elemental units as there are atoms in 0.012 kg of carbon-12. Thus, when this unit is used, it must be specified if we are talking about atoms, molecules, ions, electrons, etc. It has recently been proposed to redefine this unit using some value of Avogadro's constant (NTO).
  • Candela (cd). This is the basic unit of luminous intensity, scientifically defined as that possessed, in a given direction, by a source that emits a monochromatic radiation of 540 x 1012 Hertz. frequency, and whose energy intensity in that 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 express physical quantities mathematically.

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

The derived units are many, but we can cite 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 the physical modern, British Isaac Newton (1643-1727), is the derived unit constructed to measure the force, and expressed as kilograms per meter per second squared (kg.m / s2), from Newton's own equation for calculating the 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 the Energy, the job or the heat. It can be defined as the amount of work required to move a one coulomb charge through a voltage of one volt (volt per coulomb, VC), or as the amount of work required to produce one watt of power during one second ( watt per second, Ws).

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

Advantages and limitations of the SI

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 such as those detailed above, this no longer presents a major drawback.

On the contrary, the greatest virtue of the SI is that its base units are defined based on natural phenomena constants, which can be replicated if needed. In this way one could get to calibrate any type of instrument, starting from the scientifically reproducible fundamental unit.

In conclusion, it is a coherent system, internationally regulated and constantly recalibrated to guarantee its effectiveness.

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