In earlier time scientist of different countries were using different systems of units for measurement. Some of these systems are:-
CGS System :– In this system centimeter, gram and second are the fundamental units of length, mass and time respectively. It is a metric system of units. It is also known as Gaussian system of units.
FPS system :- In this system foot, pound and second are the fundamental units of length, mass and time respectively. It is not a metric system of units. It is also known as British.
MKS System :- In this system metre, kilogram and second are the fundamental units of length, mass and time respectively. It is also a metric system of units
International System of Units (SI) :- The system of units which is at present internationally accepted for measurement is the System Internationale d’ Units (French for International System of Units), abbreviated as SI.
The SI, with standard scheme of symbols, units and abbreviations, was developed and recommended by General conference on Weight and Measures in 1971 in France for International usage in scientific, technical, industrial and commercial work. It is based on the seven fundamental units or base units and two supplementary units.
The Seven SI Base Units:-
1.Meter (m) – Unit of Length The meter is the SI unit of length. Originally, the meter was defined based on the Earth’s meridian, but today, it is defined in terms of the speed of light in a vacuum. Specifically, one meter is the distance that light travels in a vacuum in 1/299,792,4581seconds. This definition ensures extraordinary precision and makes the meter a reliable standard for scientific measurements.
2. Kilogram (kg) – Unit of Mass The kilogram is the SI unit of mass. It was originally defined as the mass of a platinum-iridium prototype kept in France, but it has since been redefined using the Planck constant. As of 2019, the kilogram is defined as the mass corresponding to a specific value of the Planck constant, which is a fundamental physical constant. This change ensures that the kilogram is no longer dependent on a physical object.
3. Second (s) – Unit of Time The second is the SI unit of time and was originally based on the Earth’s rotation. Today, it is defined more precisely by atomic transitions. Specifically, the second is defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels in a cesium-133 atom. This atomic definition provides incredible accuracy and stability, making the second the most precise of all the SI units.
4. Ampere (A) – Unit of Electric Current The ampere is the SI unit of electric current. Previously, it was defined based on the force between two parallel wires carrying electric currents. As of 2019, it is now defined using the elementary charge (the charge of a single electron). One ampere is the current that results from the flow of 1.602176634 × 1019 elementary charges per second. This shift to a definition based on fundamental constants ensures precision.
5. Kelvin (K) – Unit of Temperature The kelvin is the SI unit of thermodynamic temperature. It was originally defined based on the triple point of water (the temperature and pressure at which water can exist in all three states (solid, liquid, and gas). In the modern definition, the kelvin is linked to the Boltzmann constant, which relates the average kinetic energy of particles in a substance to its temperature. One kelvin is equivalent to a change in energy of 1.380649 × 10−23 joules per degree.
6. Mole (mol) – Unit of Amount of Substance The mole is the SI unit that measures the amount of substance. One mole contains exactly 6.02214076×1023 elementary entities (such as atoms, molecules, or ions), a value known as Avogadro’s number. This unit allows scientists to relate macroscopic quantities of material to their atomic or molecular properties, providing a bridge between chemistry and physics.
7. Candela (cd) – Unit of Luminous Intensity The candela is the SI unit of luminous intensity, which measures the perceived power of light as seen by the human eye. Specifically, it is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540×1012 hertz and has a radiant intensity of 1/683 watts per steradian. The candela is useful in fields such as lighting and vision science.
Derived Units
Derived units are formed by combining the seven base units through multiplication or division. For example, velocity is measured in meters per second (m/s), force in newtons (N), and pressure in pascals (Pa). These units are constructed to simplify complex relationships between different physical quantities and make it easier to understand and communicate scientific data.
Importance of SI Units
1.Global Standardization: SI units ensure uniformity across the world. Whether you’re in India, the U.S., or Japan, using the same units avoids confusion and allows scientists, engineers, and industries to work collaboratively without needing to convert measurements between different systems.
2. Precision and Accuracy: By basing the SI units on fundamental constants of nature, we ensure that they remain constant, unaffected by changes in human definitions or environmental conditions. This leads to incredibly precise measurements, essential for scientific research and technology.
3. Simplicity and Ease of Use: The SI system is designed to be logical and easy to use, especially in scientific calculations. All units are either base units or derived from base units, and they often involve simple prefixes to express large or small quantities (e.g., kilometer, millimeter, nanosecond).
Following points must be remembered while using SI units :-
Even if a unit is maned after a person, the unit (in whole) is not written with a capital initial letter. Thus we write ampere (Ampere),newton (not Newton) etc.
For a unit named after a person, the symbol is acapital letter, Symbol of other units are not written in capital letters e.g. N for newton, A for ampere and m for metre.
Full stops or other punctuation marks are not to be written after the symbols of units e.g. kg m s-2 (not kg. m. s-2).
Note :-
The International System of Units provides the foundation for modern scientific measurement. By offering a standardized, precise, and universal framework for measurement, the SI system has become indispensable in science, industry, and everyday life. As technology and science progress, the SI units will continue to evolve, ensuring that our measurements remain accurate and relevant to the needs of society.
The International System of Units (SI) is the globally accepted standard for measurement. It was established in 1960 to ensure uniformity in measuring physical quantities like length, mass, time, and more. The system is built on seven base units, which are used to derive all other units for scientific, industrial, and everyday purposes.
The seven base units in the SI system are:
Meter (m) for length
Kilogram (kg) for mass
Second (s) for time
Ampere (A) for electric current
Kelvin (K) for temperature
Mole (mol) for the amount of substance
Candela (cd) for luminous intensity
SI units are important because they provide a universal standard, ensuring consistency and accuracy in measurements worldwide. This standardization allows scientists, engineers, and industries across different countries to communicate and collaborate without confusion. It also helps maintain precision in technology and scientific research.
In the modern SI system, the kilogram is no longer defined by a physical object. Since 2019, it is defined using the Planck constant, a fundamental constant of nature. This ensures that the kilogram remains stable and consistent over time, unaffected by wear or environmental changes.
Derived units are units that are formed by combining the seven base units. Examples include:
Velocity (meters per second, m/s)
Force (newton, N)
Pressure (pascal, Pa) Derived units simplify complex relationships between different physical quantities and make calculations easier.