The International System of Units (SI) is the standard metric system used around the world for scientific and technical measurements. It is based on seven fundamental physical quantities, each of which has a corresponding SI base unit. These units form the foundation for all other units of measurement in the system.

The Seven SI Base Units

1. Meter (m) – Unit of Length

• The meter is the SI unit of length. It is defined as the distance traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.

2. Kilogram (kg) – Unit of Mass

• The kilogram is the SI unit of mass. It is defined by the fixed numerical value of the Planck constant $h = 6.62607015 times 10^{-34} , text{J·s}$, and this definition was adopted in 2019, replacing the previous definition based on a physical prototype of the kilogram.

3. Second (s) – Unit of Time

• The second is the SI unit of time. It is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

4. Ampere (A) – Unit of Electric Current

• The ampere is the SI unit of electric current. It is defined by the fixed numerical value of the elementary charge $e = 1.602176634 times 10^{-19} , text{C}$, in terms of the definition of the coulomb (C), which is related to the unit of charge.

5. Kelvin (K) – Unit of Thermodynamic Temperature

• The kelvin is the SI unit of thermodynamic temperature. It is defined by the fixed numerical value of the Boltzmann constant $k = 1.380649 times 10^{-23} , text{J/K}$.

6. Mole (mol) – Unit of Amount of Substance

• The mole is the SI unit of the amount of substance. It is defined as the amount of substance that contains exactly 6.02214076 × 10²³ elementary entities (such as atoms, molecules, or ions), which is known as Avogadro's number.

7. Candela (cd) – Unit of Luminous Intensity

• The candela is the SI unit of luminous intensity. It is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² Hz and has a radiant intensity of 1/683 watt per steradian.

Summary Table of SI Base Units:

These seven units form the core of the SI system, and all other units of measurement in science and engineering (like volume, speed, pressure, etc.) are derived from combinations of these base units. For example, the unit of speed is the meter per second (m/s), which combines the base units of length and time.

SI Derived Units:

From these seven base units, many other derived units are created for quantities such as force, energy, pressure, and power. Some examples include:

• Newton (N) – Unit of force, defined as $text{kg} cdot text{m/s}^2$

• Joule (J) – Unit of energy, defined as $text{N} cdot text{m}$

• Pascal (Pa) – Unit of pressure, defined as $text{N/m}^2$

• Watt (W) – Unit of power, defined as $text{J/s}$

These derived units help describe various physical properties by combining the basic SI units, making the system versatile for measuring a wide array of scientific phenomena.