Preparation Staff asked 1 month ago The International System of Units (SI) is a globally accepted system of measurement that is based on seven fundamental physical quantities. These seven base units are defined in terms of exact and universally recognized constants or phenomena. They serve as the foundation for all other units of measurement in the system. The seven base units of the SI system are:
Meter (m) for Length: The meter is the SI unit used to measure length or distance. It is defined as the distance that light travels in a vacuum in 1/299,792,458 seconds. This definition, based on the speed of light, ensures extreme precision and consistency in length measurements.
Kilogram (kg) for Mass: The kilogram is the unit of mass in the SI system. It is defined by the Planck constant, a fundamental constant of nature, with a value of 6.62607015×10⁻³⁴ joule seconds. The kilogram is the only SI unit still defined by a physical object – a platinum-iridium cylinder that serves as the international prototype of the kilogram.
Second (s) for Time: The second is the base unit of time, and it is defined based on the vibrations of the cesium-133 atom. One second is the time it takes for 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.
Ampere (A) for Electric Current: The ampere is the base unit for electric current. It is defined by the elementary charge, which is the charge carried by a single electron. One ampere is the constant current that, if maintained in two straight parallel conductors of infinite length and negligible circular cross section, produces a force of 2 × 10⁻⁷ newtons per meter of length between the conductors.
Kelvin (K) for Temperature: The kelvin is the unit of temperature. It is defined by the Boltzmann constant, which relates the temperature of a system to the average kinetic energy of the particles. One kelvin is equivalent to a change of 1°C, and absolute zero (0 K) is the point where all thermal motion ceases.
Mole (mol) for Amount of Substance: The mole is the base unit used to measure the amount of substance. It is defined as the amount of substance that contains as many entities (atoms, molecules, ions, etc.) as there are in 12 grams of carbon-12. This number is called Avogadro’s number, approximately 6.022 × 10²³.
Candela (cd) for Luminous Intensity: The candela is the unit for measuring luminous intensity, or the perceived power of light emitted by a source in a specific direction. It is defined as the luminous intensity of a source emitting monochromatic radiation of frequency 540 × 10¹² Hz and carrying 1/683 watt per steradian.
These seven base units serve as the foundation of the entire SI system and are used to derive all other units of measurement, making the system comprehensive and consistent across all scientific disciplines.
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