SI derived unit
An SI derived unit is a measurement unit defined from the seven SI base units (or dimensionless); 22 of them have special names such as hertz, newton and pascal.
Overview
An SI derived unit is a unit of measurement in the International System of Units (SI) that can be written as a product of powers of the seven SI base units, or that is dimensionless. Derived units appear throughout science and engineering because most physical quantities (force, pressure, energy, electric potential, etc.) are not described directly by the base units. A number of derived units have been given conventional names and symbols for convenience.
Base units and expression
The SI base units are metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol) and candela (cd). Any derived unit can be expressed algebraically in terms of these. For example, the newton (N), a unit of force, is kg·m·s^-2, and the joule (J), a unit of energy, is N·m or kg·m^2·s^-2. The hertz (hertz) is defined as one cycle per second (s^-1) and is the SI unit of frequency. Some derived units are dimensionless: the radian (rad) and steradian (sr) describe plane and solid angles respectively.
Named derived units
- Radian (rad) — plane angle, dimensionless
- Steradian (sr) — solid angle, dimensionless
- Hertz (Hz) — frequency, s^-1
- Newton (N) — force, kg·m·s^-2
- Pascal (Pa) — pressure, N·m^-2
- Joule (J) — energy, N·m
- Watt (W) — power, J·s^-1
- Coulomb (C) — electric charge
- Volt (V) — electric potential
- Farad (F) — capacitance
- Ohm (Ω) — electrical resistance
- Siemens (S) — electrical conductance
- Weber (Wb) — magnetic flux
- Tesla (T) — magnetic flux density
- Henry (H) — inductance
- Degree Celsius (°C) — temperature interval (offset relative to K)
- Lumen (lm) — luminous flux
- Lux (lx) — illuminance
- Becquerel (Bq) — radioactivity (decays per second)
- Gray (Gy) — absorbed dose of ionizing radiation
- Sievert (Sv) — dose equivalent for biological effect
- Katal (kat) — catalytic activity
History and standardization
The SI system and its named derived units were progressively standardized in the 20th century and are maintained by the Bureau International des Poids et Mesures (BIPM). The set of seven base units was chosen so that most other practical units can be derived in a coherent way, meaning a derived unit is directly equal to a product of base units without extra numerical factors.
Uses, conventions and notable points
Named derived units simplify notation and communication across disciplines: saying "pascal" is often clearer than writing "kg·m^-1·s^-2." Most derived units accept SI prefixes (milli-, kilo-, mega-, etc.) where it is sensible to do so. Dimensionless derived units such as the radian are treated specially in some contexts because they represent ratios rather than independent physical dimensions. Degree Celsius has a special name but differs from kelvin by an additive offset, so care is needed when converting thermodynamic quantities. These units form the backbone of quantitative measurement in science, industry and commerce worldwide.
For further authoritative definitions and official tables of SI units, refer to international metrology resources.
Questions and answers
Q: What is a SI derived unit?
A: A SI derived unit is a unit of measurement that can be expressed as a combination of one or more SI base units or that has no dimension.
Q: How many special names do SI derived units have?
A: There are special names for 22 SI derived units.
Q: Can a SI derived unit be expressed solely in terms of SI base units?
A: Yes, a SI derived unit can be expressed as a combination of one or more SI base units.
Q: Give an example of a SI derived unit.
A: One example of a SI derived unit is the Hertz, which is defined as one per second, or s^-1.
Q: What is the Hertz?
A: The Hertz is a SI derived unit that measures the frequency of a wave, defined as one per second, or s^-1.
Q: Does every SI derived unit have a special name?
A: No, not every SI derived unit has a special name. Some SI derived units have no dimension and are expressed in terms of SI base units.
Q: How can SI derived units be expressed?
A: SI derived units can be expressed as a combination of one or more SI base units or as having no dimension.
Related articles
Author
AlegsaOnline.com SI derived unit Leandro Alegsa
URL: https://en.alegsaonline.com/art/90137