What is the International System of Units, and by what abbreviation is it internationally known?

The International System of Units is the modern form of the metric system and is the most widely used system of measurement globally. It is internationally known by the abbreviation SI, which comes from its French name, Système international d'unités.

In which sectors is the SI system primarily employed, and what is its official status worldwide?

The SI system is employed in science, technology, industry, and everyday commerce. It holds official status in nearly every country in the world, making it a universally recognized standard for measurement.

Which organization is responsible for coordinating the SI system, and what is its French abbreviation?

The SI system is coordinated by the International Bureau of Weights and Measures, which is abbreviated as BIPM from its French name, Bureau international des poids et mesures.

What are the two main components illustrated in the diagram related to the SI system?

The diagram illustrates the SI base units, which are shown in the outer ring, and the SI defining constants, which are depicted in the inner ring.

How many base units does the SI system comprise, and what is their relationship to derived units?

The SI system comprises seven base units, which form a coherent system of measurement. These base units can be combined to accommodate an unlimited number of additional quantities, known as coherent derived units, which are always represented as products of powers of the base units.

What are the seven SI base units, along with their symbols and the quantities they measure?

The seven SI base units are: the second (s) for time, the metre (m) for length, the kilogram (kg) for mass, the ampere (A) for electric current, the kelvin (K) for thermodynamic temperature, the mole (mol) for amount of substance, and the candela (cd) for luminous intensity.

How does the SI system address the need for convenient unit sizes across various applications?

To provide convenient unit sizes for different applications, the SI system offers twenty-four prefixes. These prefixes, when added to the name and symbol of a coherent unit, create additional non-coherent SI units that are always decimal (power-of-ten) multiples and sub-multiples of the coherent unit.

What was the primary motivation behind the development of the SI system?

The original motivation for developing the SI system was to address the diversity of units that had emerged within the centimetre-gram-second (CGS) systems, particularly the inconsistency between electrostatic and electromagnetic units, and the general lack of coordination among disciplines using these units.

Which international body was established by the Metre Convention of 1875, and what was its role in standardizing measurements?

The General Conference on Weights and Measures (CGPM), established by the Metre Convention of 1875, brought together various international organizations. Its role was to define and standardize a new measurement system and establish rules for writing and presenting measurements.

When was the SI system published, and what earlier system was it based upon?

The SI system was published in 1960, following an initiative that began in 1948. It is based on the metre-kilogram-second (MKS) system of units, incorporating ideas from the development of the CGS system.

What constitutes the fundamental definition of the International System of Units?

The International System of Units is fundamentally defined by a set of seven defining constants, each with a specific exact numerical value when expressed in terms of their SI units. These constants, along with their corresponding base units, derived units, and decimal-based multipliers (prefixes), form the system.

List the seven SI defining constants and their exact numerical values.

The seven SI defining constants are: the hyperfine transition frequency of caesium (ΔνCs) at 9,192,631,770 Hz; the speed of light (c) at 299,792,458 m/s; the Planck constant (h) at 6.62607015×10⁻³⁴ J·s; the elementary charge (e) at 1.602176634×10⁻¹⁹ C; the Boltzmann constant (k) at 1.380649×10⁻²³ J/K; the Avogadro constant (NA) at 6.02214076×10²³ mol⁻¹; and the luminous efficacy (Kcd) of 540 THz radiation at 683 lm/W.

How are the magnitudes of all SI units defined in relation to the defining constants?

The magnitudes of all SI units are defined by declaring that the seven defining constants have specific exact numerical values when expressed in terms of their SI units. This approach allows for new and superior measurement methods to be introduced without redefining the units themselves.

What is the definition of the second in the SI system?

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

How is the metre defined within the International System of Units?

The metre (m), the SI unit of length, is defined as the distance travelled by light in vacuum in 1/299,792,458 of a second.

What is the definition of the kilogram in the SI system?

The kilogram (kg), the SI unit of mass, is defined by setting the Planck constant (h) to 6.62607015×10⁻³⁴ J·s (where J = kg·m²·s⁻²), given the definitions of the metre and the second.

How is the ampere defined in the SI system?

The ampere (A), the SI unit of electric current, is defined as the flow of 1 / (1.602176634×10⁻¹⁹) times the elementary charge (e) per second, which is approximately 6.2415090744×10¹⁸ elementary charges per second.

What is the definition of the kelvin in the SI system?

The kelvin (K), the SI unit of thermodynamic temperature, is defined by setting the fixed numerical value of the Boltzmann constant (k) to 1.380649×10⁻²³ J·K⁻¹ (where J = kg·m²·s⁻²), given the definitions of the kilogram, the metre, and the second.

How is the mole defined in the SI system, and what must be specified when it is used?

The mole (mol), the SI unit for amount of substance, is defined as the amount of substance containing 6.02214076×10²³ elementary entities. When the mole is used, the elementary entities must be specified, which can include atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

What is the definition of the candela in the SI system?

The candela (cd), the SI unit of luminous intensity, is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5.4×10¹⁴ hertz and has a radiant intensity in that direction of 1/683 watt per steradian.

What is a 'coherent' derived unit in the SI system, and provide an example.

A coherent derived unit is an SI unit that can be represented as a product of powers of the base units with a numerical multiplier of one. For instance, the coherent derived SI unit of velocity is the metre per second (m/s).

What is the role of metric prefixes in the SI system, and how do they relate to decimal multiples?

Metric prefixes are used in the SI system to systematically create units that are decimal multiples or sub-multiples of each other over a wide range for the same physical quantity. For example, 'kilo-' signifies a factor of 1000, so 1 kilometre equals 1000 metres.

What is a 'compound unit' in the SI system, and how are prefixes applied to them?

A compound unit is formed when a prefix symbol is attached to a unit symbol, creating a new inseparable unit symbol that can be raised to a power or combined with other unit symbols. For example, g/cm³ is an SI unit of density, where cm³ is interpreted as (cm)³.

What is the rule for forming multiples of the kilogram, given its unique nature as a base unit with a prefix?

Despite the kilogram being the coherent base unit of mass, multiples and sub-multiples of mass are named as if the gram were the base unit. This means prefixes are attached to 'gram' (g), so 10⁻⁶ kg is a milligram (mg), not a microkilogram (µkg).

According to the SI Brochure, how should unit names be treated in written text?

According to the SI Brochure, unit names should be treated as common nouns of the context language. This means they should be typeset in the same character set as other common nouns and follow the usual grammatical and orthographical rules of that language, typically starting with a lowercase letter unless at the beginning of a sentence or in a heading.

What is the difference in spelling for certain SI units and prefixes between US English and International English?

US English uses the spelling 'deka-', 'meter', and 'liter', while International English uses 'deca-', 'metre', and 'litre'. Additionally, the unit 't' (10³ kg) is called 'metric ton' in US English and 'tonne' in International English.

What is the distinction between the definition of an SI unit and its 'realisation'?

The definition of an SI unit establishes its fundamental basis by assigning exact numerical values to defining constants. Its 'realisation' is the practical procedure by which this definition is used to determine the value and associated uncertainty of a quantity of the same type as the unit.

What are 'mises en pratique' in the context of SI units?

The BIPM publishes 'mises en pratique' (French for 'putting into practice' or 'implementation') for each base unit. These documents describe the current best practical methods for realizing the unit, allowing for improvements in measurement techniques without altering the unit's fundamental definition.

What is the International System of Quantities (ISQ), and how does it relate to SI units?

The International System of Quantities (ISQ) refers to the quantities and equations that provide the context for defining SI units. It is based on the quantities underlying the seven SI base units, with other quantities derived from them through clear equations. The ISQ defines the physical quantities that are measured using SI units.

Which three international organizations regulate and develop the SI system?

The SI system is regulated and continuously developed by three international organizations established under the Metre Convention of 1875: the General Conference on Weights and Measures (CGPM), the International Committee for Weights and Measures (CIPM), and the International Bureau of Weights and Measures (BIPM).

What is the 'SI Brochure', and who publishes it?

The 'SI Brochure' is a document that collects all decisions and recommendations concerning SI units. It is published in French and English by the BIPM and is periodically updated, with its writing and maintenance handled by one of the CIPM's committees.

What was the CGS system, and what principle did it formalize?

The CGS (centimetre-gram-second) system of units was developed in 1874 by scientists like James Clerk Maxwell and William Thomson. It formalized the concept of a 'coherent' system of units, where base units combine to define derived units without requiring extra factors.

What problem did Giovanni Giorgi address regarding electrical units at the turn of the 20th century?

At the end of the 19th century, there were three different systems for electrical measurements (CGS-based electrostatic, CGS-based electromagnetic, and an International system). Giovanni Giorgi resolved the difficulty of expressing electrical units in terms of length, mass, and time by advocating for a fourth base unit, such as electric current, voltage, or electrical resistance, alongside the existing three.

Which electrical quantity was chosen as the fourth base unit, leading to the MKSA system?

Electric current, with the named unit 'ampere', was chosen as the fourth base unit. When combined with the MKS system, this new system became known as MKSA and was approved in 1946.

What significant decision was made by the 10th CGPM in 1954 regarding the SI system?

In 1954, the 10th CGPM defined an international system that included six base units: the metre, kilogram, second, ampere, degree Kelvin, and candela, laying the groundwork for the modern SI.

When was the International System of Units (SI) formally adopted, and what was the final base unit added?

The International System of Units, abbreviated SI, was adopted by the 11th CGPM in 1960. The mole was later added as the seventh base unit in 1971.

What was the primary issue with the International Prototype of the Kilogram (IPK) that led to the 2019 redefinition of the SI?

The International Prototype of the Kilogram (IPK) was the only physical artifact defining a base unit, making other units indirectly dependent on it. Over time, a significant divergence in mass was observed between the IPK and its official copies, indicating an inherent instability that undermined the reliability of the entire metric system for precision measurements.

What was the fundamental change in how the SI system is defined after the 2019 redefinition?

Before 2019, the SI was defined through its seven base units. After the 2019 redefinition, the SI is defined by fixing the numerical values of seven defining constants. This means all units, both base and derived, can be constructed directly from these fundamental constants, eliminating reliance on physical artifacts.

Why is the distinction between base units and derived units still maintained in the SI system after the 2019 redefinition?

The distinction between base units and derived units is retained because it is considered useful and historically well-established. Additionally, the ISO/IEC 80000 series of standards, which define the International System of Quantities (ISQ), specifies base and derived quantities that correspond to these SI units.

What are some examples of non-SI units that are accepted for use with the SI system?

The CIPM recognizes and accepts the use of several non-SI units alongside SI units due to their deep embedding in history and culture. Examples include the minute, hour, day (for time), the degree of angle, the litre, the tonne, the dalton, the electronvolt, the neper, and the bel/decibel.

Why are certain unit name variations, such as 'watt-peak' or 'atomic second', considered unacceptable in the SI system?

Such variations are considered unacceptable because the SI mandates that any information concerning a physical quantity or its measurement conditions must be presented separately and not mixed with the unit name itself. The unit should stand alone to specify the quantity without additional descriptive modifiers.

What is the value of one minute in SI units?

One minute (min) is equal to 60 seconds (s) in SI units.

How is one hour expressed in SI units?

One hour (h) is equal to 60 minutes, which converts to 3600 seconds (s) in SI units.

What is the SI equivalent of one day?

One day (d) is equivalent to 24 hours, or 1440 minutes, which translates to 86,400 seconds (s) in SI units.

What is the definition of an astronomical unit (au) in SI units?

One astronomical unit (au) is defined as 149,597,870,700 metres (m).

How is one degree of angle expressed in radians, the SI derived unit for plane angle?

One degree (°) is expressed as (π / 180) radians (rad).

What is the SI equivalent of one hectare?

One hectare (ha) is equivalent to 1 hm², which is 10,000 m² in SI units.

How is the litre defined in terms of SI units?

One litre (L) is defined as 1 dm³, which is equivalent to 1000 cm³ or 0.001 m³ in SI units.

What is the SI equivalent of one tonne?

One tonne (t) is equivalent to 1 megagram (Mg), which is 1000 kilograms (kg) in SI units.

What is the value of one dalton (Da) in kilograms?

One dalton (Da) is approximately 1.66053906892×10⁻²⁷ kg.

International System Of Units Wiki: Defining Reality: An In-Depth Guide to the International System of Units (SI)

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An Introduction to the SI

The Global Standard

The International System of Units, universally known by its French abbreviation SI (Système international d'unités), is the modern iteration of the metric system. It stands as the world's most widely adopted system of measurement, holding official status in nearly every country. Its application is fundamental across science, technology, industry, and daily commerce, providing a coherent and universal framework for measurement.

A Coordinated System

The SI is meticulously coordinated by the International Bureau of Weights and Measures (BIPM). It comprises a coherent set of units beginning with seven base units. From these, an unlimited number of derived units can be formed. To handle quantities of vastly different magnitudes, the system incorporates a set of twenty-four prefixes, allowing for decimal multiples and sub-multiples of the coherent units.

From Artifacts to Ideals

The development of the SI reflects a decades-long progression towards a more abstract and idealized framework. Historically, units were defined by physical artifacts, such as the standard kilogram prototype. The modern SI defines units by fixing the values of fundamental constants of nature. This ensures that as technology advances, the realization of units can improve without altering the fundamental definitions, eliminating the risks associated with physical artifacts being lost, damaged, or changed.

The Seven Defining Constants

The Cornerstone of Modern Metrology

The most fundamental feature of the modern SI is that it is defined by seven defining constants. The magnitudes of all SI units are established by assigning exact numerical values to these constants. This represents a paradigm shift from physical artifacts to unchanging principles of nature. These constants range from fundamental constants like the speed of light to technical constants like luminous efficacy.

The Seven SI Defining Constants
Symbol	Defining Constant	Exact Value
Δν_Cs	Hyperfine transition frequency of ¹³³Cs	9,192,631,770 Hz
c	Speed of light in vacuum	299,792,458 m/s
h	Planck constant	6.62607015 × 10⁻³⁴ J·s
e	Elementary charge	1.602176634 × 10⁻¹⁹ C
k	Boltzmann constant	1.380649 × 10⁻²³ J/K
N_A	Avogadro constant	6.02214076 × 10²³ mol⁻¹
K_cd	Luminous efficacy of 540 THz radiation	683 lm/W

The Seven Base Units

The Foundation of Measurement

The SI framework selects seven units to serve as base units, which correspond to seven fundamental physical quantities. While all SI units can now be derived directly from the defining constants, the concept of base units is retained for its historical significance and practical utility. They provide a preferred set for expressing and analyzing the relationships between all other units in the system.

SI Base Units
Unit Name	Symbol	Quantity Name	Definition Basis
second	s	Time	The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
metre	m	Length	The distance travelled by light in vacuum in 1/299,792,458 of a second.
kilogram	kg	Mass	Defined by setting the Planck constant h to 6.62607015 × 10⁻³⁴ J·s, given the definitions of the metre and the second.
ampere	A	Electric Current	The flow of 1 / (1.602176634 × 10⁻¹⁹) times the elementary charge e per second.
kelvin	K	Thermodynamic Temperature	Defined by setting the Boltzmann constant k to 1.380649 × 10⁻²³ J·K⁻¹, given the definitions of the kilogram, metre, and second.
mole	mol	Amount of Substance	The amount of substance containing exactly 6.02214076 × 10²³ elementary entities (the Avogadro constant, N_A).
candela	cd	Luminous Intensity	The luminous intensity of a source emitting monochromatic radiation of frequency 5.4 × 10¹⁴ hertz with a radiant intensity of 1/683 watt per steradian.

Derived Units

Building on the Foundation

Derived units are formed by combining the base units through multiplication and division. There is no limit to the number of derived units. When a derived unit is formed solely from base units without any numerical factor other than one, it is called a coherent derived unit. For example, the metre per second (m/s) is the coherent derived unit for velocity. This coherence ensures that physical equations maintain their form without extra numerical factors when using SI units.

Special Names and Symbols

For convenience and to honor the work of influential scientists, twenty-two coherent derived units have been given special names and symbols. These include well-known units like the newton (N) for force, the joule (J) for energy, and the watt (W) for power. These special names can be used in combination with base units and other derived units to express further quantities, simplifying complex expressions.

22 SI Derived Units with Special Names
Name	Symbol	Quantity	In SI Base Units
hertz	Hz	frequency	s⁻¹
radian	rad	plane angle	m·m⁻¹ (dimensionless)
steradian	sr	solid angle	m²·m⁻² (dimensionless)
newton	N	force	kg·m·s⁻²
pascal	Pa	pressure, stress	kg·m⁻¹·s⁻²
joule	J	energy, work, heat	kg·m²·s⁻²
watt	W	power, radiant flux	kg·m²·s⁻³
coulomb	C	electric charge	s·A
volt	V	voltage, electric potential	kg·m²·s⁻³·A⁻¹
farad	F	capacitance	kg⁻¹·m⁻²·s⁴·A²
ohm	Ω	electrical resistance	kg·m²·s⁻³·A⁻²
siemens	S	electrical conductance	kg⁻¹·m⁻²·s³·A²
weber	Wb	magnetic flux	kg·m²·s⁻²·A⁻¹
tesla	T	magnetic flux density	kg·s⁻²·A⁻¹
henry	H	inductance	kg·m²·s⁻²·A⁻²
degree Celsius	°C	Celsius temperature	K
lumen	lm	luminous flux	cd·sr
lux	lx	illuminance	cd·sr·m⁻²
becquerel	Bq	radioactivity	s⁻¹
gray	Gy	absorbed dose	m²·s⁻²
sievert	Sv	dose equivalent	m²·s⁻²
katal	kat	catalytic activity	mol·s⁻¹

Scaling the Universe: SI Prefixes

The Power of Ten

A key feature of any metric system is the use of prefixes to create decimal multiples and submultiples of units. The SI provides a standardized set of 24 prefixes that represent powers of ten, from 10³⁰ (quetta) down to 10⁻³⁰ (quecto). This allows for concise and convenient expression of quantities across an immense range of scales, from the astronomical to the subatomic. For example, instead of writing 1,000 metres, we simply write 1 kilometre (km).

SI Prefixes
Name	Symbol	Factor	Name	Symbol	Factor
quetta	Q	10³⁰	deci	d	10⁻¹
ronna	R	10²⁷	centi	c	10⁻²
yotta	Y	10²⁴	milli	m	10⁻³
zetta	Z	10²¹	micro	μ	10⁻⁶
exa	E	10¹⁸	nano	n	10⁻⁹
peta	P	10¹⁵	pico	p	10⁻¹²
tera	T	10¹²	femto	f	10⁻¹⁵
giga	G	10⁹	atto	a	10⁻¹⁸
mega	M	10⁶	zepto	z	10⁻²¹
kilo	k	10³	yocto	y	10⁻²⁴
hecto	h	10²	ronto	r	10⁻²⁷
deca	da	10¹	quecto	q	10⁻³⁰

The Evolution of Measurement

Early Systems & Unification

The concept of a coherent system of units emerged in the 19th century with the centimetre–gram–second (CGS) system. However, inconsistencies arose, particularly with electrical units, leading to multiple competing systems. In 1901, Giovanni Giorgi proposed adding a fourth base unit for an electrical quantity to the metre-kilogram-second (MKS) system. This resolved the anomalies and laid the groundwork for a unified system.

The Birth of the SI

Following the Metre Convention of 1875, international efforts to standardize measurement intensified. In 1948, the 9th General Conference on Weights and Measures (CGPM) commissioned a study for a single, practical system. This led to the 11th CGPM officially adopting the name "International System of Units" (SI) in 1960, initially with six base units. The mole was added as the seventh base unit in 1971.

The 2019 Redefinition

A monumental shift occurred on May 20, 2019. The SI was redefined by fixing the numerical values of seven fundamental physical constants. This change retired the International Prototype of the Kilogram (IPK), the last physical artifact used to define a base unit. By tying all units to constants of nature, the SI became a more robust, stable, and truly universal system, capable of supporting future scientific and technological advancements with unprecedented precision.

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References

Despite the prefix "kilo-", the kilogram is the coherent base unit of mass, and is used in the definitions of derived units. Nonetheless, prefixes for the unit of mass are determined as if the gram were the base unit.

Electric potential difference is also called "voltage" in many countries, as well as "electric tension" or simply "tension" in some countries.

from French: Bureau international des poids et mesures

Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, German Empire, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States, and Venezuela.

A full list of references for this article are available at the International System of Units Wikipedia page

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This page was generated by an Artificial Intelligence and is intended for informational and educational purposes only. The content is based on a snapshot of publicly available data from Wikipedia and may not be entirely accurate, complete, or up-to-date.

This is not a substitute for official documentation. For any scientific, engineering, or commercial application requiring precise measurements, you must consult the official publications from the International Bureau of Weights and Measures (BIPM) or your national metrology institute (such as NIST in the United States). Never disregard official standards or professional advice because of something you have read on this website.

The creators of this page are not responsible for any errors or omissions, or for any actions taken based on the information provided herein.

Defining Reality

💡 Dive in with Flashcard Learning! 💡

An Introduction to the SI 🌐

🏛️ The Global Standard

📜 A Coordinated System

⚖️ From Artifacts to Ideals

The Seven Defining Constants ✨

🔑 The Cornerstone of Modern Metrology

The Seven Base Units 🧱

🏗️ The Foundation of Measurement

Derived Units ⚙️

➕ Building on the Foundation

🏷️ Special Names and Symbols

Scaling the Universe: SI Prefixes 🌌

🔢 The Power of Ten

The Evolution of Measurement 📈

🔬 Early Systems & Unification

🌍 The Birth of the SI

💡 The 2019 Redefinition

Teacher's Corner 🧑‍🏫

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📜 Important Notice

Dive in with Flashcard Learning!

An Introduction to the SI

The Global Standard

A Coordinated System

From Artifacts to Ideals

The Seven Defining Constants

The Cornerstone of Modern Metrology

The Seven Base Units

The Foundation of Measurement

Derived Units

Building on the Foundation

Special Names and Symbols

Scaling the Universe: SI Prefixes

The Power of Ten

The Evolution of Measurement

Early Systems & Unification

The Birth of the SI

The 2019 Redefinition

Teacher's Corner

True or False?

Test Your Knowledge!

Gamer's Corner

Discover other topics to study!

References

Feedback & Support

Disclaimer

Important Notice