What are SI units?
- SI stands for the ‘Système lnternational (SI) D’Unités’, which translates from the French as the ‘International System of Units’
- Represent a set of fundamental base units, derived units and prefixes used throughout the world:
- Based on metric units with the decimal system
- Definitions are not static and undergo frequent evolution
- The latest change in 2019 saw all definitions become based upon “constants” of physical sciences
- The need to create highly accurate and precise definitions has led to very complex definitions
What are the base SI units and how are they defined?
Unit
Unit Symbol
Base Measure
Physical Constant
Definition
Second
s
Time
Hyperfine transition frequency of caesium (ΔνCs)
The time taken by 9,192,631,770 oscillations of the light emitted by a caesium-133 atom
Metre
m
Length
Speed of light (c)
The length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second
Mole
mol
Amount of Substance
Avagadro's constant - a value of 6.02214076 × 1023
The amount of substance that contains the same number of particles as there are atoms in 0.012 kg (12 g) of carbon-12, which isequal to Avagadro's dumber
Ampere
A
Electric Current
Elementary charge (e) - the charge of a single proton
Defined by taking the elementary charge e to be a fixed numerical value of 1.602176634×10 A⋅s
Candela
cd
Luminous Intensity
Luminous efficacy of monochromatic radiation of frequency 540×1012 Hz (Kcd)
Defined by taking the fixed numerical value of Kcd, to be 683 expressed in the units cd⋅sr⋅kg–1⋅m–2⋅s3
Kilogram
kg
Mass
Plancks Constant (h) - relationship between a photons energy and frequency
Defined by taking Plancks constant (h) to be a fixed numerical value of 6.62607015×10−34 m2⋅kg⋅s–1
Kelvin
K
Temperature
Boltzmann's Constant (k) - relationship between a particles kinetic energy and temperature
Defined by taking the Boltzmann constant k to be a fixed numerical value of 1.380649×10−23 kg⋅m2⋅s–2⋅K–1)