What are SI units?
- SI stands for the ‘Système lnternational (SI) D’Unités’, which translates from the French as the ‘International System of Units’
- There are three types of SI units:
- Base SI units
- Based on metric units with the decimal system
- Definitions are based upon “constants” of physical sciences
- Definitions not static and undergo frequent evolution – the latest change was in 2019
- The need to create highly accurate and precise definitions has led to very complex definitions
- Supplementary SI units:
- Used along with base units to form the derived SI units
- Contains only two, purely geometric units, the unit of plane angle (the radian) and the unit of solid angle (the steradian)
- Derived SI units:
- Obtained through equations containing the seven base units and supplementary units
- Base SI units
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)