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)
What are the commonly used derived SI units and how are they defined?
Unit
Unit Symbol
Base Measure
Base Units
Definition
Hertz
Hz
Frequency
s−1
The frequency of one cycle per second
Newton
N
Force
kg.m.s−2
The force required to give 1 kilogram an acceleration of 1 metre per second per second
Pascal
Pa
Pressure
N.m−2
The pressure of 1 Newton per square metre
Joule
J
Energy or Work
N.m
The energy expended when the point of application of a force of 1 Newton moves 1 metre in the direction of the force
Watt
W
Power
J.s−1
1 joule per second
Volt
V
Electrical Potential
kg⋅m2⋅s-3⋅A−1
The difference of electrical potential between two points of a conductor carrying a constant current of 1 ampere, when the power dissipated between these points is 1 watt
Ohm
Ω
Electrical Resistance
kg⋅m2⋅s−3⋅A−2
The resistance when one volt is applied across a conductor and produces a current of 1 ampere
Coulomb
Q
Charge
s⋅A
The quantity of electricity transported in one second by a current of 1 ampere
Farad
F
Capacitance
kg−1⋅m−2⋅s4⋅A2
The capacitance if a potential difference of 1 volt is present across its plates, when a charge of 1 coulomb is held by them.
Henry
H
Inductance
kg⋅m2⋅s−2⋅A−2
The inductance in a circuit when an electric current that is changing at 1 ampere per second results in an electromotive force of 1 volt across the inductor:
Weber
Wb
Magnetic Flux
kg⋅s−2⋅A−1
The magnetic flux that, linking a circuit of one turn, would produce in it an electromotive force of 1 volt if it were reduced to zero at a uniform rate in 1 second
Tesla
T
Magnetic Flux Density
kg⋅m2⋅s−2⋅A−1
The magnetic flux density equal to 1 weber per square metre