- Focus on Physics

Units & Measurement

SI Units
Name Symbol Quantity Definition
metre m distance The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
kilogram kg mass The kilogram is equal to the mass of the international prototype of the kilogram.
second s time The second is 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.
ampere A electric current The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length.
kelvin K temperature The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
mole mol amount of substance The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
candela cd intensity of light The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Derived units with special names
Name Symbol Based on SI units Derived quantity
radian rad angle
hertz Hz frequency
newton N force
pascal Pa N/m2 pressure
joule J N·m energy, work, heat
watt W J/s power,
coulomb C electric charge, quantity of electricity
volt V W/A potential difference,

electromotive force

farad F C/V capacitance
ohm Omega V/A resistance
siemens S A/V conductance
weber Wb V·s magnetic flux
tesla T Wb/m2 magnetic flux density
henry H Wb/A inductance
degree Celsius °C Celsius temperature
becquerel Bq activity (radioactive)

Derived units with no special names
Derived quantity Name Symbol
acceleration meter per second squared m/s2
area square meter m2
angular velocity radian per second rad/s
angular acceleration radian per second squared rad/s2
density kilogram per cubic meter kg/m3
electric field strength volt per meter V/m
heat capacity joule per kelvin J/K
moment of force newton meter N·m
permittivity farad per meter F/m
permeability henry per meter H/m
specific heat capacity joule per kilogram kelvin J/(kg·K)
speed meter per second m/s
thermal conductivity watt per meter kelvin W/(m·K)
velocity meter per second m/s
volume cubic meter m3
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