﻿ F-II Annex 2, 1.1 SI base units - Guidelines for Examination
1.1
SI base units
 Quantity Unit Name Symbol Length metre m Mass kilogram kg Time second s Electric current ampere A Thermodynamic temperature kelvin K Amount of substance mole mol Luminous intensity candela cd

Definitions of SI base units:

– Unit of length

The metre is the length of the path travelled in a vacuum by light during 1/299792458 seconds.

– Unit of mass

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

– Unit of 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.

– Unit of 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 one metre apart in a vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per metre of length.

– Unit of thermodynamic temperature

The kelvin, unit of thermodynamic temperature, is the fraction 1/273,16 of the thermodynamic temperature of the triple point of water.

This definition relates to water having the isotopic composition defined by the following amount-of-substance ratios: 0,00015576 mole of 2H per mole of 1H, 0,0003799 mole of 17O per mole of 16O and 0,0020052 mole of 18O per mole of 16O.

– Unit of 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 kg 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.

– Unit of luminous intensity

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic rays with a frequency of 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.