Guidelines for Examination

SI units and their decimal multiples and submultiples

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.

1.1.1

Special name and symbol of the SI derived unit of temperature for expressing Celsius temperature

Quantity	Unit
	Name	Symbol
Celsius temperature	degree Celsius	C

Celsius temperature t is defined as the difference t = T-T_o between the two thermodynamic temperatures T and T_o where T_o = 273.15 K. An interval of or difference in temperature may be expressed either in kelvins or in degrees Celsius. The unit of 'degree Celsius' is equal to the unit 'kelvin'.

1.2

SI derived units

1.2.1

Deleted

1.2.2

General rule for SI derived units

Units derived coherently from SI base units are given as algebraic expressions in the form of products of powers of the SI base units with a numerical factor equal to 1.

1.2.3

SI derived units with special names and symbols

Quantity	Unit		Expression
	Name	Symbol	In other SI units	In terms of SI base units
Plane angle	radian	rad		m.m^-1
Solid angle	steradian	sr		m².m^-2
Frequency	hertz	Hz		s^-1
Force	newton	N		m.kg.s^-2
Pressure, stress	pascal	Pa	N.m^-2	m^-1.kg.s^-2
Energy, work; quantity of heat	joule	J	N.m	m².kg.s^-2
Power⁽¹⁾, radiant flux	watt	W	J.s^-1	m².kg.s^-3
Quantity of electricity, electric charge	coulomb	C		s.A
Electric potential, potential difference, electromotive force	volt	V	W.A^-1	m².kg.s^-3.A^-1
Electric resistance	ohm	Ω	V.A^-1	m².kg.s^-3.A^-2
Conductance	siemens	S	A.V^-1	m^-2.kg^-1.s³.A²
Capacitance	farad	F	C.V^-1	m^-2.kg^-1.s⁴.A²
Magnetic flux	weber	Wb	V.s	m².kg.s^-2.A^-1
Magnetic flux density	tesla	T	Wb.m^-2	kg.s^-2.A^-1
Inductance	henry	H	Wb.A^-1	m².kg.s^-2.A^-2
Luminous flux	lumen	lm	cd.sr	cd
Illuminance	lux	lx	lm.m^-2	m^-2.cd
Activity (of a radionuclide)	becquerel	Bq		s^-1
Absorbed dose, specific energy imparted, kerma, absorbed dose index	gray	Gy	J.kg^-1	m².s^-2
Dose equivalent	sievert	Sv	J.kg^-1	m².s^-2
Catalytic activity	katal	kat		mol.s^-1

⁽¹⁾

Special names for the unit of power: the name volt-ampere (symbol 'VA') is used to express the apparent power of alternating electric current, and var (symbol 'var') is used to express reactive electric power.

Units derived from SI base units may be expressed in terms of the units listed in this annex.

In particular, derived SI units may be expressed by the special names and symbols given in the above table. For example, the SI unit of dynamic viscosity may be expressed as m^-1.kg.s^-1 or N.s.m^-2 or Pa.s.

1.3

Prefixes and their symbols used to designate certain decimal multiples and submultiples

Factor	Prefix	Symbol	Factor	Prefix	Symbol
10²⁴	yotta	Y	10^-1	deci	d
10²¹	zetta	Z	10^-2	centi	c
10¹⁸	exa	E	10^-3	milli	m
10¹⁵	peta	P	10^-6	micro	μ
10¹²	tera	T	10^-9	nano	n
10⁹	giga	G	10^-12	pico	p
10⁶	mega	M	10^-15	femto	f
10³	kilo	k	10^-18	atto	a
10²	hecto	h	10^-21	zepto	z
10¹	deca	da	10^-24	yocto	y

The names and symbols of the decimal multiples and submultiples of the unit of mass are formed by attaching prefixes to the word 'gram' and their symbols to the symbol 'g'.

Where a derived unit is expressed as a fraction, its decimal multiples and submultiples may be designated by attaching a prefix to units in the numerator or the denominator, or in both these parts.

Compound prefixes, that is to say prefixes formed by the juxtaposition of several of the above prefixes, may not be used.

1.4

Special authorised names and symbols of decimal multiples and submultiples of SI units

Quantity	Unit
	Name	Symbol	Value
Volume	litre	l or L⁽¹⁾	1 l = 1 dm³ = 10^-3 m³
Mass	tonne	t	1 t = 1 Mg = 10³ kg
Pressure, stress	bar	bar	1 bar = 10⁵ Pa

⁽¹⁾

The two symbols 'l' and 'L' may be used for the litre unit.

The prefixes and their symbols listed in F-II, Annex 2, 1.3 may be used in conjunction with the units and symbols contained in this table.

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