Impact on base unit definitions Proposed redefinition of SI base units

1 impact on base unit definitions

1.1 second
1.2 metre
1.3 kilogram
1.4 ampere
1.5 kelvin
1.6 mole
1.7 candela

impact on base unit definitions

the ccu proposal recommended text of definitions of base units either refined or rewritten changing emphasis explicit-unit explicit-constant type definitions. explicit-unit type definitions define unit in terms of specific example of unit—for example in 1324 edward ii defined inch being length of 3 barleycorns , since 1889 kilogram has been defined being mass of international prototype kilogram. in explicit-constant definitions, constant of nature given specified value , definition of unit emerges consequence. example, in 1983, speed of light defined 7008299792458000000♠299792458 metres per second and, since second had been independently defined, length of metre derived.

the current (2008) , proposed (2016) definitions given below.

second

the proposed definition of second same current definition, difference being conditions under measurements made more rigorously defined.

current definition: second duration of 7009919263177000000♠9192631770 periods of radiation corresponding transition between 2 hyperfine levels of ground state of caesium-133 atom.
proposed definition: second, symbol s, si unit of time. defined taking fixed numerical value of caesium frequency Δνcs, unperturbed ground-state hyperfine transition frequency of caesium 133 atom, 7009919263177000000♠9192631770 when expressed in unit hz, equal s.

metre

the proposed definition of metre same current definition, difference being additional rigour in definition of second propagate metre.

current definition: metre length of path travelled light in vacuum during time interval of 1/7008299792458000000♠299792458 of second.


proposed definition: metre, symbol m, si unit of length. defined taking fixed numerical value of speed of light in vacuum c 7008299792458000000♠299792458 when expressed in unit m⋅s, second defined in terms of caesium frequency Δνcs.

kilogram

a watt balance being used measure planck constant in terms of international prototype kilogram.

the definition of kilogram undergoing fundamental change—the current definition defines kilogram being mass of international prototype kilogram artefact, not constant of nature while new definition relates equivalent energy of photon via planck constant.

current definition: kilogram unit of mass; equal mass of international prototype of kilogram.


proposed definition: kilogram, symbol kg, si unit of mass. defined taking fixed numerical value of planck constant h 6966662607015000000♠6.62607015×10 when expressed in unit j⋅s, equal kg⋅m⋅s, metre , second defined in terms of c , Δνcs.

one consequence of change new definition makes definition of kilogram dependent on definitions of second , metre.

ampere

the definition of ampere undergoing major overhaul—the current definition, difficult realise high precision in practice, being replaced definition more intuitive , easier realise.

current definition: ampere constant current which, if maintained in 2 straight parallel conductors of infinite length, of negligible circular cross-section, , placed 1 m apart in vacuum, produce between these conductors force equal 6993200000000000000♠2×10 newton per metre of length.
proposed definition: ampere, symbol a, si unit of electric current. defined taking fixed numerical value of elementary charge e 6981160217663400000♠1.602176634×10 when expressed in unit c, equal a s, second defined in terms of Δνcs.

since current definition contains reference force has dimensions mlt follows in si kilogram, metre , second, base units representing these dimensions, must defined before ampere can defined. other consequences of current definition in si value of vacuum permeability (μ0) fixed @ 6993400000000000000♠4π×10 h⋅m. since speed of light in vacuum (c) fixed, follows relationship

c

2


=


1


μ

0



ε

0







{\displaystyle c^{2}={\frac {1}{\mu _{0}\varepsilon _{0}}}}

that vacuum permittivity (ε0) has fixed value, , from

z

0


=




μ

0



ε

0







{\displaystyle z_{0}={\sqrt {\frac {\mu _{0}}{\varepsilon _{0}}}}}

that impedance of free space (z0) likewise has fixed value.

one consequence of proposed changes definition of ampere definition no longer dependent on definitions of kilogram , metre, still dependent on definition of second. in addition vacuum permeability, vacuum permittivity , impedance of free space, which, in current definition have exact values will, in future, subject experimental error.

kelvin

the definition of kelvin undergo fundamental change if proposal accepted. rather using triple point of water fix temperature scale, proposal recommends energy equivalent given boltzmann s equation used.

current definition: kelvin, unit of thermodynamic temperature, 1/273.16 of thermodynamic temperature of triple point of water.
proposed definition: kelvin, symbol k, si unit of thermodynamic temperature. defined taking fixed numerical value of boltzmann constant k 6977138064900000000♠1.380649×10 when expressed in unit j⋅k, equal kg⋅m⋅s⋅k, kilogram, metre , second defined in terms of h, c , Δνcs.

one consequence of change new definition makes definition of kelvin depend on definitions of second, metre, , kilogram.

mole

a near-perfect sphere of ultra-pure silicon – part of avogadro project, international avogadro coordination project determine avogadro s number

the current definition of mole links kilogram. proposed definition break link making mole specific number of entities of substance in question.

current definition: mole amount of substance of system contains many elementary entities there atoms in 0.012 kilogram of carbon-12. when mole used, elementary entities must specified , may atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
proposed definition: mole, symbol mol, si unit of amount of substance of specified elementary entity, may atom, molecule, ion, electron, other particle or specified group of such particles. defined taking fixed numerical value of avogadro constant na 7023602214076000000♠6.02214076×10 when expressed in unit mol.

one consequence of change current defined relationship between mass of c atom, dalton, kilogram, , avogadro s number no longer valid. 1 of following must change:

the mass of c atom 12 dalton
the number of dalton in gram numerical value of avogadro constant

the draft si brochure assumes first remain true, mean second no longer true. molar mass constant, while still great accuracy remaining equal 1 g/mol, no longer equal that.

candela

the proposed definition of candela same current definition, has been rephrased difference being additional rigour in definition of second , metre propagate candela.

current definition: candela luminous intensity, in given direction, of source emits monochromatic radiation of frequency 7014540000000000000♠540×10 hz , has radiant intensity in direction of 1/683 watt per steradian.
proposed definition: candela, symbol cd, si unit of luminous intensity in given direction. defined taking fixed numerical value of luminous efficacy of monochromatic radiation of frequency 7014540000000000000♠540×10 hz, kcd, 683 when expressed in unit lm⋅w, equal cd⋅sr⋅w, or cd⋅sr⋅kg⋅m⋅s, kilogram, metre , second defined in terms of h, c , Δνcs.




^ mills, ian (september–october 2011). part ii—explicit-constant definitions kilogram , mole . chemistry international. 33 (5): 12–15. issn 0193-6484. 
^ travenor, robert (2007). smoot s ear – measure of humanity. yale university press. pp. 35–36. isbn 978-0-300-14334-8. 
^ cite error: named reference basedefs invoked never defined (see page).
^ cite error: named reference brochure9_2016 invoked never defined (see page).
^ bipm watt balance . international bureau of weights , measures. 2012. retrieved 2013-03-28. 
^ taylor, barry n (november–december 2011). current si seen perspective of proposed new si . journal of research of national institute of standards , technology. national institute of standards , technology (nist). 116 (6): 797–80. doi:10.6028/jres.116.022. 
^ unit of electric current (ampere) . historical context of si. nist. retrieved 2015-09-07. 
^ orfanidis, sophocles j. (31 august 2010). electromagnetic waves , antennas (pdf). ece department, rutgers university. 1.3 constitutive relations. retrieved 2013-06-24. 
^ cite error: named reference chyla invoked never defined (see page).

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