Le, Grand K, a cylinder comprising of platinum-iridium, placed in an underground vault in Paris, will no longer be the standard that the scientists will measure their one kg with!
On Friday, at the Palace of Versailles, scientists from all around the world voted to what is a major change in the world of Standard Units(Système international). Along with kilogram, the units which will undergo change are the ampere(unit of current), the Kelvin(unit of temperature) and the mole(a measure of a chemical substance).
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The International System of Units(SI) is a set of seven units which can be used to measure all the other SI units. With now all the seven definitions being dependent on scientific constants, a huge leap has taken place in the history of mankind. The new definitions will come in force on 20 May 2019.
Here, we list down current definitions of the “seven units”:
Second:
Second is defined as the time it takes a caesium-133 atom to radiate for 9,192,631,770 periods when a transition takes place between its two hyperfine levels. Second is the SI unit of time.
Metre:
The SI unit of distance, the metre is defined as the length of the path travelled by light vacuum in 1/299,792,458 seconds.
Kilogram:
Say bye-bye to Grand K, Kg has been redefined. The kilogram like the other units now depends solely on a constant. The dimensions of Planck’s constant include kilogram and that is exactly what has been used. Consequently, it now depends on metre and second as a little setback.
When expressed in the unit Joule-second, Planck constant h is 6.62607015×10−34 Joule-second, which equals kg.m−2.s−1 , here metre and seconds have the above definitions, defining kg as a result.
Ampere:
Ampere’s definition has undergone major changes too. It is now defined in a similar way to kilogram. The charge of an elementary charge, e, has been taken as 1.602176634×10−19 C.
Coulomb has the dimensions of ampere.second(A.s), defining ampere in the way, taking second’s definition from above.
Kelvin:
Earlier, defined with the help of triple point(pressure and temperature at which all the three states are simultaneously present) of water, Kelvin’s definition has undergone a complete make-over.
Now it is defined with physical constant, Boltzmann constant. A similar procedure has been followed, only this time, three units required their usage, kilogram, seconds and metres.
Candela:
A source that emits monochromatic radiation of frequency 540 x 1012, having a radiant intensity of 1/683 watt per steradian is defined as one Candela. Candela is the base unit of luminous intensity.
Mole:
Earlier, the unit which was defined with the weight of carbon-12 now uses the most popular of the constants, the Avogadro constant(NA). A mole is the fixed numerical value of constant, NA, when expressed in the unit mol-1.
