How to Understand the Changes in the New SI Units of Measurement?

A new redefined system of SI unit
of measurements have come into force on World Metrology Day, May 20. This was
based on a resolution adopted by 60 nations on November 16.2018 in Versailles,
France. The new accuracy in measurements will be a boon for scientists and
those involved in world trade, but for the common man buying rice, bananas and
vegetables, the new system makes no difference although it may sound too
scientific and behind comprehension. Here is an attempt to make it simple for
anyone.
Why was this redefinition needed
and what is the significance of it for the common man?
Until now, the unit of weight,
kilogramme was defined by the weight of a metal cylinder kept in a vault in
Paris. The cylinder made of platinum-iridium alloy is known by the name the
International Prototype of Kilogram (IPK) or Le Grand K. The problem with this
standard was that the material could always loose or gain mass due to external
factors. Comparison of copies of the cylinder sent across the world also
revealed the changing mass.
Therefore, scientists have now devised units of measurements
that are based on fundamental constants or quantum standards.
The
redefined kilogram
The kilogram has been redefined
using a fundamental constant called the Planck's constant. The new kilogram is
an electrical or electronic kilogram as it is measured on a kibble balance. It
measures an object very accurately calculating the electric current and voltage
needed to produce a compensating force. Using a Planck's constant, the three units
of measurement, meter, kilogram and second are measured using the speed of
light.
The value of kilogram doesn't
change under the new system. However, the redefinition improves its reliability
and enables far more accurate mass measurements.
Kelvin
for Temperature
Kelvin is the SI unit of
temperature. The redefined Kelvin is not dependent on the triple point of
water. Triple point refers to the temperature and pressure in which a material
can coexist in three states of matter, namely, solid, liquid and gas.
Temperature of any substance is a measure of how hot or cold it is with respect
to the triple point of water (273.16 k). The redefined Kelvin is based on the
Boltzmann constant which is a fundamental constant. It is denoted in kb and is
used to estimate the relation between temperature and energy. We owe this
development to Ludwig Boltzmann (1844-1906) who used statistical mechanics to
find out how much energy is expended when an object is raised to a certain
temperature. Although the molecules will be moving at different speeds, using
statistical mechanics it was possible to estimate the varying speeds, he
thought. Using Boltzmann constant expressed in joules per kelvin. it is now
possible to find out how much kinetic energy measured in joules (E) is related
to temperature (T). The corresponding equation is E=kbT. One joule is the
energy used by 100 watt light bulb in 0.01 seconds. Now there are accurate
methods to measure Boltzmann constant using acoustic thermometry. It has been
revised to SI as 1.380649 x 10 raised to-23 JK raised to -1.
Redefinition
of mole
The mole is the unit of amount of
substance. Present definition of a mole is the amount of substance that
contains as many elementary entities such as atoms, molecules, ions etc as
there are in 12 grams of carbon 12(6.023 x1023). This is based on the
definition adopted in 1971 which is dependent on the mass of the kilogram. Now
this looks out of date when we have a more accurate Avagadro number. It is was formulated by a scientist Amedeo
Avogadro. It is based on the proportion of constituent particles rather than
mass of a sample. The Avogadro constant is 6.02214076 x 10 raised to 23 which
is now considered as one mole.
The new measurement will be
useful to measure chemical composition of atmosphere accurately and with lower
uncertainty thereby enabling government to formulate appropriate pollution
control policies.
Redefinition
of ampere
The ampere is used to measure of
electric current and we frequently use in daily life. Or in simple terms the
measure of amount of electric charge in motion per unit time, For eg. we may be
using a 5 amps socket for regular use devices and 15 amps for heavy duty
electrical devices. The definition of ampere may look quite complex and mind
boggling to the layman.
The ampere is that constant
current which, if maintained in two straight parallel conductors of infinite
length, of negligible circular cross-section, and placed meter apart in vacuum, would produce between
these conductors a force equal to 2 x 10-7 newton per meter of length. In
reality where do you get these long wires and vacuum chambers? Also the
electrical quantity is defined in mechanical terms.
The new ampere is based on
elementary electric charge (e) or in other words the amount of electric charge
in a single electron (negative) or proton (positive). The electric charge will
be denoted by coulomb, another SI unit that is equal to 6.241 x 10 raised to 18
electric charges. So what does one ampere of current mean? It is equal to one
coulomb of charge that travels across a point in a second.
By quantifying the ampere in
terms of the coulomb, the new definition directly ties the ampere to an exactly
fixed constant of nature — the elementary electric charge. The new definition
for ampere - It is defined by taking the
fixed numerical value of the elementary charge e to be 1.602176634 × 10-19 when
expressed in the unit C, which is equal to A-s.
Photo Courtesy: Steve Buissinne, Pixabay.com
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