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May 29, 2019 Wednesday 11:17:35 PM IST

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

Guest Column

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



Lolex Damian

Lolex Damian has done MSc Physics with specialisation in Electronics and MPhil in Physics from Cochin University of Science and Technology. She has 14 years of experience in teaching. She encourages students to reflect on their own learning and to pursue imaginative activities and questions. She believes in teaching based on demonstrations and activities. She is currently a Physics teacher at Rajagiri Public School, Kalamassery.

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