**Why kelvin selected as an international unit of temperature**

On this occasion I will share the physics, why kelvin made international units for temperature. we have seen that the temperature unit that we often use is Celsius. But why scientists chose kelvin, instead Celsius. before explaining even longer. Read the history below.

Thomson was very curious about the nature of the temperature. In particular he was interested in Charles' law, written by French physicist Jacques Charles-Alexandre-CÃ©sar. His law states that when the gas is cooled from 0˚ Celsius, reduced volume 1/273 for each degree drop. Although this law has been proved and proved again, it served disturbing implications: if the temperature is reduced to -273 ° C, the volume of gas will be reduced to 0 (loss 273/273). No scientist can explain how a problem that does not take volume - that is, until Thomson.

Thomson suggested in 1848 that it is gas' energy of motion, not its volume, which decreased by falling temperatures. At 273 ° C, the energy level will reach zero; molecule will stop moving, and it will effectively take no space. Since there is no further reduction in temperature can occur, Thomson labeled -273 ° C as absolute zero.

The concept of absolute zero are very useful in the creation and verification of many theories of thermodynamics. Thomson quickly introduced a new temperature scale, called the absolute scale, with the lowest point of this final as zero. Basically, the return to work of the scale with all reduced with the temperature 273 degrees; on this scale, now known as the Kelvin scale, the freezing point of water stands at 273 K, the boiling point at 373 K. Thomson 's scale will eventually become an instrumental tool's James Clerk Maxwell developed the kinetic theory of gases. Thomson in 1851 continues to enhance its reputation as Britain's dominant thermophysicist introducing the idea that all energy flows down, disappearing into the environment as heat. The concept was explored by the degradation of Rudolf Clausius and re-introduced in a clearer and more explicit form as the Second Law of Thermodynamics, better known as entropy.

**A. kelvin scale can measure temperatures below 0 degrees.**

Kelvin scale when compared with the scale of Celsius and Fahrenheit can measure temperatures below 0 degrees. if known P1 is the pressure of the gas to the temperature of the ice point (0˚ C) and P2 is the pressure of the gas to the temperature of the steam point (100 ° C) then a slash drawn to the left until it intersects the axis T ° C (temperature), we will find that when the pressure of gas = 0, then the large temperature = -273.15 ° C

Absolute zero or the absolute zero is the temperature at which the molecules are not moving (relative to other molecules as a whole). Located at low temperatures has some consequences of thermodynamics, for example, at zero absulot all molecular motion does not stop but did not have enough energy to move to another system. Therefore, it can be said that at a temperature of 0 kelvin molecular energy is worth at least. By an international agreement, absolute zero is defined as a temperature of 0 on the Kelvin scale, and -273.15 ° on the Celsius scale.

**B. Kelvin can determine the condition of the substance to be measured more universal.**

Thermometers besides Kelvin, ie Celsius, Reamur, and Fahrenheit points above and below the point determined by the boiling point and freezing point of water. Yes, for Reamur Celsius and freezing point of water is 0 while at the beginning of Fahrenheit 32. As for the boiling point of water on the Celsius thermometer is 100, on Reamur is 80, and for Fahrenheit 212. So, of the three thermometers are identified the scale of the thermometer while pengkalibrasiannya (reference) remains the same, namely water conditions. Therefore, when applied to a more universal system (eg gas in very low temperatures) as do the calculations physical (eg kinetic energy) will produce a number that is less pretty.

Example: What is the kinetic energy of the hydrogen gas when the temperature is -100 degrees Celsius?

Answer: As we know that the formula for calculating the gas kinetic energy is:

Ek = 3 / 2.k.T

So if we use the Celsius temperature type will produce:

Ek = 3/2 k (-100) = -150.k

When used Fahrenheit:

Ek = 3/2. k. (-148) = -222.k

As for Reamur:

Ek = 3 / 2.k.. (- 80) = -120.k

If we look at the value of the kinetic energy from the temperatures will be negative. Well, the problem is, what is the significance of the value of the negative kinetic energy?

Yes, that's the problem we encounter when using other than the Kelvin temperature. We would be possible to get the value of the kinetic energy is negative. In contrast, if we use the Kelvin temperature. The scale used on the Kelvin temperature is not determined by water conditions but is determined by the condition of molecular motion. At the point Kelvin below, namely 0 K indicates that at that temperature thermodynamically will stop moving. So it can be said that (perhaps) there are no objects that have a temperature below 0 K it. That's why physicists prefer Kelvin compared to other temperatures. A value of 0 in Kelvin is absolute. Therefore, by using Kelvin we would not find the kinetic energy of a substance is negative. Yes, just try to do about the above with Kelvin.

So, using the Kelvin temperature we can determine the condition of the substances which we will measure them more universal. Both in the measurement termiknya and mechanics. As a sign to show that Kelvin is the standard reference for the distinguished way of writing Kelvin compared to other temperatures. In writing it is not necessary to use Kelvin degrees unlike other units that before the letter was preceded by degrees. In addition, the reason for writing is to show other than Kelvin unit is the size scale, while Kelvin is the unit of measure.

In addition to the reasons above, there are also other reasons why Kelvin temperature unit used internationally, namely because there is already agreement that CGPM 13, 1967, with various random as follows:

· Calculations based on a matrix of water and has an accuracy of up to 108 assessed accurately and precisely.

· The use in chemistry and engineering are more likely to ice temperature. The use celcius bother of having to use a minus figure. Kelvin does not need to use minus numbers are certainly simplify the calculation.

source:

http://yuanttiandho.blogspot.co.id/2012/11/kenapa-sih-harus-si-kelvin.html

http://zeladesniaworld.blogspot.co.id/2015/03/alasan-kelvin-dijadikan-satuan-si.html

https://id.answers.yahoo.com/question/index?qid=20090828232029AARubzR

A few of my posts today, may be useful.

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