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769311-6-24CQ AID: 419796 | 28/05/2021
We need to define absolute thermodynamic temperature scale using kelvin plank
statement.
We know,
According to kelvin plank statement, it is necessary to have a temperature difference to
extract work from any cyclic device or heat engine.
We also know that the efficiency of all reversible heat engines operating between same
temperature levels is the same, and it is independent of working substance.
Therefore, for a reversible cycle, the efficiency will only depend upon the temperatures at
which heat is transferred.
Let efficiency of heat engine = η
rev
( )
rev 1 2
f T ,Th =
Where T
1
= temperature of source
T
2
= temperature of sink.
f is the function of temperatures at which heat engine is running.
We also know
( )
1
rev 1 2
2
Q
f T ,T 1
Q
h

= =


Where Q
1
= heat supplied from the source
Q
2
= the heat rejected to the sink.
Hence we can say that
( )
1
12
2
Q
F T ,T
Q

=


Choosing a function of temperatures is completely arbitrary but kelvin proposed the
simplest possible way to define this arbitrary function.
And in this way kelvin defined the absolute thermodynamic temperature T.
According to kelvin
( )
1
12
2
T
F T ,T
T
=
Absolute thermodynamic temperature scale is also known as Kelvin scale. The kelvin
temperature scale is independent of any specific characteristics of substance.
Now we know that every temperature scale has a fixed reference point. In defining the
absolute thermodynamic temperature scale, kelvin took the triple point of water as a
slandered reference point.
The arbitrary assigned value for triple point temperature of water (T
t
) =273.16 kelvin
So from equation 1, we can write
tt
TQ
TQ
=
t
Q
T 273.16
Q
=
Where T is the absolute thermodynamic temperature.
Some extra points to note:
Q plays the role of thermometric property.
It is impossible to get an absolute 0 temperature because for that Q must be 0 and that’s
impossible according to 2
nd
law of thermodynamics.

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769311-6-24CQ AID: 419796 | 28/05/2021 We need to define absolute thermodynamic temperature scale using kelvin plank statement. We know, According to kelvin plank statement, it is necessary to have a temperature difference to extract work from any cyclic device or heat engine. We also know that t ...
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