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By the end of this section, you will be able to:
  • Contrast the second law of thermodynamics statements according to Kelvin and Clausius formulations
  • Interpret the second of thermodynamics via irreversibility

Earlier in this chapter, we introduced the Clausius statement of the second law of thermodynamics, which is based on the irreversibility of spontaneous heat flow. As we remarked then, the second law of thermodynamics can be stated in several different ways, and all of them can be shown to imply the others. In terms of heat engines, the second law of thermodynamics may be stated as follows:

Second law of thermodynamics (kelvin statement)

It is impossible to convert the heat from a single source into work without any other effect.

This is known as the Kelvin statement of the second law of thermodynamics    . This statement describes an unattainable “ perfect engine    ,” as represented schematically in [link] (a). Note that “without any other effect” is a very strong restriction. For example, an engine can absorb heat and turn it all into work, but not if it completes a cycle . Without completing a cycle, the substance in the engine is not in its original state and therefore an “other effect” has occurred. Another example is a chamber of gas that can absorb heat from a heat reservoir and do work isothermally against a piston as it expands. However, if the gas were returned to its initial state (that is, made to complete a cycle), it would have to be compressed and heat would have to be extracted from it.

The Kelvin statement is a manifestation of a well-known engineering problem. Despite advancing technology, we are not able to build a heat engine that is 100 % efficient. The first law does not exclude the possibility of constructing a perfect engine, but the second law forbids it.

Part a shows schematic of a perfect heat engine with a downward arrow Q at T subscript h and a right arrow W where Q equals W. Part b shows schematic of a perfect refrigerator with an upward arrow Q at T subscript c and an upward arrow Q at T subscript h.
(a) A “perfect heat engine” converts all input heat into work. (b) A “perfect refrigerator” transports heat from a cold reservoir to a hot reservoir without work input. Neither of these devices is achievable in reality.

We can show that the Kelvin statement is equivalent to the Clausius statement if we view the two objects in the Clausius statement as a cold reservoir and a hot reservoir. Thus, the Clausius statement becomes: It is impossible to construct a refrigerator that transfers heat from a cold reservoir to a hot reservoir without aid from an external source . The Clausius statement is related to the everyday observation that heat never flows spontaneously from a cold object to a hot object. Heat transfer in the direction of increasing temperature always requires some energy input . A “ perfect refrigerator ,” shown in [link] (b), which works without such external aid, is impossible to construct.

To prove the equivalence of the Kelvin and Clausius statements, we show that if one statement is false, it necessarily follows that the other statement is also false. Let us first assume that the Clausius statement is false, so that the perfect refrigerator of [link] (b) does exist. The refrigerator removes heat Q from a cold reservoir at a temperature T c and transfers all of it to a hot reservoir at a temperature T h . Now consider a real heat engine working in the same temperature range. It extracts heat Q + Δ Q from the hot reservoir, does work W , and discards heat Q to the cold reservoir. From the first law, these quantities are related by W = ( Q + Δ Q ) Q = Δ Q .

Questions & Answers

determining dimensional correctness
determine dimensional correctness of,T=2π√L/g
somebody help me answer the question above
calculate the heat flow per square meter through a mineral roll insulation 5cm thick if the temperature on the two surfaces are 30degree Celsius and 20 degree Celsius respectively. thermal conduction of mineral roll is 0.04
akuribire Reply
what are the elementary compositions of a cell?
jackson Reply
when a current pass through a material does the velocity varies
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gurpreet Reply
I'm not sure about it, but I think it's possible. If you add some form of energy to the system, it's a possibility. Also, if you change the pression or the volume of the system, you'll increase the kinetic energy of the system, increasing the gas temperature. I don't know if I'm correct.
For example, if you get a syringe and close the tip(sealing the air inside), and start pumping the plunger, you'll notice that it starts getting hot. Again, I'm not sure if I am correct.
you are right for example an adiabatic process changes all variables without external energy to yield a temperature change. (Search Otto cycle)
when a current pass through a material does the velocity varies
yes at adiabatic compression temperature increase
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Ssempala Reply
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why do we use integration?
Monalisa Reply
To know surfaces below graphs.
To find a Primitive function. Primitive function: a function that is the origin of another
what is laps rate
Г=-dT/dZ that is simply defination
what is z
to find the area under a graph or to accumulate .e.g. sum of momentum over time is no etic energy.
Z is alt.,dZ altv difference
what is the Elasticty
Heeran Reply
it is the property of the by virtue of it regains it's original shape after the removal of applied force (deforming force).
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Practice Key Terms 3

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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