Second Law of Thermodynamics

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The two demerits for first law of thermodynamics are;

a)      First Law doesn’t tells about how much heat is converted into work.

b)      It also doesn’t tell about the direction of heat transfer.

Because of this reason, we are forced for the second law which shows direction and amount of heat.  There are w statements in second law.

Kelvin Planck statement

“It is impossible for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at a single fixed temperature.”

The term efficiency is arising from the Kelvin Planck statement.  Efficiency is the ratio of the output energy to input energy.

Consider a heat engine works between 2 heat energy Q₁ and Q₂ and does a work output W_net 

      Where W_net     =          Q₁-Q₂

      Q₁                    =          Input heat

      Q₂                    =          Heat lost to sink

      Then efficiency=         Output

                                          Input

                              =          Wnet

                                             Q₁

                              =          Q₁ – Q₂

                                               Q₁

                              =          1 – Q₂

                                                Q₁

All heat engine will have a heat transfer Q₂ with the sink. In most of the case, the sink being the atmosphere.

If an engine which violates a Kelvin Planck statement, then Q₂ for that engine is zero.

Therefore     Efficiency    =    100%

Such a heat engine with 100% efficiency is called Perpetual  Motion Machine of the second kind or PMM2.

Claussius statement

“It is impossible to construct a device which operating in a cycle will produce no effect other than the transfer of heat from a cooler to a hotter body”.

This law gives us the direction of heat transfer that is from hot bodies to cold bodies.

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