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Second Law of Thermodynamics.

9.1

Second Law of Thermodynamics

Carnot cycle efficiency

-Heat engines

- Refrigerators

Second Law of Thermodynamics

Heat engines (Kelvin-Planck Law)

Refrigerators (Claussius Statement)

Entropy Principle

Three Statements of the second law

Heat Engines

A heat engine takes in heat

at a high temperature and

exhausts heat at a low

temperature.

In the process of heat flow

some of the input heat is

converted to work

First Law (for a cycle)

Q = Q

+ Q

= W

Second Law ( puts limits on Q

and Q

)

Kelvin-Planck Statement of

the second law of thermodynamics

Perfect heat engine

It is impossible to construct a heat engine operating in a cycle that

extracts heat from a reservoir and delivers an equal amount of work.

No perfect heat engine

Efficiency of a heat engine

The efficiency is the

fraction of the heat input

at high temperature converted

to work.

−

In calculating efficiencies Q

and Q

are taken as positive quantities.

(i.e. the magnitude of the heat)

=−

The second law says that a heat engine cannot be 100% efficient.

Carnot Cycle

Sadi Carnot, a French engineer (1796-1832) proposed a

cycle set the limits to the efficiency of a heat engine operating

between two temperatures.

The cycle consists of 4 reversible steps.

1. Isothermal expansion at T

2. Adiabatic expansion from T

to T

3. Isothermal compression at T

4. Adiabatic compression from T

to T

1 2 3 4

Resumo do Conteúdo

Página 1 - Carnot Cycle

1Second Law of Thermodynamics. 9.1Second Law of ThermodynamicsCarnot cycle efficiency-Heat engines- RefrigeratorsSecond Law of ThermodynamicsHeat engi

Página 2 - Stirling Engine

2Heat engine.Heat engine undergoing the Carnot cycle.Work is done by the gas in expansionWork must be done on the gas to compress it to the initial st

Página 3 - Real Heat Engines

3Refrigerator and heat pumpA heat engine run in reverse is a refrigerator and heat pump.Work is done to move heat from a cold temperature source to a

Página 4 - 5.4 1.62 10 8.7 10== =

4How to improve the efficiencyof a heat engine.Increase ThThis requires high temperature materialsDecrease TcThis requires efficient heat transfer..Co

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