Chapter 20: The Second Law Of Thermodynamics Do Processes . - Uwyo.edu

Chapter 20: The Second Law of Thermodynamics Do processes have preferential processtendency/direction? What are “reversible” and “irreversible”processes? How do we compute the disorder change? How do we use a heat engine or a refrigerator?

Do processes have preferential processtendency/direction?The First Law of Thermodynamics 𝑈 𝑄 𝑊For a processpp121p12p1V1V2V1VT 25 CT 5 CV2V

What are “reversible” and “irreversible”processes?Metal box at 70 CIce at 0 CMetal box at 0 CIce at 0 CMetal box at 40 Cwater at 40 CMetal box at 0 Cwater at 0 CReversible process: the universe unchanged after the restoration of the process

Free expansion vs isothermal expansionFree expansion (W 0)Q 0, since it is fastDU 0, by First lawWRestoringThe universe changed (the environment doeswork to the system and gain heat from it)IrreversibleQIsothermal expansion (DT 0)ReversibleWSlow, keep thermalequilibrium at all timeQRestoringWThe universe unchanged (W 0 and Q 0 forthe system and the environment.)Q

Examples of irreversible process Moving object slows down due to friction Flow of electric current through a resistance Spontaneous mixing of matter of varyingcomposition

How do we compute the change of disorder?Natural processes tend to increase disorder of the ngentropy.html

http://comic.hmp.is.it/comic/entropy/

Entropy: A quantity describes level of disorder𝑑𝑄𝑑𝑆 𝑇2 𝑆 1𝑑𝑄𝑇Microscopic view of Entropy: calculating the level of disorderw1S𝑘 𝑙𝑛 13H1T4𝑘 𝑙𝑛 42H2T6𝑘 𝑙𝑛 61H3T4𝑘 𝑙𝑛 41𝑘 𝑙𝑛 1configuration4H4T𝑆 𝑘 𝑙𝑛 𝑤w: number of possiblestate/configuration

The Second Law of ThermodynamicsThe Second Law of Thermodynamics:Δ𝑆 0Reversible process:Δ𝑆 0Irreversible process:Δ𝑆 0

Quiz:A hot piece of iron is thrown into the ocean and itstemperature eventually stabilizes. Which of the followingstatements concerning this process is correct?A. The change in the entropy of the iron-oceansystem is zero.B. The ocean gains less entropy than the iron loses.C. The entropy gained by the iron is equal to theentropy lost by the ocean.D.The entropy lost by the iron is equal to theentropy gained by the ocean.E. The ocean gains more entropy than the ironloses.

Some process are impossible:The Second Law of ThermodynamicsKelvin statementIt is impossible for any system to undergo a process in which it absorbsheat from a reservoir at a single temperature and converts the heatcompletely into mechanical work, with the system ending in the samestate in which it began.Clausius statementIt is impossible for any process to have as its sole result the transfer ofheat from a cooler to a hotter body.𝑄𝐶𝑄𝐻𝑊𝑄𝐶𝑄𝐶𝑄𝐻 𝑄𝐶Equivalent𝑊

How do we use a heat engine or arefrigerator? Both heat engine and refrigerator perform in cyclic processes. In other words, theinitial and final states are the same. DU 0 During the cycling, some parts of the cycle gain heat, and some lose heat. 𝑈 0 𝑄 𝑊𝑄𝐻𝑊𝑄𝐶Heat Engine𝑄 𝑄𝑖𝑛 𝑄𝑜𝑢𝑡 𝑊𝑄𝐻𝑄𝐶Refrigerator𝑊

Heat Engine𝑄𝐻𝑊𝑄𝐶Engine efficiency:𝑊𝑒 𝑄𝐻𝑄𝐻 𝑄𝐶𝑄𝐶(𝑒 1 )𝑄𝐻𝑄𝐻

Otto Cycle (two adiabatic, and two isochoric processes)𝑄𝐶𝑒 1 𝑄𝐻d to aΔ𝑈 𝑈𝑎 𝑈𝑑 𝑄 𝑄𝐶b to cΔ𝑈 𝑈𝑐 𝑈𝑏 𝑄 𝑄𝐻Note:Δ𝑈 𝑛𝐶𝑉 Δ𝑇𝑒 1 1𝑟 𝛾 1

Diesel Cycle (two adiabatic, one isochoric,and one isobaric processes)

Carnot Cycle (two adiabatic, and twoisothermal processes)An ideal cycle that maximize the efficiency𝑄𝐶𝑒 1 𝑄𝐻a to b𝑄 𝑄𝐻 𝑊 𝑏𝑝𝑑𝑉𝑎c to d𝑑𝑄 𝑄𝐶 𝑊 𝑇𝐶𝑒 1 𝑇𝐻𝑝𝑑𝑉𝑐

Coefficient of performance of refrigerator𝑄𝐻𝑄𝐶Coefficient of performance:𝑊𝑄𝐶𝑄𝐶𝐾 𝑊𝑄𝐻 𝑄𝐶𝑄𝐶𝐻𝐾 𝑊𝑃 typical air conditioners have heat removal rates H of 5,000 to 10,000 Btu/h( 1,500 – 3,000 W); and require electric power input of about 600 to 1,200 W.Thus, typically, K 3.

Reversible Irreversible. Examples of irreversible process . Both heat engine and refrigerator perform in cyclic processes. In other words, the initial and final states are the same. DU 0 During the cycling, some parts of the cycle gain heat, and some lose heat.