heat transfer and the 2 nd law of thermodynamics

Heat Transfer and theHeat Transfer and the22ndnd Law of Law of

ThermodynamicsThermodynamics

Warm Up 1

• 10 A sample liquid is cooled from 150°C to 30°C, causing the liquid to change into a solid. Which of the following has occurred?

• F Chemical composition has changed.

• G A physical change has occurred.

• H A new compound has formed.

• J Evaporation of a solvent has occurred.

Heat TransferHeat Transfer

• Heat flows from _______________.Heat flows from _______________.

• To reverse the process, work must be To reverse the process, work must be done.done.

Heat TransferHeat Transfer• The amount of heat absorbed (Q) is The amount of heat absorbed (Q) is

equal to the amount of heat released equal to the amount of heat released (-Q).(-Q).

Q = - QQ = - Q

mcmcΔΔT = mc-(T = mc-(ΔΔT) ; ( T) ; ( -Tf-Tf + Ti) + Ti)

mc (Tf-Ti) = mc (Ti mc (Tf-Ti) = mc (Ti –Tf–Tf ) )

• The final temperature for both will be The final temperature for both will be the same. (Thermal Equilibrium)the same. (Thermal Equilibrium)

Heat TransferHeat Transfer

Positive Q value if Positive Q value if

-energy is absorbed, added, required, -energy is absorbed, added, required, neededneeded

Negative Q value ifNegative Q value if

- energy is released, removed- energy is released, removed

Heat TransferHeat Transfer

• ExampleExample

A 50 g metal bolt (cA 50 g metal bolt (cboltbolt= 899 J/kg = 899 J/kg ooC) is C) is dropped into a 150 ml of water. The initial dropped into a 150 ml of water. The initial temperature of the water is 21 temperature of the water is 21 oo C and they C and they both reached an equilibrium temperature of both reached an equilibrium temperature of 25 25 oo C. What is the initial temperature of the C. What is the initial temperature of the bolt?bolt?

Remember: energy absorbed by the water is Remember: energy absorbed by the water is equal to energy released by the metal bolt.equal to energy released by the metal bolt.

SolutionSolution Q water = - Q bolt (released energy)Q water = - Q bolt (released energy)

mmwwccww (Tf - Ti) = m (Tf - Ti) = mbbccbb (Ti-Tf) (Ti-Tf)(0.15 kg)(4186 J/kg(0.15 kg)(4186 J/kgooC)(25 C)(25 ooC – 21 C – 21 ooC) = (0.05 kg)(899 J/kg C) = (0.05 kg)(899 J/kg ooC)(C)(Ti Ti - 25 - 25

ooC)C)

(0.15 kg)(4186 J/kg(0.15 kg)(4186 J/kgooC)(4 C)(4 ooC) = (0.05 C) = (0.05 kgkg)(899 J/)(899 J/kgkg ooC) (C) (Ti Ti - 25 - 25 ooC)C)

2511.6 J = (44.95 J/ 2511.6 J = (44.95 J/ ooC) (C) (Ti Ti - 25 - 25 ooC)C)

2511.6 2511.6 JJ = ( = (Ti Ti - 25 - 25 ooC)C)

(44.95 (44.95 JJ/ / ooC)C)

55.88 55.88 ooC = C = Ti Ti - 25 - 25 ooCC

55.88 55.88 ooC + 25 C + 25 ooC = C = TiTi

80.88 80.88 ooC = C = TiTi

22ndnd Law of Thermodynamics Law of Thermodynamics

• The Law of Increased EntropyThe Law of Increased Entropy

–In a closed system the entropy of In a closed system the entropy of the system will either remain the system will either remain constant or increase.constant or increase.

Law of Heat Transfer and EntropyLaw of Heat Transfer and Entropy

• Entropy – disorderliness Entropy – disorderliness of things (gas particles of things (gas particles have more entropy than have more entropy than solids and liquids)solids and liquids)

• Heat flows from higher Heat flows from higher temperature to lower temperature to lower temperature.temperature.

• To reverse the process, To reverse the process, work will be done.work will be done.

Law of Heat Transfer and EntropyLaw of Heat Transfer and EntropyExamples: Examples: 1.1. Ice cream easily melts but to refreeze it you Ice cream easily melts but to refreeze it you

need a heat pump (fridge) to do it.need a heat pump (fridge) to do it.

2.2. Coffee on the table will cool down eventually Coffee on the table will cool down eventually at room temperature. What kind of work is at room temperature. What kind of work is needed to warm it up?needed to warm it up?

EntropyEntropy

• By doing work on a system you can By doing work on a system you can decrease the system’s entropy because decrease the system’s entropy because the work you are doing is creating orderthe work you are doing is creating order

• It takes a lot more work to put your room in It takes a lot more work to put your room in order than to mess it up.order than to mess it up.

• You will need an AC pump to cool down You will need an AC pump to cool down (reduce the entropy) of a room in Houston (reduce the entropy) of a room in Houston during summer time.during summer time.

Light bulb momentLight bulb moment

• To reverse the process of Heat To reverse the process of Heat Transfer work must be done to do it.Transfer work must be done to do it.

• To reverse or to minimize the entropy To reverse or to minimize the entropy (disorderliness), work must be done.(disorderliness), work must be done.

Exit Ticket

• If energy cannot be created and heat only flows from high to low, what would happen to the universe far, far, far in the future?

Universal Thermal EquilibriumUniversal Thermal Equilibrium

• ““Heat Death”Heat Death”

• Eventually everything in the universe will Eventually everything in the universe will be at thermal equilibrium as energy is be at thermal equilibrium as energy is dissipated (lost).dissipated (lost).

• If everything is at the same temperature, If everything is at the same temperature, work will be impossible.work will be impossible.

Work