# Chapter 12 The Laws of Thermodynamics - PowerPoint PPT Presentation

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Chapter 12 The Laws of Thermodynamics. Heat and work Thermodynamic cycle. Heat and work Work is done by the system: Work is done on the system :. The first law of thermodynamics Work and heat are path-dependent quantities

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Chapter 12 The Laws of Thermodynamics

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#### Presentation Transcript

Chapter 12

The Laws of Thermodynamics

Heat and work

Thermodynamic cycle

• Heat and work

• Work is done by the system:

• Work is done on the system :

• The first law of thermodynamics

• Work and heat are path-dependent quantities

• Quantity Q + W = ΔEint(change of internal energy) is path-independent

• 1st law of thermodynamics: the internal energy of a system increases if heat is added to the system or work is done on the system

• The first law of thermodynamics

• Adiabatic process: no heat transfer between the system and the environment

• Isochoric (constant volume) process

• Free expansion:

• Cyclical process:

Chapter 12

Problem 18

Consider the cyclic process depicted in the figure. If Q is negative for the process BC and ΔEint is negative for the process CA, what are the signs of Q, W, and ΔEint that are associated with each process?

• Work done by an ideal gas at constant temperature

• Isothermal process – a process at a constant temperature

• Work (isothermal expansion)

• Work done by an ideal gas at constant volume and constant pressure

• Isochoric process – a process at a constant volume

• Isobaric process – a process at a constant pressure

• Molar specific heat at constant volume

• Heat related to temperature change:

• Internal energy change:

• Molar specific heat at constant pressure

• Heat related to temperature change:

• Internal energy change:

Free expansion of an ideal gas

• Time direction

• Irreversible processes – processes that cannot be reversed by means of small changes in their environment

• Configuration

• Configuration – certain arrangement of objects in a system

• Configuration for N spheres in the box, with n spheres in the left half

• Microstates

• Microstate – one of the ways to prepare a configuration

• An example of 4 different microstates for 4 spheres in the box, with 3 spheres in the left half

• Multiplicity

• Multiplicity ( W ) – a number of microstates available for a given configuration

• From statistical mechanics:

Multiplicity

Multiplicity

Multiplicity

Multiplicity

• Entropy

• For identical spheres all microstates are equally probable

• Entropy ( S ), see the tombstone:

• For a free expansion of

• 100 molecules

• Entropy is growing for

• irreversible processes in

• isolatedsystems

• Entropy

• Entropy, loosely defined, is a measure of disorder in the system

• Entropy is related to another fundamental concept – information. Alternative definition of irreversible processes – processes involving erasure of information

• Entropy cannot noticeably decrease in isolated systems

• Entropy has a tendencyto increase in open systems

• Entropy in open systems

• In open systems entropy can decrease:

• Chemical reactions

• Molecular self-assembly

• Creation of information

• Entropy in thermodynamics

• In thermodynamics, entropy for open systems is

• For isothermal process, the change in entropy:

• For adiabatic process, the change in entropy:

• The second law of thermodynamics

• In closed systems, the entropy increases for irreversible processes and remains constant for reversible processes

• In real (not idealized) closed systems the process are always irreversible to some extent because of friction, turbulence, etc.

• Most real systems are open since it is difficult to create a perfect insulation

Nicolas Léonard

(1796–1832)

• Engines

• In an ideal engine, all processes are reversible and no wasteful energy transfers occur due to friction, turbulence, etc.

• Carnot engine:

• Carnot engine (continued)

• Carnot engine on the p-V diagram:

• Carnot engine on the T-S diagram:

• Engine efficiency

• Efficiency of an engine (ε):

• For Carnot engine:

• Perfect engine

• Perfect engine:

• For a perfect Carnot engine:

• No perfect engine is possible in which a heat from a thermal reservoir will be completely converted to work

• Gasoline engine

• Another example of an efficient engine is a gasoline engine:

Chapter 12

Problem 31

In one cycle, a heat engine absorbs 500 J from a high-temperature reservoir and expels 300 J to a low-temperature reservoir. If the efficiency of this engine is 60% of the efficiency of a Carnot engine, what is the ratio of the low temperature to the high temperature in the Carnot engine?

• Heat pumps (refrigerators)

• In an ideal refrigerator, all processes are reversible and no wasteful energy transfers occur due to friction, turbulence, etc.

• Performance of a refrigerator (K):

• For Carnot refrigerator :

• Perfect refrigerator

• Perfect refrigerator:

• For a perfect Carnot refrigerator:

• No perfect refrigerator is possible in which a heat from a thermal reservoir with a lower temperature will be completely transferred to a thermal reservoir with a higher temperature

Questions?

Chapter 12

Problem 36

6.06 kJ/K

• Answers to the even-numbered problems

• Chapter 12

• Problem 56

• −4.9 × 10−2 J

• 16 kJ

• 16 kJ