Chapter 12 The Laws of Thermodynamics

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# Chapter 12 The Laws of Thermodynamics - PowerPoint PPT Presentation

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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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)
• 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:
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:
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