The Laws of Thermodynamics Chapter 15 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:

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

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 tendency to increase in open systems

Entropy in open systems In open systems entropy can decrease: Chemical reactions

Entropy in open systems In open systems entropy can decrease: Chemical reactions Molecular self-assembly

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 processes 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

Engines In an ideal engine, all processes are reversible and no wasteful energy transfers occur due to friction, turbulence, etc. Carnot engine: Nicolas Léonard Sadi Carnot (1796–1832)

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:

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

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