ch15 - The Laws of Thermodynamics Chapter Outline GENERAL...

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Unformatted text preview: The Laws of Thermodynamics Chapter Outline GENERAL PHYSICS The First Law of Thermodynamics PHY 302K The Second Law of Thermodynamics Chapter 15: The Laws of Thermodynamics Heat Engines Entropy Maxim Tsoi Physics Department, The University of Texas at Austin http://www.ph.utexas.edu/~tsoi 302K - Ch.15 302K - Ch.15 Work in Thermodynamic Processes The First Law of Thermodynamics Energy can be transferred to a system by heat and/or by work Energy conservation law • Work done on a gas at • Ideal gas is compressed constant pressure: slowly – piston pushed down by force F W Fy PV • P=const: Isobaric process • If a system undergoes a change from an initial state to a final state, where Q is the energy transferred to the system by heat and W is the work done on the system, the change in the internal energy of the system is • The area under the U U f U i Q W graph in a PV diagram is equal in magnitude to the work done on the gas Arrow on the PV graph points toward larger volumes the work is negative! 302K - Ch.15 Example 1 302K - Ch.15 The First Law of Thermodynamics The First Law of Thermodynamics Isochoric processes Isobaric processes V const W PV 0 • Volume remains constant • Work done on the gas: • For an ideal gas undergoing an isochoric process • Pressure remains constant U Q • Heat transferred to gas: • Transfer by heat • The change in the internal energy of a monatomic gas U nRT nCV T 3 2 • Molar specific heat at constant volume of a monatomic ideal gas 302K - Ch.15 P const W PV Q U W 5 nRT 2 • Work done on an expanding gas: Q nC P T • Molar specific heat at constant pressure of a monatomic ideal gas CP 5 R 2 CV R 3 2 302K - Ch.15 1 The First Law of Thermodynamics The First Law of Thermodynamics Adiabatic processes Isothermal processes Q0 • No heat transfer T const W Q • Temperature remains constant W U • Work done on the gas: • Work done on the gas: • For an ideal gas undergoing an isothermal process U 0 • For an ideal gas undergoing an adiabatic process • The work done on the gas PV const Vf W nRT ln V i C P / CV • Adiabatic index 302K - Ch.15 nCV T Example 2 302K - Ch.15 Heat Engines Heat Engines Cyclic process Refrigerators and heat pumps • Heat engine converts heat to other forms of energy (electrical, mechanical) • Heat pump heat engine operating in reverse (refrigerator, air conditioner) • Heat engine carries some working substance through a cyclic process (1) energy is injected into (1) energy is transferred by the engine U 0 Q W heat from a source at a high T (2) Resulting in energy being extracted from the Weng W Qh Qc (2) work is done by the engine cold reservoir (3) and transferred to the (3) energy is expelled by the hot reservoir engine by heat to a source at lower T Cooling mode • Thermal efficiency e Weng Qh 1 Qc • Coefficient of performance Qh 302K - Ch.15 Q COP c W Heating mode COP Qh W 302K - Ch.15 Heat Engines Heat Engines Second law of thermodynamics The Carnot engine • No real engine operating between two energy reservoirs can be more • No heat engine operating in a cycle can absorb energy from a reservoir efficient that a Carnot engine operating between the same two reservoirs and use it entirely for the performance of an equal amount of work • Thermal efficiency of a Carnot engine • We can’t get a greater amount of energy out of a cyclic process than we put in e 1 Weng W Qh Qc Qc T 1 c Qh Th • Third law it’s impossible to lower T of • We can’t break even e Weng Qh 1 Qc Qh a system to 0oK in a finite number of steps 1 • All real engines operate irreversibly, due to friction and the brevity of their cycles less efficient than the Carnot engine 302K - Ch.15 302K - Ch.15 Example 3 2 Entropy SUMMARY State variable describing the thermodynamic state of a system The Laws of Thermodynamics • If a system undergoes a change from an initial state to a final state, where Q is the energy transferred to the system by heat and W is the work done on • Let Qr be the energy absorbed or expelled during a reversible, constant temperature process between two equilibrium states. Than the change in entropy during any constant temperature process connecting the two the system, the change in the internal energy of the system is U U f U i Q W equilibrium states is defined as S Qr T • The entropy of the Universe increases in all natural processes ! • Entropy 302K - Ch.15 S Qr T 302K - Ch.15 3 ...
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This note was uploaded on 09/01/2009 for the course PHY M taught by Professor Staff during the Spring '09 term at University of Texas at Austin.

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