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Chapter 12 The Laws of Thermodynamics

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Work in Thermodynamic Processes Work is an important energy transfer mechanism in thermodynamic systems Work is an important energy transfer mechanism in thermodynamic systems Heat is another energy transfer mechanism Heat is another energy transfer mechanism

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Work in a Gas Cylinder, cont. A force is applied to slowly compress the gas A force is applied to slowly compress the gas The compression is slow enough for all the system to remain essentially in thermal equilibrium The compression is slow enough for all the system to remain essentially in thermal equilibrium W = - P ΔV W = - P ΔV This is the work done on the gas This is the work done on the gas

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More about Work on a Gas Cylinder When the gas is compressed When the gas is compressed ΔV is negative ΔV is negative The work done on the gas is positive The work done on the gas is positive When the gas is allowed to expand When the gas is allowed to expand ΔV is positive ΔV is positive The work done on the gas is negative The work done on the gas is negative When the volume remains constant When the volume remains constant No work is done on the gas No work is done on the gas

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PV Diagrams Used when the pressure and volume are known at each step of the process Used when the pressure and volume are known at each step of the process The work done on a gas that takes it from some initial state to some final state is the negative of the area under the curve on the PV diagram The work done on a gas that takes it from some initial state to some final state is the negative of the area under the curve on the PV diagram This is true whether or not the pressure stays constant This is true whether or not the pressure stays constant

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Additional Notes About the First Law The First Law is a general equation of Conservation of Energy The First Law is a general equation of Conservation of Energy There is no practical, macroscopic, distinction between the results of energy transfer by heat and by work There is no practical, macroscopic, distinction between the results of energy transfer by heat and by work Q and W are related to the properties of state for a system Q and W are related to the properties of state for a system

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Fig. 12.13, p. 374 Slide 22 T A <T B <T C B D Air Conditioner/Refrigerator

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Fig. 12.13, p. 374 Slide 22

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