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# chapter12 - Chapter 12 The Laws of Thermodynamics First Law...

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Chapter 12 The Laws of Thermodynamics

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First Law of Thermodynamics The First Law of Thermodynamics tells us that the internal energy of a system can be increased by Adding energy to the system Doing work on the system There are many processes through which these could be accomplished As long as energy is conserved
Second Law of Thermodynamics Constrains the First Law Establishes which processes actually occur Heat engines are an important application

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Work in Thermodynamic Processes Assumptions Dealing with a gas Assumed to be in thermodynamic equilibrium Every part of the gas is at the same temperature Every part of the gas is at the same pressure Ideal gas law applies
Work in a Gas Cylinder The gas is contained in a cylinder with a moveable piston The gas occupies a volume V and exerts pressure P on the walls of the cylinder and on the piston

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Work in a Gas Cylinder, cont. A force is applied to slowly compress the gas The compression is slow enough for all the system to remain essentially in thermal equilibrium W = - P ΔV This is the work done on the gas where P is the pressure throughout the gas
More about Work on a Gas Cylinder When the gas is compressed ΔV is negative The work done on the gas is positive When the gas is allowed to expand ΔV is positive The work done on the gas is negative When the volume remains constant No work is done on the gas

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Notes about the Work Equation The pressure remains constant during the expansion or compression This is called an isobaric process The previous work equation can be used only for an isobaric process
PV Diagrams 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 equal in magnitude to the area under the curve on the PV diagram This is true whether or not the pressure stays constant

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PV Diagrams, cont. The curve on the diagram is called the path taken between the initial and final states The work done depends on the particular path Same initial and final states, but different amounts of work are done
First Law of Thermodynamics Energy conservation law Relates changes in internal energy to energy transfers due to heat and work Applicable to all types of processes Provides a connection between microscopic and macroscopic worlds

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First Law, cont. Energy transfers occur due to By doing work Requires a macroscopic displacement of an object through the application of a force By heat Occurs through the random molecular collisions Both result in a change in the internal energy, U, of the system
First Law, Equation If a system undergoes a change from an initial state to a final state, then U = U f U i = Q + W Q is the energy transferred to the system by heat W is the work done on the system U is the change in internal energy

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chapter12 - Chapter 12 The Laws of Thermodynamics First Law...

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