Chapter_4 - Student Study Guide for 5th edition of...

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Student Study Guide for 5 th edition of Thermodynamics by Y. A. Ç engel & M. A. Boles 4- 1 Chapter 4 -1 Chapter 4: Energy Analysis of Closed Systems The first law of thermodynamics is an expression of the conservation of energy principle. Energy can cross the boundaries of a closed system in the form of heat or work. Energy transfer across a system boundary due solely to the temperature difference between a system and its surroundings is called heat. Work energy can be thought of as the energy expended to lift a weight. Closed System First Law A closed system moving relative to a reference plane is shown below where z is the elevation of the center of mass above the reference plane and G V is the velocity of the center of mass. For the closed system shown above, the conservation of energy principle or the first law of thermodynamics is expressed as Total energy entering the system Total energy leaving the system The change in total energy of the system F H G I K J F H G I K J = F H G I K J or Heat Work z Closed System Reference Plane, z = 0 G V
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Student Study Guide for 5 th edition of Thermodynamics by Y. A. Ç engel & M. A. Boles 4- 2 Chapter 4 -2 EE E in out system −= According to classical thermodynamics, we consider the energy added to be net heat transfer to the closed system and the energy leaving the closed system to be net work done by the closed system. So QW E net net system Where 2 1 () net in out net out in other b b QQ Q WW W W WP d V =− + = Normally the stored energy, or total energy, of a system is expressed as the sum of three separate energies. The total energy of the system , E system , is given as E Internal energy Kinetic energy Potential energy EU K E P E = + = Recall that U is the sum of the energy contained within the molecules of the system other than the kinetic and potential energies of the system as a whole and is called the internal energy. The internal energy U is dependent on the state of the system and the mass of the system.
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Student Study Guide for 5 th edition of Thermodynamics by Y. A. Ç engel & M. A. Boles 4- 3 Chapter 4 -3 For a system moving relative to a reference plane, the kinetic energy KE and the potential energy PE are given by 2 0 0 2 V V z z mV KE mV dV PE mg dz mgz = = == G G G GG The change in stored energy for the system is ∆∆∆ EUK E P E =+ + Now the conservation of energy principle, or the first law of thermodynamics for closed systems, is written as QW UK EP E net net −=+ + ∆∆ If the system does not move with a velocity and has no change in elevation, the conservation of energy equation reduces to U net net −= We will find that this is the most commonly used form of the first law. Closed System First Law for a Cycle
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Student Study Guide for 5 th edition of Thermodynamics by Y. A. Ç engel & M. A. Boles 4- 4 Chapter 4 -4 Since a thermodynamic cycle is composed of processes that cause the working fluid to undergo a series of state changes through a series of processes such that the final and initial states are identical, the change in internal energy of the working fluid is zero for whole numbers of cycles.
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This note was uploaded on 09/17/2009 for the course MAE 301 taught by Professor Hassan during the Fall '08 term at N.C. State.

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Chapter_4 - Student Study Guide for 5th edition of...

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