Chapter_2 - 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 2-1 Chapter 2-1 Chapter 2: Energy, Energy Transfer, and General Energy Analysis We will soon learn how to apply the first law of thermodynamics as the expression of the conservation of energy principle. But, first we study the ways in which energy may be transported across the boundary of a general thermodynamic system. For closed systems (fixed mass systems) energy can cross the boundaries of a closed system only in the form of heat or work. For open systems or control volumes energy can cross the control surface in the form of heat, work, and energy transported by the mass streams crossing the control surface. We now consider each of these modes of energy transport across the boundaries of the general thermodynamic system. Energy Consider the system shown below moving with a velocity G V at an elevation Z relative to the reference plane. The total energy E of a system is the sum of all forms of energy that can exist within the system such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear. The total energy of the system is normally thought of as the sum of the internal energy, kinetic energy, Z General System CM 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 2-2 Chapter 2-2 and potential energy. The internal energy U is that energy associated with the molecular structure of a system and the degree of the molecular activity (see Section 2-1 of text for more detail). The kinetic energy KE exists as a result of the system's motion relative to an external reference frame. When the system moves with velocity G V the kinetic energy is expressed as KE m V kJ = G 2 2 () The energy that a system possesses as a result of its elevation in a gravitational field relative to the external reference frame is called potential energy PE and is expressed as PE mgZ kJ = where g is the gravitational acceleration and z is the elevation of the center of gravity of a system relative to the reference frame. The total energy of the system is expressed as EUK EP E k J =+ + or, on a unit mass basis, e E m U m KE m PE m kJ kg u V gZ ==+ + + G 2 2 where e = E/m is the specific stored energy, and u = U/m is the specific internal energy. The change in stored energy of a system is given by
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Student Study Guide for 5 th edition of Thermodynamics by Y. A. Ç engel & M. A. Boles 2-3 Chapter 2-3 ∆∆∆ EUK E P E k J =+ + () Most closed systems remain stationary during a process and, thus, experience no change in their kinetic and potential energies. The change in the stored energy is identical to the change in internal energy for stationary systems. If KE = PE = 0, ∆∆ EU k J = Energy Transport by Heat and Work and the Classical Sign Convention Energy may cross the boundary of a closed system only by heat or work.
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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_2 - Student Study Guide for 5th edition of...

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