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ME2320_CHAPTERII

ME2320_CHAPTERII - CH II ME2320 Thermo I Energy Energy...

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CH. II ME2320 Thermo I Energy, Energy Transfer, and General Energy Analysis 14 ENERGY, ENERGY TRANSFER, AND GENERAL ENERGY ANALYSIS 1. INTRODUCTION The postulate of the first law of Thermodynamics states that energy cannot be created nor destroyed only transformed . This statement leads us immediately to ask: Are there different kinds of energy? Can energy take different forms? If there are different kinds of energy and energy can take different forms, how these changes occur? Effectively, there are many different forms of energy and under specific circumstances energy can transform to a different form. Also, as any known process, these transformations are not 100 % efficient (indeed not even nearly). As we will, study later in the course, these last sentences implicitly contain references to the first and second law of thermodynamics. In the following sections we will study the fundamental theory required to analyze the energy interactions in a global perspective for any system. This is, we will concentrate on the global thermodynamic performance of a system rather that analyzing the particular mechanisms by which energy is transferred. 2. FORMS OF ENERGY The total energy, E , contained in a system is equal to the sum of all the different forms of energy present in it. Then the total energy of a system per unit mass is denoted by e and is equal to ) 1 . 2 ( m E e = The science of thermodynamics has been structured in such a way that deals with changes in the energy content of a system rather than absolute values. This is convenient since the reference energy value can be set conveniently. The different forms of energy can be adequately grouped as: Forms of energy Thermal Mechanical Kinetic Potential Nuclear Chemical Electric Magnetic

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CH. II ME2320 Thermo I Energy, Energy Transfer, and General Energy Analysis 15 The sum of all the microscopic forms of energy is called internal energy and denoted by U . In general, the internal energy of a system is composed of the translational energy, rotational kinetic energy, and vibrational kinetic energy of the molecules that form the system. The spinning of the electrons about their axes also contributes to the internal energy of the system (spin energy). On the other hand, the macroscopic energy of a system is related to motion and external effects such as gravity, electricity, magnetism, and surface tension. The kinetic energy (KE) is related to the motion of the system relative to some frame of reference. Thus, when a system of mass m moves at a velocity V its kinetic energy is given by ) 2 . 2 ( ] [ 2 1 2 J mV KE = Then the kinetic energy on a unit mass basis is ) 3 . 2 ( 2 1 2 = kg J V ke The potential energy (PE) is related to the energy that a system contains due to its elevation relative to a given reference and is expressed as ) 4 . 2 ( ] [ J mgh PE = Similarly as for the kinetic energy, the potential energy per unit mass of a system is expressed as ) 5 . 2 ( = kg J gh pe
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