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ME 311 Chapter 2 lecture

ME 311 Chapter 2 lecture - Chapter 2 ENERGY ENERGY TRANSFER...

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Chapter 2 ENERGY, ENERGY TRANSFER, AND GENERAL ENERGY ANALYSIS
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INTRODUCTION Adiabatic = no heat transfer 2 As a result of the conversion of electric energy consumed by the device to heat, the room temperature will rise .
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FORMS OF ENERGY Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy, E of a system. Macroscopic forms of energy : Those a system possesses as a whole with respect to some outside reference frame. Kinetic energy, KE : The energy that a system possesses as a result of its motion relative to some reference frame. Potential energy, PE: The energy that a system possesses as a result of its elevation in a gravitational field. Microscopic forms of energy : Those related to the molecular structure of a system and the degree of the molecular activity. Internal energy, U : The sum of all the microscopic forms of energy. 3
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4 Total energy of a system Energy of a system per unit mass (specific energy) Potential energy Kinetic energy Energy flow rate (control volume) Considering only KE, PE, and Internal Energy (U)
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Some Physical Insight to Internal Energy Sensible energy: The portion of the internal energy of a system associated with the motion of the molecules. Latent energy: The internal energy associated with the phase of a system. Chemical energy: The internal energy associated with the atomic bonds in a molecule. Nuclear energy: The tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself. Internal Energy = Sensible + Latent + Chemical + Nuclear Thermal Energy = Sensible + Latent
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6 The macroscopic kinetic energy is an organized form of energy and is much more useful than the disorganized microscopic kinetic energies of the molecules. The only two forms of energy interactions associated with a closed system are heat transfer and work . The difference between heat transfer and work: An energy interaction is heat transfer if its driving force is a temperature difference. Otherwise it is work.
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Mechanical Energy of a Fluid 7 Mechanical energy: The form of energy that can be converted to mechanical work completely and directly by an ideal mechanical device such as an ideal turbine. Kinetic and potential energies: The familiar forms of mechanical energy. Mechanical energy of a flowing fluid per unit mass Rate of mechanical energy of a flowing fluid Flow work term: The work required to push a unit mass across a control volume boundary.
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Mechanical energy change of a fluid during incompressible flow per unit mass 8 Rate of mechanical energy change of a fluid during incompressible flow 2 1
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ENERGY TRANSFER BY HEAT 9 Energy can cross the boundaries of a closed system in the form of heat and/or work.
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ME 311 Chapter 2 lecture - Chapter 2 ENERGY ENERGY TRANSFER...

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