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ERT 206/4 THERMODYNAMICS SEM 2 (2011/2012) Energy can exist in lightforms: numerous Macroscopic forms of energy light Thermal Mechanical Kinetic Potential Electric Magnetic Chemical Nuclear Those related to the molecularlight structure of a system and the degree of the molecular activity. Their sum constitutes the total energy, E of a system The total energy of a system on a unit mass: Elight e (kJ/kg) m Microscopic forms of energy Related to the motion & the influence of some external effects such as : gravity, magnetion, electricity & surface tension Internal energy, U light The sum of all the microscopic forms of energy. Usually ignored. Kinetic energy, KE Potential energy, PE The energy that a system possesses light as a result of its motion relative to some reference frame. The energy that a system possesses light as a result of its elevation in a gravitational field. Kinetic energy, KE Potential energy, PE Velocity Gravitational acceleration PE mgz mV 2 KE light 2 (kJ) Elevation of the center of light gravity of a system relative to some arbitrarily selected reference level Kinetic energy per unit mass: V2 ke 2 Kinetic energy per unit mass: pe gz (kJ/kg) Total energy of a system mV 2 E U KE PE U mgz 2 light (kJ) Total energy of a system per unit mass: V2 e u ke pe u gz 2 (kJ/kg) Flow of steam in a pipe Energy flow rate Mass flow rate light ρV Ac Vavg m (kg/s) e Em light (kJ/s @ kW) The various forms of microscopic energies light that make up sensible energy. The internal energy of a system is the sum of all light forms of the microscopic energies Sensible & latent energy Thermal Energy Chemical energy Nuclear energy INTERNAL ENERGY Sensible energy The portion of the internal energy light with the of a system associated kinetic energies of the molecules. Latent energy The internal light energy associated with the phase of a system. Chemical energy The internal light energy associated with the atomic bonds in a molecule. Nuclear energy The tremendous amount of lightwith the strong energy associated bonds within the nucleus of the atom itself. Wind Energy A site evaluated for a wind farm is observed to have steady winds at a speed of 8.5 m/s. Determine the wind energy (a) per unit mass, (b) for a mass of 10 kg, and (c) for a flow rate of 1154 kg/s for air. c) a) V 2 8.5 m/s 2 1 J/kg e ke 36.1 J/kg 2 2 2 2 1 m /s b) E me (10 kg)(36.1 J/kg) 361 J 1 kW e (1154 kg/s)(36.1 J/kg) E m 41.7 kW 1000 J/s The total energy of a system, can be contained or stored in a system, and thus can be viewed as the static forms of energy. The forms of energy not stored in a system can be viewed as: the dynamic forms of energy @ energy interactions. are recognized at the system boundary aslight they cross it Represent the energy gained or lost by a systemlight during a process. 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 : 1) 2) heat transfer work. The difference is An energy interaction is heat transfer if its driving force is a temperature difference. Otherwise it is work. Heat The form of energy that is transferred between light two systems (or a system and its surroundings) by virtue of a temperature difference. Energy can cross the boundaries of a closed system in the form of heat and work. Temperature difference is the driving force for heat transfer. The larger the light temperature difference, the higher is the rate of heat transfer. Energy is recognized as heat transfer onlylight as it crosses the system boundary. Heat transfer per unit mass lightprocess Adiabatic light Q q (kJ/kg) m A process during which light there is no heat transfer Amount of heat transfer when heat transfer rate changes with time light dt Q Q t2 (kJ) The system is well insulated so that only a negligible light amount of heat can pass through the boundary t1 Amount of heat transfer when heat transfer rate is constant light Δt QQ (kJ) During an adiabatic process, a system exchanges no heat with its surroundings. Both the system and surroundings are at the same temperature and therefore light there is no driving force (temp diff) for heat transfer • Kinetic theory: Treats molecules as tiny balls that are in motion and thus possess kinetic energy. • Heat: The energy associated with the random motion of atoms and molecules. Heatlight Transfer Mechanisms Conduction: The transfer of energy from the more energetic particles of a light substance to the adjacent less energetic ones as a result of interaction between particles. Convection: The transfer of energy between a solid surface and the adjacent fluid that is in light motion, and it involves the combined effects of conduction and fluid motion. Radiation: The transfer of energy due to the emission of light electromagnetic waves (or photons). light ViDeO Work: The energy transfer associated with a force acting through a distance. – A rising piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated with work interactions Work done per unit mass Power is the work done per unit time (kW) Specifying the directions of heat and work. 12 • The first law of thermodynamics (the conservation of energy principle) provides a sound basis for studying the relationships among the various forms of energy and energy interactions. • The first law states that energy can be neither created nor destroyed during a process; it can only change forms. Energy cannot be created or destroyed; it can only change forms. 13 The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process. 15 Esystem Internal, kinetic, and potential energy changes light 16 Ein and Eout • • • Heat transfer Work transfer Mass flow A closed mass involves only heat transfer and work. (kJ) For constant rates, the total quantities during a time interval Δt are related to the quantities per unit time light The energy content of a control volume can be changed by mass flow as well as heat and work interactions. Cooling of a Hot Fluid in a Tank A rigid tank contains a hot fluid that is cooled while being stirred by a paddle wheel. Initially, the internal energy of the fluid is 800 kJ. During the cooling process, the fluid loses 500 kJ of heat, and the paddle wheel does 100 kJ of work on the fluid. Determine the final internal energy of the fluid. Neglect the energy stored in the paddle wheel. Ans: U2=400 kJ