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thermodynamics

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  hermodynamics: 1) it deals with the equilibrium states of matter & precludes the existence of a temp. gradient. 2) when a system changes from one equilibrium state to another, thermodynamics helps to determine the quantity of work & heat interactions. it describes how much heat exchange during a process is required but doesn't hint on how the same could be achieved. heat transfer: 1) it is inherently a non equilibrium process since a temp. gradient must exist for the heat transfer to occur 2) it helps to predict the distribution of temp across a surface and to determine the temp. gradient that exists between different surfaceshermodynamics: 1) it deals with the equilibrium states of matter & precludes the existence of a temp. gradient. 2) when a system changes from one equilibrium state to another, thermodynamics helps to determine the quantity of work & heat interactions. it describes how much heat exchange during a process is required but doesn't hint on how the same could be achieved. heat transfer: 1) it is inherently a non equilibrium process since a temp. gradient must exist for the heat transfer to occur 2) it helps to predict the distribution of temp across a surface and to determine the temp. gradient that exists between different surfaces Heat transfer   describes the exchange of  thermal energy, between physical systems depending on thetemperature and pressure, by dissipating heat. Systems which are not isolated may decrease in entropy. Most objects emit infrared thermal radiation near room temperature. The fundamental modes of heat transfer are conduction  or diffusion , convection , advection  and radiation . The exchange of  kinetic energy of particles through the boundary between two systems which are at different temperatures from each other or from their surroundings. Heat transfer always occurs from a region of high temperature to another region of lower temperature. Heat transfer changes the internal energy of both systems involved according to the First Law of Thermodynamics. [1]  The Second Law of Thermodynamicsdefines the concept of thermodynamic entropy, by measurable heat transfer. Thermal equilibrium is reached when all involved bodies and the surroundings reach the same temperature.Thermal expansion is the tendency of matter to change in volume in response to a change in temperature  eat transfer - the molecules (or atoms at a smaller level) of a substance which are on average moving faster are hotter (more heat). When these molecules come in contact with another substance with slower molecules they impact each other (either physically or through forces) and transfer some of their speed (kinetic energy) to the slower molecules. Thus the temperature decreases in the first substance and increases in the second substance. Heat transfer is energy in transit due to temperature difference . Whenever there exists a temperature difference in a medium or between media, heat transfer must occur. The basic requirement for heat transfer is the presence of temperature difference . There can be no net heat transfer between two mediums that are at the same temperature. The temperature difference is the driving force for heat transfer, just as the voltage differenceis the driving force for electric current flow and pressure difference is the driving force for fluid flow. The rate of heat transfer in a certain  direction depends on the magnitude of the temperature gradient (the temperature difference per unit length or the rate of change of temperature) in that direction. The larger the temperature gradient, the higher the rate of heat transfer. THERMODYNAMICS AND HEAT TRANSFER:   Thermodynamics is concerned with the amount of heat transfer as a system undergoes a process from onanother, and it gives no indication about how long the process will take. A thermodynamic analysis simply temust be transferred to realize a specified change of state to satisfy the conservation of energy principle. In practice, we are concerned with the rate of heat transfer (heat transfer per unit time) than we are with the aFor example, we can determine the amount of heat transferred from a thermos flask as the hot milk inside coby a thermodynamic analysis alone. But, a designer of the thermos flask is primarily interested in how long it milk inside cools to 85 o C, and a thermodynamic analysis cannot answer this question. Determining the ratesfrom a system and thus the time of cooling or heating, as well as the variation of temperature, is the subject oThermodynamics deals with equilibrium states and changes from one equilibrium state to another. Heat transf deals with systems that lack thermal equilibrium, and thus it is a non-equilibriumphenomenon. Therefore, the cannot be based on the principles of thermodynamics alone. However, the laws of thermodynamics lay the fraof heat transfer. The first law requires that the rate of energy transfer into a system be equal to the rate of incr that system. Thesecond law requires that heat be transferred in the direction of decreasing temperature. It is ancurrent flowing in the direction of decreasing voltage or the fluid flowing in the direction of decreasing pressur    1234567891011      PHYSICAL ORIGINS AND RATE EQUATIONS:  It is important to understand the physical mechanisms which underlie the heat transfer modes and that we equations that quantify the amount of energy being transferred per unit time. Conduction:  Conduction can be imagined as a atomic or molecular activity which involves the transfer of energy from thless energetic particles of a substance due to interactions between the particles. Explanation:  The physical mechanism of conduction is explained as follows: Consider a gas in which there exists a temperature gradient and assume that there is no bulk motion . The gasbetween two surfaces that are maintained at different temperatures, as shown in Figure 1.2. The temperature atwith the energy of gas molecules in proximity to the point. This energy is related to the random translational internal rotational and vibrational motions, of the molecules. Figure 1.2 Association Of Conduction Heat Transfer With Diffusion Of Energy DuTo Molecular Activity   Higher temperatures are associated with higher molecular energies, and when neighboring molecules collide,doing, a transfer of energy from the more energetic to the less energetic molecules must occur. In the presegradient, energy transfer by conduction must then occur in the direction of decreasing temperature. This tr Figure 1.2. The hypothetical plane at  x  o  is constantly being crossed by molecules from above and below due However, molecules from above are associated with a larger temperature than those from below, in whia net transfer of energy in the positive  x- direction. Hence, the net transfer of energy by random molecular mas diffusion of energy. It is possible to quantify heat transfer processes in terms of appropriate rate equations . These equations may amount of energy being transferred per unit time. The rate equation for heat conduction is known as Fourier's for the one dimensional plane wall shown in Figure below, having a temperature distribution T  (  x  ) is given by The heat flux (W/m 2 ) is the heat transfer rate in the  x -direction per unit area perpendicular to the directiproportional to the the temperature gradient, dT/dx , in this direction. The proportionality constant k is a trans  the thermal conductivity (W/m.K) and is a characteristic of the wall material. The minus sign is a consequence is transferred in the direction of decreasing temperature. Figure 1.3 One Dimensional Heat Transfer By Conduction   Under the steady state conditions shown in Figure 1.3, where the temperature distribution is linear  , the tempeexpressed as (1.2) and the heat flux then (1.3) or This equation provides a heat flux , that is, the rate of heat transfer per unit area. The heat rate by conductionwall of area  A is then the product of the flux and the area q  x  =   .A.  (1.4) 1234567891011  
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