Enclosure, adiabatic

The mixing, by diffusion, of two gases or liquids in an adiabatic enclosure, in which case there is no absorption or production of heat, but, nevertheless, an increase of entropy. 

An enclosure, such that the equilibrium of a system contained within it can only be disturbed by mechanical means, is adiabatic, otherwise it is diathermic. For instance, stirring, or the passage of an electric current, constitute mechanical means." A system Kf] in an adiabatic enclosure is adiabatically isolated, but this does not preclude mechanical interactions with the surroundings. Its transitions are then called adiabatic. 

As a general case, let us consider a set of n processes enclosed in an adiabatic enclosure shown in Fig. 13 (a). Such a system is called a process system. In this paper, the boundary of process systems is represented by broken lines, as shown in Fig. 13. 

The fallacy of this view arises in conjunction with a concept of adiabatic processes. Adiabatic enclosures are ideal partitions which separate regions of thermodynamic interest from the remainder of the universe in particular, no heat transfer of any type can occur across those boundaries. In the present example, however, the walls of the container are in intimate contact with the gas which is being compressed. Thus, these walls cannot be considered as part of the adiabatic partition which separates the container plus contents from the remainder of the universe. 

An adiabatic enclosure is filled with supercooled water and allowed to stand. After a while some ice is observed to form. Obviously, the process is spontaneous, yet it appears that there is an increase in order, hence, a reduction in entropy. Can you think of a way that gets you around this apparent violation of the Second Law ... 

In an adiabatic enclosure at 298 K 2.00 moles of CH (g) are mixed with 5.00 moles of 02(g). A spark is produced in the mixture and the CH4 is completely burned in the oxygen to C02 and H20. Assume ideal gas behavior and compute the final temperature of the gas mixture. What approximations are used in arriving at the final result The following data are relevant ... 

Fig. 1.7.1. Schematic diagram depicting a system S in an adiabatic enclosure that contains a movable piston P r and t2 are retractable release pins, and si and S2 are stops that arrest the motion of the piston under the influence of a gravitational field.

Fig. 5.21 Body with any directional spectral absorptivity a x in an adiabatic enclosure...

This important empirical result has, of course, been confirmed by many later workers and using several substances other than water. Without discussing the great mass of evidence, we shall therefore make a preliminary formulation of the first law of thermodynamics as follows the change of a body inside an adiabatic enclosure from a given initial state to a given final state involves the same amount of work by whatever means the process is carried out.f... 

Finally, it may be remarked that when we speak of a possible or impossible process the notion of time s passage is implicit. It is a question of whether a state A can precede or succeed another state B, in an adiabatic enclosure. The decision which of these states is later than or earlier than the other is based primarily on the subjective time sense of the human observer— which is not to say however that, once the second law has been seen to be true for all isolated systems, we cannot choose one such system as defining the time direction for all the rest. This can be done without reducing the law to a tautology. ... 

Changes which take place in a system spontaneously and of their, own accord are called natural processes. Examples are the equalization of temperature between two pieces of metal, the mixing of two gases and all processes which can occur spontaneously within an adiabatic enclosure. From what has been said in the last section it seems that such changes can never be reversed in their entirety, for it is known from experience that the system in question can he restored to its original condition only by transferring a quantity of heat elsewhere. In this respect natural processes are said to be irreversible. In brief, a cycle of changes A- B- A on a particular... 

Now the defining equation (1 13) for an entropy change is only applicable imder reversible conditions. We shall therefore suppose that, after the original process A->B has taken place within the adiabatic enclosure, the reverse change of state B- A is carried out under conditions which are known to be reversible. In the general case, as we have noted previously, this return process cannot be carried out adiabatically. Let it be devised in such a way that any heat, which must be absorbed by the system is taken in from a single heat bath at a temperature T (Fig. 8). 

As we have seen, the only changes which can take place in an adiabatic system are those in which the entropy either increases or remains constant. The same applies, of course, if the system is completely isolated, that is, if it does no work, as well as being within an adiabatic enclosure, so that the internal energy is constant. Thus whenever a system could change from a state of lower to one of higher entropy, dthin an enclosure of constant volume and energy, it is possible, in a thermodynamic sense, for this change to occur. 

Finally, we discussed the fact that certain changes are impossible under adiabatic conditions. The possibility or impossibility of the change A- B depends on the characteristics of the states A and B. There was thus shown to be a new function of state, the entropy, such that if S Sji the change is a possible one, within an adiabatic enclosure. The same empirical basis allowed also of the definition of a thermodynamic temperature, T, which is independent of the properties of any particular substance. 

Calculate the quantity of electrical work needed to heat 2.000mol of aluminum from 300.00 K to 400.00 K at 1 atm in an adiabatic enclosure. 

SOFCs cannot function at temperatures lower than around 700°C, beyond which the standard materials used as electrolytes become conductive of 0 ions. Also, the stack needs to be placed in an adiabatic enclosure which is raised to the stack s nominal temperature before it can produce electrical power. Thus, the system cannot be started up at ambient temperature. The nominal operating temperature is between 750 and 850°C approximately, depending on the manufacturers of the stacks. 

Consider a system which undergoes a spontaneous (irreversible) change from state A to state B within an adiabatic enclosure. Since the process is irreversible (spontaneous), the entropy change (Sg - S ) cannot be evaluated through Eq.3.10.1. 

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