need to answer the questions after reading the article. The Concept of Reveraibi
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need to answer the questions after reading the article.
The Concept of Reveraibility 19 tion. This is good for all sorts of trivial applications of thermodynamics, but it can also be used to illustrate a number of very important concepts; we will use it exten- sively for this purpose. energy, and we write our First Law equation as The work W is simply the work done by the force F as it moves a distance As. This work is negative (done on the system); thus we write W--F As. Therefore, Imagine that we have such a piston-and-cylinder com- bination and that a weight w is placed on the piston to hold a gas under compression in the eylinder. This initial state of the system is shown on the left in Fig. 2-5. We will In this equation Q is not heat transfer between the blocks, but heat transfer from both blocks to their surroundings. We now have an entirely proper equation and have avoided any embarrassment. On the other hand, we cannot by thermodynamics alone evaluate , 2, or Q. Thermo- dynamics just tells us what terms we need to take into sccount, and it gives us an equation relating the terms. With this illustration I've tried to make two major points First, we must use judgment in the selection of a system if we expect to use the formalism by which the First Law is normally expressed. Second, many problems cannot be solved by thermodynamies alone. START HERE Another apparently simple device encountered endlessly in thermodynamics is the piston-and-cylinder combination, as shown in Fig. 2-4 (we usually consider a gas to be trapped in the cylinder). If embarrassments of the kind encountered with the sliding block are to be avoided here, we must assume that the piston moves in the cylinder without frie- Figure 2-5 assume that the piston is so perfectly lubricated that it can move without friction in the cylinder. In addition, we will assume that the piston and cylinder are constructed of a special material that is a perfect heat insulator. Thus there can be no heat transfer between the gas and its surround- ings. Any such process is said to be adiabatic. We wish to use the compressed gas in the cylinder to accomplish useful work, and the question is how to carry out a process so that we may obtain the maximum possible Figure 2-4Explanation / Answer
a) The author divides the mass into fine grains of sand so as to be able to remove the mass bit by bit which will allow the piston to do reversible work.
b) according to the handout, reversibility is an imaginary process which is not possible to execute practically. In this process the piston must do infinitesimal work each time so as to be able to maintain balance and equilibrium between the gravitational force and the upward thrust by the gas. only then it is possible to reverse this work.
c) The author imagines a heap of sand placed on top of the piston. If one flicks one grain at a time from this heap, there will be a very minute change in the mass, resulting in a very minute reduction in the gravitational force, i.e increment in the upthrust by the piston. Now if we put the grain back into the heap, there will be a compression of the same amount as there was expansion on removal of the grain. This is how gas expansion can be reversed.
d) The conditions are:- the surface between the piston and cylinder must be friction less and the work done must be reversible, i.e. there should be a balance between the gravitational force and the upward thrust by the piston.
e) "Force =Pressure x area" holds good only if there is a balance between the gravitaional force and the upward thrust on the piston by the gas. This is only possible if the change in mass is infinitesimal. i.e. the work done is reversible.
f) The amount of time required to execute a reversible process is infinite, which is indeed impractical.
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