Piston-cylinder example reversible

The nature of reversible processes is illustrated by the example of a simple expansion of gas in a piston/cylinder arrangement. The apparatus shown in Fig. 2.2 is imagined to exist in an evacuated space. The gas trapped inside the cylinder is chosen as the system all else is the surroundings. Expansion processes result when mass is removed from tlie piston. F or simplicity, assume that the piston slides within the cylinder without friction and that the piston and cylinder neither absorb nor transmit heat. Moreover, because the density of the gas in the cylinder is low and because the mass of gas is small, we ignore the effects of gravity on the contents of the... 

IRREVERSIBLE EXPANSION OF AN IDEAL GAS Consider a gas confined within a piston-cylinder arrangement and held at constant temperature in a heat bath. Suppose the external pressure is abruptly reduced and held constant at the new lower value. The gas immediately expands against the piston until its internal pressure declines to match the new external pressure. The total entropy of system plus surroundings will increase during this expansion. In preparation for a quantitative example, a general comparison of irreversible and reversible processes connecting the same initial and final states provides insight into why the total entropy increases in a spontaneous process. 

Equation (4.23) is known as the first law of thermodynamics for a closed system.] Keep in mind that a system may do work, or have work done on it, without some obvious mechanical device such as a pump, shaft, and so on, being present. Often the nature of the work is implied rather than explicity stated. For example, a cylinder filled with gas enclosed by a movable piston implies that the surrounding atmosphere can do work on the piston or the reverse a batch fuel cell does no mechanical work, unless it produces bubbles, but does deliver a current at a potential difference electromagnetic radiation can impinge on or leave a system and so forth. 

Figure 2.16 An ideal gas in a piston-cylinder assembly undergoing a reversible, adiabatic expansion. In this example, is constant. See if you can predict the signs of At/, (), and W for this process in the table.

EXAMPLES. Calculation of Entropy Change for an Irreversible, Isothermal Compression A piston-cylinder device initially contains 0.50 of an ideal gas at 150 kPa and 20°C. The gas is subjected to a constant external pressure of 400 kPa and compressed in an isothermal process. Assume the surroundings are at 20 C. Take Cp = 25R and assume the ideal gas model holds. (a) Determine the heat transfer (in kj) during the process. (b) What is the entropy change of the system, surroundings, and universe (c) Is the process reversible, irreversible, or impossible ... 

This is a short but critically important section. When a system is at equilibrium, it has no tendency to change in either direction (forward or reverse) and will remain in its state until it is disturbed from outside the system. For example, when a block of metal is at the same temperature as its surroundings, it is in thermal equilibrium with them, and energy has no tendency to flow into or out of the block as heat. When a gas confined to a cylinder by a piston has the same pressure as the surroundings, the system is in mechanical equilibrium with the surroundings, and the gas has no tendency to expand or contract (Fig. 7.21). When a solid, such as ice, is in contact with its liquid form, such as water, at certain conditions of temperature and pressure (at 0°C and 1 atm for water), the two states of matter are in physical equilibrium with each other, and there is no tendency for one form of matter to change into the other form. Physical equilibria, which include vaporization as well as melting, are dealt with in detail in Chapter 8. When a chemical reaction mixture reaches a certain composition, it seems to come to a halt. A mixture of substances at chemical equilibrium has no tendency either to produce... 

Now let s consider another example, the expansion of an ideal gas at constant temperature (referred to as an isothermal process). In the cylinder-piston arrangement of Figure 19.5, when the partition is removed, the gas expands spontaneously to fill the evacuated space. Can we determine whether this particular isothermal expansion is reversible or irreversible Because the gas expands into a vacuum with no external... 

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