Author: yasir
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The Second Law of Thermodynamics
Here is a puzzle. We saw in Sample Problem 20-1 that if we cause the reversible process of Fig. 20-3 to proceed from (a) to (b) in that figure, the change in entropy of the gas—which we take as our system—is positive. However, because the process is reversible, we can just as easily make it proceed from (b) to…
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Change in Entropy
Let’s approach this definition of change in entropy by looking again at a process that we described in Sections 18-11 and 19-11: the free expansion of an ideal gas. Figure 20-1a shows the gas in its initial equilibrium state i, confined by a closed stopcock to the left half of a thermally insulated container. If we open the stopcock, the gas rushes to…
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Irreversible Processes and Entropy
The one-way character of irreversible processes is so pervasive that we take it for granted. If these processes were to occur spontaneously (on their own) in the wrong way, we would be astonished. Yet none of these wrong-way events would violate the law of conservation of energy. For example, if you were to wrap your hands around a cup…
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Entropy and the Second Law of Thermodynamics
What in the world gives direction to time? When a bag of popcorn is heated in a microwave oven, the corn kernels explode into fluffy, edible structures that are just the right snack for a football game. If, however, you then decided to remove thermal energy from the popped kernels by sticking them in a…
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The Adiabatic Expansion of an Ideal Gas
We saw in Section 17-4 that sound waves are propagated through air and other gases as a series of compressions and expansions; these variations in the transmission medium take place so rapidly that there is no time for energy to be transferred from one part of the medium to another as heat. As we saw in Section 17-11,…
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A Hint of Quantum Theory
We can improve the agreement of kinetic theory with experiment by including the oscillations of the atoms in a gas of diatomic or polyatomic molecules. For example, the two atoms in the O2 molecule of Fig. 19-11b can oscillate towardand away from each other, with the interconnecting bond acting like a spring. However, experiment shows that such oscillations…
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Degrees of Freedom and Molar Specific Heats
As Table 19-2 shows, the prediction that agrees with experiment for monatomic gases but fails for diatomic and polyatomic gases. Let us try to explain the discrepancy by considering the possibility that molecules with more than one atom can store internal energy in forms other than translational kinetic energy. Figure 19-11 shows common models of helium (a monatomic molecule, containing a…
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The Molar Specific Heats of an Ideal Gas
In this section, we want to derive from molecular considerations an expression for the internal energy Eint of an ideal gas. In other words, we want an expression for the energy associated with the random motions of the atoms or molecules in the gas. We shall then use that expression to derive the molar specific heats of…
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The Distribution of Molecular Speeds
The root-mean-square speed vrms gives us a general idea of molecular speeds in a gas at a given temperature. We often want to know more. For example, what fraction of the molecules have speeds greater than the rms value? What fraction have speeds greater than twice the rms value? To answer such questions, we need to know…
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Mean Free Path
We continue to examine the motion of molecules in an ideal gas. Figure 19-4 shows the path of a typical molecule as it moves through the gas, changing both speed and direction abruptly as it collides elastically with other molecules. Between collisions, the molecule moves in a straight line at constant speed. Although the figure shows the…