Furthermore, the system does not affect the entropy of its surroundings, since heat transfer between them does not occur. Thus the reversible process changes neither the total entropy of the system nor the entropy of its surroundings. Sometimes this is stated as follows: Reversible processes do not affect the total entropy of the universe. Real processes are not reversible, though, and they do change total entropy.
We can, however, use hypothetical reversible processes to determine the value of entropy in real, irreversible processes. Example 1 illustrates this point. Spontaneous heat transfer from hot to cold is an irreversible process. See Figure 3. Figure 3. Remember that the total change in entropy of the hot and cold reservoirs will be the same whether a reversible or irreversible process is involved in heat transfer from hot to cold.
So we can calculate the change in entropy of the hot reservoir for a hypothetical reversible process in which J of heat transfer occurs from it; then we do the same for a hypothetical reversible process in which J of heat transfer occurs to the cold reservoir. This produces the same changes in the hot and cold reservoirs that would occur if the heat transfer were allowed to occur irreversibly between them, and so it also produces the same changes in entropy.
First, for the heat transfer from the hot reservoir,. There is an increase in entropy for the system of two heat reservoirs undergoing this irreversible heat transfer. We will see that this means there is a loss of ability to do work with this transferred energy. Entropy has increased, and energy has become unavailable to do work.
It is reasonable that entropy increases for heat transfer from hot to cold. The decrease in entropy of the hot object is therefore less than the increase in entropy of the cold object, producing an overall increase, just as in the previous example. This result is very general:. There is an increase in entropy for any system undergoing an irreversible process.
With respect to entropy, there are only two possibilities: entropy is constant for a reversible process, and it increases for an irreversible process. There is a fourth version of the second law of thermodynamics stated in terms of entropy :. The total entropy of a system either increases or remains constant in any process; it never decreases. For example, heat transfer cannot occur spontaneously from cold to hot, because entropy would decrease. Entropy is very different from energy.
Entropy is not conserved but increases in all real processes. Reversible processes such as in Carnot engines are the processes in which the most heat transfer to work takes place and are also the ones that keep entropy constant. Thus we are led to make a connection between entropy and the availability of energy to do work. What does a change in entropy mean, and why should we be interested in it? One reason is that entropy is directly related to the fact that not all heat transfer can be converted into work.
Example 2 gives some indication of how an increase in entropy results in less heat transfer into work. Figure 4. The increase in entropy caused by the heat transfer to a colder reservoir results in a smaller work output of J. There is a permanent loss of J of energy for the purpose of doing work.
There is J less work from the same heat transfer in the second process. This result is important. The same heat transfer into two perfect engines produces different work outputs, because the entropy change differs in the two cases.
In the second case, entropy is greater and less work is produced. Entropy is associated with the un availability of energy to do work.
When entropy increases, a certain amount of energy becomes permanently unavailable to do work. The energy is not lost, but its character is changed, so that some of it can never be converted to doing work—that is, to an organized force acting through a distance. For instance, in Example 2, J less work was done after an increase in entropy of 9.
In the early, energetic universe, all matter and energy were easily interchangeable and identical in nature. Gravity played a vital role in the young universe.
Although it may have seemed disorderly, and therefore, superficially entropic, in fact, there was enormous potential energy available to do work—all the future energy in the universe. As the universe matured, temperature differences arose, which created more opportunity for work.
For this system, the most probable distribution is confirmed to be the one in which the matter is most uniformly dispersed or distributed between the two flasks. The spontaneous process whereby the gas contained initially in one flask expands to fill both flasks equally therefore yields an increase in entropy for the system.
A similar approach may be used to describe the spontaneous flow of heat. The hot object is comprised of particles A and B and initially contains both energy units. The cold object is comprised of particles C and D , which initially has no energy units. Distribution a shows the three microstates possible for the initial state of the system, with both units of energy contained within the hot object.
If one of the two energy units is transferred, the result is distribution b consisting of four microstates. If both energy units are transferred, the result is distribution c consisting of three microstates. And so, we may describe this system by a total of ten microstates. As for the previous example of matter dispersal, extrapolating this treatment to macroscopic collections of particles dramatically increases the probability of the uniform distribution relative to the other distributions.
And, again, this spontaneous process is also characterized by an increase in system entropy. What is the change in entropy for a process that converts the system from distribution a to c? The sign of this result is consistent with expectation; since there are more microstates possible for the final state than for the initial state, the change in entropy should be positive. Check Your Learning Consider the system shown in Figure 3.
What is the change in entropy for the process where all the energy is transferred from the hot object AB to the cold object CD? Consider the phase changes illustrated in Figure 4. In the solid phase, the atoms or molecules are restricted to nearly fixed positions with respect to each other and are capable of only modest oscillations about these positions.
In the liquid phase, the atoms or molecules are free to move over and around each other, though they remain in relatively close proximity to one another. This increased freedom of motion results in a greater variation in possible particle locations, so the number of microstates is correspondingly greater than for the solid.
Now consider the vapor or gas phase. The atoms or molecules occupy a much greater volume than in the liquid phase; therefore each atom or molecule can be found in many more locations than in the liquid or solid phase. According to kinetic-molecular theory, the temperature of a substance is proportional to the average kinetic energy of its particles. The maximum disorder of a system occurs when it is at thermal equilibrium , therefore, this is what all isolated systems will tend to over time.
Entropy can also be described as a system's thermal energy per unit temperature that is unavailable for doing useful work. Shown in Figure 1, this is represented as the "energy quality", which decreases as the entropy of a system increases.
In a reversible thermodynamic process, such as a Carnot engine , the change in entropy over a full cycle must be equal to zero.
This can be explored in more detail on the Hyperphysics website. Since entropy is increasing, and the Second law entails that out of this increase comes disorder, randomness and simplicity, how is it that there is so much order and complexity around us? This is a major confusion in understanding entropy, and it is important to distinguish between an isolated closed and non-isolated open system. Open systems are free to interact with their environment, and therefore energy can be added to or removed from them.
The student knows that changes occur within a physical system and applies the laws of conservation of energy and momentum. The student is expected to: G analyze and explain everyday examples that illustrate the laws of thermodynamics, including the law of conservation of energy and the law of entropy.
Teacher Support The meaning of entropy is difficult to grasp, as it may seem like an abstract concept. Figure Eventually, the components of the liquid will reach thermal equilibrium, as predicted by the second law of thermodynamics—that is, after heat transfers energy from the warmer liquid to the colder ice. Jon Sullivan, PDPhoto. The reverse process is impossible. The random motions of the gas molecules will prevent them from returning altogether to the corner. Teacher Support [AL] Ask students what would happen if the second law of thermodynamics were not true.
Some of them refreeze in the winter, but the second law of thermodynamics predicts that it would be extremely unlikely for the water molecules contained in these particular floes to reform in the distinctive alligator-like shape they possessed when this picture was taken in the summer of Patrick Kelley, U. Coast Guard, U. Geological Survey. Entropy Associated with Disorder Find the increase in entropy of 1. The systematic arrangement of molecules in a crystal structure is replaced by a more random and less orderly movement of molecules without fixed locations or orientations.
Its entropy increases because heat transfer occurs into it. Entropy is a measure of disorder. If What is the increase in entropy when 3.
Teacher Support Use these questions to assess student achievement of the section's learning objectives. What is entropy? Entropy is a measure of the potential energy of a system. Entropy is a measure of the net work done by a system. Entropy is a measure of the heat transfer of energy into a system. Which forms of energy can be used to do work? Only work is able to do work. Only heat is able to do work. Only internal energy is able to do work.
Heat, work, and internal energy are all able to do work. What is the statement for the second law of thermodynamics? It decreases. It must remain constant.
The entropy of the system cannot be predicted without specific values for the temperatures.
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