Less irreversibility = less change in entropy

In summary, a reversible process has a net change in entropy of zero, but the individual entropy changes of the system and surroundings may not be zero. The degree of irreversibility cannot be determined solely based on the entropy change, but can be better evaluated by comparing the entropy change of the system with the sum of Q/T at the interface between the system and surroundings. The larger the difference, the more irreversible the process is.
  • #1
pyroknife
613
3
I have a conceptual question that I am trying to clear up.

A reversible process means that the net change in entropy is zero.

If we have two scenarios, where case 1 has a ΔS= 0.1 J/K and case 2 has a ΔS= 0.5 J/K.
Is it a correct statement to say that case 1 is "less irreversible (i.e., more reversible)" than case 2?

Or do I have to think of this in a more binary(black & white) kind of way, where as long as ΔS≠0, the degree of irreversibility is the same?
 
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  • #2
It doesn't matter how you call it. Higher entropy change means higher loss and low efficiency process.
 
  • #3
When you say that the net change in entropy is zero for a reversible process, you are referring to the entropy change of the combination of system plus surroundings. The entropy change for either the system or the surroundings does not have to be zero as long as their sum is zero.

Entropy is an intensive property, meaning that it depends on the amount of material involved. You can have a very large system that operates very nearly reversibly, and a smaller system that operates very irreversibly, and, in both cases the change in entropy can be about the same. So entropy change (of combined system and surroundings) is not a good indicator of irreversibility. A better indicator of irreversibility is to compare the entropy change of the system with the sum of Q/T at the interface between the system and surroundings. The greater the difference in these quantities, the more irreversible the process is.

Chet
 

Related to Less irreversibility = less change in entropy

1. How does reducing irreversibility affect entropy?

Reducing irreversibility decreases the amount of disorder or randomness in a system, which leads to a smaller change in entropy. This is because with less irreversibility, the movement of particles and energy is more organized and predictable, resulting in a smaller change in the distribution of energy and therefore a smaller change in entropy.

2. Why is less change in entropy desirable?

In many cases, less change in entropy means that a system is more stable and efficient. For example, in a chemical reaction, a smaller change in entropy means that the reaction is less likely to spontaneously reverse, making it more useful for practical applications. In thermodynamics, systems with less change in entropy are often more efficient at converting energy into work.

3. Can entropy ever decrease?

Yes, while the second law of thermodynamics states that the total entropy of a closed system will always increase over time, it is possible for local regions within a system to experience a decrease in entropy. However, this decrease must be offset by a larger increase in entropy elsewhere in the system in order to maintain the overall increase in entropy.

4. Is there a limit to how much entropy can change?

According to the third law of thermodynamics, the change in entropy of a system approaches zero as the temperature approaches absolute zero (0 Kelvin). This means that there is a theoretical limit to how much entropy can change in a system, and that entropy can never reach absolute zero.

5. How does entropy relate to the arrow of time?

The concept of entropy is closely linked to the arrow of time, which refers to the unidirectional flow of time from past to future. This is because entropy tends to increase over time, leading to a more disordered and unpredictable universe. This is known as the "arrow of time" or the "direction of increasing entropy." Therefore, the decrease of entropy in a system can indicate the reversal of time, which is highly improbable and rarely occurs in nature.

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