Understanding Entropy in Irreversible Processes: A Thermodynamic Perspective

In thermodynamics, the second law plays a critical role in understanding the behavior of energy and the concept of entropy. Specifically, this law states that the total entropy of an isolated system can never decrease over time. This article explores what happens to entropy when heat is removed from a system during an irreversible process and how the overall entropy of the universe changes.

Entropy and Irreversible Processes

When heat is removed from a system in an irreversible process, the entropy of the system itself will decrease due to the loss of thermal energy. However, because the process is irreversible, the entropy of the surroundings will increase by a greater amount than the decrease in the system's entropy. As a result, the overall entropy of the combined system and surroundings will increase. This aligns with the second law of thermodynamics, which dictates that the total entropy of the universe (system and surroundings together) must not decrease.

Factors Affecting Entropy Change

The change in entropy for an irreversible process is affected by two primary factors: heat exchange between the system and the surroundings, and entropy generation. Entropy generation is always positive, as it accounts for the energy lost to irreversibilities in the system.

Three outcomes are possible for the change in entropy:

Entropy Increases: This occurs when the entropy generation is greater than the heat exchange during the process. Entropy Decreases: This happens when the entropy generation is less than the heat exchange during the process. Entropy Remains Constant: This is the case when the entropy generation is equal to the heat exchange during the process.

In this context, it's important to note that the heat exchange is always negative because heat is being removed from the system.

The entropy change can be calculated using the formula:

S Q/T Sgeneration

Where S is the change in entropy, Q is the heat transfer, and T is the temperature. Sgeneration represents the entropy generated during the process due to irreversibilities.

Implications for Isentropic Processes

From the above explanation, we can derive a significant conclusion. If the entropy change is zero, meaning the entropy of the system remains constant, then the process is isentropic. This is true even if the process is irreversible. Therefore, any reversible adiabatic process is isentropic, but not all isentropic processes are reversible—they can also be irreversible.

Overall Entropy and System Surroundings

If net heat is removed from the system, the entropy of the system will reduce. However, since the process is irreversible, the increment in entropy of the surroundings will be greater than the decrement in the system's entropy. As a result, the overall entropy of the universe (system surroundings) will increase.

Understanding these principles is crucial for engineers and scientists working in thermodynamics and related fields. By mastering the nuances of entropy and irreversibility, one can design more efficient systems and processes, leading to significant advancements in various industries, including energy, engineering, and materials science.