Question

How do you distinguish between internal and external irreversibilities?

Short answer

Provide an example for each. Answer: Internal irreversibilities occur within a system due to non-ideal behaviors of the components or processes, such as friction, heat conduction, and fluid resistance. An example of internal irreversibility is heat transfer in a heat engine, where some heat is lost due to non-zero heat conduction resistance. External irreversibilities are those that occur outside of the system but still affect its ability to reverse a process, typically due to interactions between the system and its surroundings. An example of external irreversibility is energy loss due to temperature differences between a system and its surroundings, such as a hot object cooling down in a cooler environment.

Step-by-step solution

1. Understand the irreversibilities in a thermodynamic system

Irreversibilities are the causes of a process in a thermodynamic system being non-reversible, or unable to return to its original state without an additional energy input or external force. They can be classified into two types: internal irreversibilities and external irreversibilities.

2. Define internal irreversibilities

Internal irreversibilities are those that occur within a system due to non-ideal behaviors of the components or processes within the system. These include factors like friction, heat conduction, and fluid resistance. Due to these factors, the system may lose some energy and may not be able to return to its original state without additional inputs. For a process to be reversible, it has to be carried out quasi-statically; that is, very slowly so that the system remains in thermodynamic equilibrium at all times.

3. Provide an example of internal irreversibility

An example of internal irreversibility is the heat transfer in a heat engine. In a real heat engine, the heat transfer between the hot and cold reservoirs always happens with a finite temperature difference, which in turn creates a non-zero heat conduction resistance. This means that some heat will be lost in the process, making it impossible for the heat engine to return to its original state without compensating for the lost heat.

4. Define external irreversibilities

External irreversibilities are those that occur outside of the system but still have an effect on the system's ability to reverse a process. These are usually due to interactions between the system and its surroundings. Examples include heat transfer between the system and its environment, or the effect of gravity on the system.

5. Provide an example of external irreversibility

An example of external irreversibility is the energy lost due to temperature differences between a system and its surroundings. When a hot object is placed in a cooler environment, heat is transferred from the object to the surroundings due to the temperature difference. This irreversible heat transfer prevents the object from returning exactly to its original temperature without an additional energy input. In conclusion, understanding the differences between internal and external irreversibilities is crucial in thermodynamics for assessing the efficiency of processes and explaining why some processes are irreversible. Internal irreversibilities are associated with non-ideal behaviors within the system, while external irreversibilities arise from the interaction between the system and its surroundings.

Key concepts

Thermodynamic System

In the context of thermodynamics, a system is any portion of the physical universe chosen for analysis. Everything outside this system is known as the surroundings. A system is defined by its boundary, which may be a real or an imaginary enclosure separating it from the surroundings. The systems are broadly classified into isolated, closed, or open systems, based on the nature of interaction with their surroundings.

Isolated systems do not exchange any matter or energy with the surroundings, while closed systems allow energy transfer but not matter. On the other hand, open systems can exchange both energy and matter. For any thermodynamic analysis, characterizing the system correctly is the starting point. Internal and external irreversibilities both occur within these defined systems and affect the processes occurring in them.

Understanding how a thermodynamic system functions and how it is affected by its surroundings is essential for grasping the concepts of internal and external irreversibilities. Every real-world system experiences some form of irreversibility, making it vital for students to understand the basic classification and behavior of thermodynamic systems to fully comprehend deeper concepts such as reversible and irreversible processes.

Reversible Processes

A reversible process in thermodynamics is an idealized concept in which a system changes its state in such a way that the process can be reversed without leaving any trace on the surrounding environment. In such a process, the system is always in thermodynamic equilibrium with its surroundings, meaning the process is infinitely slow with infinitesimal changes in state variables.

For a process to be reversible, it must have no internal irreversibilities such as friction or unrestrained expansion. Additionally, there must not be any external irreversibilities; the system must not exchange heat or work with the surroundings across a finite difference in temperature or pressure.

Understanding the intricacies behind reversible processes is crucial because they set the upper limit on efficiency for engineering cycles. Any real process deviates from this ideal and is thus irreversible, due to natural phenomena like friction or spontaneous heat flow, which are forms of internal and external irreversibilities. This concept forms the foundation for second law analyses in thermodynamics.

Heat Transfer

Heat transfer is the physical act of thermal energy being exchanged between physical systems depending on the temperature and the properties of the conducting medium. There are three main modes of heat transfer: conduction, convection, and radiation. Conduction is heat transfer through direct contact, convection involves fluid motion, and radiation happens through electromagnetic waves.

In the context of irreversibilities, the manner in which heat is transferred plays a significant role. When heat flows due to a temperature gradient, as stated in the exercise example, some energy is lost due to the resistance to heat flow, which is an example of internal irreversibility. Heat loss to the surrounding environment, often unavoidable in actual applications, is a typical example of an external irreversibility.

Moreover, the second law of thermodynamics introduces the concept of entropy, which increases in real irreversible processes. The entropy generation is usually associated with some form of heat transfer. Studying these heat transfer processes in detail, along with understanding how they contribute to the irreversibility of a system, is pivotal for students to appreciate why certain systems are more efficient than others.