The assumption of constant pressure and volume during ideal heat addition/subtraction in a cycle is a simplification often used in the analysis of thermodynamic cycles, such as the Carnot cycle. This assumption allows for a more straightforward analysis and mathematical modeling of the system.
Here are a few reasons why this assumption is made:
Simplification of Calculations: Assuming constant pressure and volume simplifies the mathematical equations involved in analyzing the system. It allows for the direct application of equations like the ideal gas law and simplifies the integration of heat transfer calculations.
Idealization of Processes: In an idealized thermodynamic cycle, such as the Carnot cycle, the assumption of constant pressure and volume is used to represent reversible processes. Reversible processes are hypothetical processes that can be traversed in both directions without any irreversibilities or losses. While real processes may not perfectly adhere to these assumptions, they serve as a useful model for understanding the fundamental principles and efficiency limits of heat engines.
Focus on Key Concepts: By assuming constant pressure and volume, the analysis can focus on key thermodynamic concepts, such as the relationship between heat transfer, work, and temperature. This allows for a clearer understanding of how energy is transferred and converted within the system.
Practical Applications: Constant pressure and volume assumptions can provide reasonable approximations for certain practical applications. For example, in internal combustion engines, assuming constant volume combustion or constant pressure expansion can be useful for analyzing and designing engine performance.
However, it's important to note that in real-world systems, maintaining constant pressure and volume during heat addition/subtraction processes may not be practical or achievable. Real systems may have varying pressure and volume due to factors like heat losses, non-ideal behavior of fluids, friction, and other inefficiencies. Therefore, while the constant pressure and volume assumptions simplify analysis, they should be used with caution and in the appropriate context when modeling or studying real-world systems.