A generalized and explicit conceptual statement of the principle of the second law of thermodynamics

Abstract

The Clausius Inequality, the fundamental statement of the second law of thermodynamics, focuses on the net entropy rejection by heat transfer out of a closed system operating in a cyclic manner. Recently, the Clausius Inequality was re-stated to correctly apply to systems regardless of the form of heat transfer, and in particular systems with radiative transfer. This is important for many engineering systems because all matter emits thermal radiation continuously due to non-zero temperature. Other statements of the second law also generally refer to heat transfer and heat engines. Of even greater importance, the focus is on equilibrium processes and relations, whereas actual engineering systems are overwhelmingly non-equilibrium by nature. Although not explicitly stated in the Clausius Inequality, the second law principle encompasses the universal approach to obtain uniformity or equilibrium, a state of maximum entropy or disorder. By nature this approach to equilibrium is through non-equilibrium processes that continuously produce entropy as they occur. This paper attempts to capture a more generalized conceptual statement of the second law, that explicitly states various aspects of the second law principle for the purposes of engineering education; a statement that applies to any scenario, system or process.