Entropy, exergy

Entropy (Depletion of Exergy) and Two Laws of Thermodynamics by Mark Lindley The te e t op as oi ed in the 1850s when the oldest explicit and mathematically formulated versions of the First and Second Laws of Thermodynamics were published. Subsequent discoveries have led to broader formulations of these concepts. The First Law is about the fact that there are different kinds of energy and it can be transformed from one kind to another, but cannot be destroyed. (The meaning of this distinction between transforming and destroying energy has broadened as more and more kinds of energy have been investigated. For instance, nothing was known in the 1850s about nuclear energy.) A familiar example of the First Law in operation is when you come in from the cold and rub your hands to warm them up. The muscular energy is potential chemical energy when stored in your muscles, but is transformed into kinetic energy when you use them to rub your hands, and some of that kinetic energy is converted into heat owing to friction in the process of rubbing. (The rubbing also stimulates your cardio-vascular system to convey warm blood to your hands, but that effect takes a little longer to become operative.) Although we can sense the heat, it is not a substance but a manifestation of thermal energy: molecules stirring about aimlessly – i.e. in gases (which are therefore volatile), in liquids (which are therefore pourable) or in solid matter (which can therefore expand even without a chemical change) – or smaller kinds of particles stirring about aimlessly in a form of matter that physi ists all plas a (which has othi g to do ith lood plas a ; there is plasma in a fire). The aimlessness is a crucial consideration. In contrast to physical work, which transfers energy into coherent motion, heat transfers it into incoherent motion (even though the resulting expansion of a gas can perform work in certain special circumstances such as in steam engines and internal-combustion engines). The Second Law was originally a quantification of the fact that thermal energy is spontaneously diffused from warmer to cooler stuff but not the other way. A modern, and is thus sto ked ith ualit oade state e t of it is that if a isolated s ste is ot i e uili iu e e g -wise, (energy-wise), it will evolve in such a way as to entail losses of quality until equilibrium within its enclosure is attained. The unifying overall concept for such losses of ualit is increase of e t op . E t op is ou d to i ease i an isolated system until equilibrium is reached. So, for instance, if our universe is an irretrievably isolated system, then its destiny is the complete dissipation of the original sum of disequilibrium due to the Big Bang. However, such cosmology has nothing to do with economics, i.e. with the material aspects of human life on the Earth situated as it is in the solar system. I have referred to isolated systems in order to give brief but clear definitions. The Earth qualifies as a system – because it has a boundary (the atmosphere) – but hardly as an isolated one. It gets lots of radiation from the sun every day. E eg is a term, introduced in the 1950s, for energy that is available at any given moment from within a given system (such as the Earth) to achieve work. The amount of exergy within an isolated system at any one moment depends on how much was there when it became isolated and on how much decrease has meanwhile taken place. Equilibrium means zero exergy and maximum entropy. Co su a le e e g is e e g e t a ted fo use i the e o o . Here is more about the difference between energy and exergy: Energy, though sometimes loosely defined as ability to produce work, is more correctly defined as motion or ability to produce motion (even if no ability to produce work is thereby gained). Energy cannot be destroyed. Exergy, which is exactly defined as work or ability to produce work, is conserved in reversible processes, but is lost in processes which the Second Law renders irreversible. (For a simple, abstract example of the difference between energy and exergy, imagine two nearly adjacent isolated systems which have each reached equilibrium and which happen to be exactly alike except maybe for their temperatures, and imagine now that the boundary between them is eradicated, ending their mutual isolation and causing them to merge into one bigger isolated system. This change would yield exergy if, but only if, their temperatures were unequal, whereas the total amount of energy would in either case be simply the sum of the amounts in the two systems before they merged.) The te h i al te s i ludi g o k that I ha e used here are among the many which have, in physics, meanings codified exactly by equations for quantitative relationships among them. The equations are easy to find i Pie e Pe ot’s A to Z of Thermodymanics (Oxford Univ. Press, 1998; translated from his Dictionnaire de thermodynamique, Paris 1994). Market economists mimic physics in their theorizing, but without actually using any of it. Their concept of work, for instance, excludes unpaid housework, a mother caring for her baby, her husband cutting up firewood or repairing the house, siblings or neighbors helping each other, charitable work, etc.