## Metadata - Author: [[Peter Atkins]] - Full Title: The Laws of Thermodynamics - Category: #books ## Highlights - the zeroth law implies the existence of a criterion of thermal equilibrium: if the temperatures of two systems are the same, then they will be in thermal equilibrium when put in contact through conducting walls and an observer of the two systems will have the excitement of noting that nothing changes. ([Location 242](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=242)) - From the outside, from the viewpoint of an observer stationed, as always, in the surroundings, temperature is a property that reveals whether, when closed systems are in contact through diathermic boundaries, they will be in thermal equilibrium—their temperatures are the same—or whether there will be a consequent change of state—their temperatures are different—that will continue until the temperatures have equalized. From the inside, from the viewpoint of a microscopically eagle-eyed observer within the system, one able to discern the distribution of molecules over the available energy levels, the temperature is the single parameter that expresses those populations. As the temperature is increased, that observer will see the population extending up to higher energy states, and as it is lowered, the populations relax back to the states of lower energy. At any temperature, the relative population of a state varies exponentially with the energy of the state. That states of higher energy are progressively populated as the temperature is raised means that more and more molecules are moving (including rotating and vibrating) more vigorously, or the atoms trapped at their locations in a solid are vibrating more vigorously about their average positions. Turmoil and temperature go hand in hand. ([Location 362](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=362)) - The amount of energy that is transferred as heat into or out of the system can be measured very simply: we measure the work required to bring about a given change in the adiabatic system, and then the work required to bring about the same change of state in the diathermic system (the one with thermal insulation removed), and take the difference of the two values. That difference is the energy transferred as heat. A point to note is that the measurement of the rather elusive concept of ’heat’ has been put on a purely mechanical foundation as the difference in the heights through which a weight falls to bring about a given change of state under two different conditions (Figure 7). We are within a whisper of arriving at the first law. Suppose we have a closed system and use it to do some work or allow a release of energy as heat. Its internal energy falls. We then leave the system isolated from its surroundings for as long as we like, and later return to it. We invariably find that its capacity to do work—its internal energy—has not been restored to its original value. ([Location 439](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=439)) - the internal energy of an isolated system is constant. That is the first law of thermodynamics, or at least one statement of it, for the law comes in many equivalent forms. ([Location 450](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=450)) - In everyday language, heat is both a noun and a verb. Heat flows; we heat. In thermodynamics heat is not an entity or even a form of energy: heat is a mode of transfer of energy. It is not a form of energy, or a fluid of some kind, or anything of any kind. Heat is the transfer of energy by virtue of a temperature difference. Heat is the name of a process, not the name of an entity. ([Location 460](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=460)) - A final point is that the molecular interpretation of heat and work elucidates one aspect of the rise of civilization. Fire preceded the harnessing of fuels to achieve work. The heat of fire—the tumbling out of energy as the chaotic motion of atoms—is easy to contrive for the tumbling is unconstrained. Work is energy tamed, and requires greater sophistication to contrive. Thus, humanity stumbled easily on to fire but needed millennia to arrive at the sophistication of the steam engine, the internal combustion engine, and the jet engine. ([Location 510](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=510)) - The first law is essentially based on the conservation of energy, the fact that energy can be neither created nor destroyed. Conservation laws—laws that state that a certain property does not change—have a very deep origin, which is one reason why scientists, and thermodynamicists in particular, get so excited when nothing happens. There is a celebrated theorem, Noether’s theorem, proposed by the German mathematician Emmy Noether (1882-1935), which states that to every conservation law there corresponds a symmetry. Thus, conservation laws are based on various aspects of the shape of the universe we inhabit. In the particular case of the conservation of energy, the symmetry is that of the shape of time. Energy is conserved because time is uniform: time flows steadily, it does not bunch up and run faster then spread out and run slowly. Time is a uniformly structured coordinate. If time were to bunch up and spread out, energy would not be conserved. Thus, the first law of thermodynamics is based on a very deep aspect of our universe and the early thermodynamicists were unwittingly probing its shape. ([Location 640](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=640)) - As we have seen for the zeroth and first laws, the formulation and interpretation of a law of thermodynamics leads us to introduce a thermodynamic property of the system: the temperature, T, springs from the zeroth law and the internal energy, U, from the first law. Likewise, the second law implies the existence of another thermodynamic property, the entropy (symbol S). To fix our ideas in the concrete at an early stage it will be helpful throughout this account to bear in mind that whereas U is a measure of the quantity of energy that a system possesses, S is a measure of the quality of that energy: low entropy means high quality; high entropy means low quality. ([Location 658](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=658)) - A final point in this connection, one that will pervade this chapter, is that power in science springs from abstraction. Thus, although a feature of nature may be established by close observation of a concrete system, the scope of its application is extended enormously by expressing the observation in abstract terms. Indeed, we shall see in this chapter that although the second law was established by observations on the lumbering cast-iron reality of a steam engine, when expressed in abstract terms it applies to all change. To put it another way, a steam engine encapsulates the nature of change whatever the concrete (or cast-iron) realization of that change. All our actions, from digestion to artistic creation, are at heart captured by the essence of the operation of a steam engine. ([Location 665](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=665)) - The first giant, William Thomson, later Lord Kelvin (1824-1907), reflected on the essential structure of heat engines. Whereas lesser minds might view the heat source as the crucial component, or perhaps the vigorously reciprocating piston, Kelvin—as we shall slightly anachronistically call him—saw otherwise: he identified the invisible as indispensible, seeing that the cold sink—often just the undesigned surroundings—is essential. Kelvin realized that to take away the surroundings would stop the heat engine in its tracks. To be more precise, the Kelvin statement of the second law of thermodynamics is as follows (Figure 9): no cyclic process is possible in which heat is taken from a hot source and converted completely into work. In other words, Nature exerts a tax on the conversion of heat into work, some of the energy supplied by the hot source must be paid into the surroundings as heat. ([Location 703](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=703)) - the Clausius statement of the second law of thermodynamics (Figure 9): heat does not pass from a body at low temperature to one at high temperature without an accompanying change elsewhere. In other words, heat can be transferred in the ‘wrong’ (non-spontaneous) direction, but to achieve that transfer work must be done. ([Location 725](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=725)) - For our initial encounter with the concept, we shall identify entropy with disorder: if matter and energy are distributed in a disordered way, as in a gas, then the entropy is high; if the energy and matter are stored in an ordered manner, as in a crystal, then the entropy is low. ([Location 797](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=797)) - We begin by proposing the following as a statement of the second law: the entropy of the universe increases in the course of any spontaneous change. The key word here is universe: it means, as always in thermodynamics, the system together with its surroundings. There is no prohibition of the system or the surroundings individually undergoing a decrease in entropy provided that there is a compensating change elsewhere. ([Location 816](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=816)) - The first law and the internal energy identify the feasible change among all conceivable changes: a process is feasible only if the total energy of the universe remains the same. The second law and entropy identify the spontaneous changes among these feasible changes: a feasible process is spontaneous only if the total entropy of the universe increases. ([Location 850](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=850)) - This last point raises the question of the value of the entropy at the absolute zero of temperature (at T = 0). According to the Boltzmann distribution, at T =0 only the lowest state (the ‘ground state’) of the system is occupied. That means that we can be absolutely certain that in a blind selection we will select a molecule from that single ground state: there is no uncertainty in the distribution of energy, and the entropy is zero. ([Location 880](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=880)) - The concept of entropy is the foundation of the operation of heat engines, heat pumps, and refrigerators. We have already seen that a heat engine works because heat is deposited in a cold sink and generates disorder there that compensates, and in general more than compensates, for any reduction in entropy due to the extraction of energy as heat from the hot source. The efficiency of a heat engine is given by the Carnot expression. We see from that expression that the greatest efficiency is achieved by working with the hottest possible source and the coldest possible sink. Therefore, in a steam engine, a term that includes steam turbines as well as classical piston-based engines, the greatest efficiency is achieved by using superheated steam. The fundamental reason for that design feature is that the high temperature of the source minimizes the entropy reduction of the withdrawal of heat (to go unnoticed, it is best to sneeze in a very busy street), so that least entropy has to be generated in the cold sink to compensate for that decrease, and therefore that more energy can be used to do the work for which the engine is intended. A refrigerator is a device for removing heat from an object and transferring that heat to the surroundings. This process does not occur spontaneously because it corresponds to a reduction in total entropy. Thus, when a given quantity of heat is removed from a cool body (a quiet library, in our sneeze analogy), there is a large decrease in entropy. When that heat is released into warmer surroundings, there is an increase in entropy, but the increase is smaller than the original decrease because the temperature is higher (it is a busy street). Therefore, overall there is a net decrease in entropy. We used the same argument in the discussion of Clausius’s statement of the second law, which applies directly to this arrangement. A crude restatement of Clausius’s statement is that refrigerators don’t work unless you turn them on. ([Location 935](https://readwise.io/to_kindle?action=open&asin=B005DKR456&location=935))