Course Notes, 6.050J/2.110J Information and Entropy - Chapter 12, Temperature

Chapter 12 Temperature In previous chapters of these notes we introduced the Principle of Maximum Entropy as a technique for estimating probability distributions consistent with constraints. In Chapter 8 we discussed the simple case that can be done analytically, in which there are three prob­ abilities, one constraint in the form of an average value, and the fact that the probabilities add up to one. There are, then, two equations and three unknowns, and it is straightforward to express the entropy in terms of one of the unknowns, eliminating the others, and find the maximum. This approach also works if there are four probabilities and two average-value constraints, in which case there is again one fewer equation than unknown. In Chapter 9 we discussed a general case in which there are many probabilities but only one average constraint, so that the entropy cannot be expressed in terms of a single probability. The result previously derived using the method of Lagrange multipliers was given. In Chapter 11 we looked at the implications of the Principle of Maximum Entropy for physical systems that adhere to the multi-state model motivated by quantum mechanics, as outlined in Chapter 10. We found that the dual variable β plays a central role. Its value indicates whether states with high or low energy are occupied (or have a higher probability of being occupied). From it all the other quantities, including the expected value of energy and the entropy, can be calculated. In this chapter, we will interpret β further, and will define its reciprocal as (to within a scale factor) the temperature of the material. Then we will see that there are constraints on the efficiency of energy conversion that can be expressed naturally in terms of temperature. 12.1 Temperature Scales A heat engine is a machine that extracts heat from the environment and produces work, typically in mechanical or electrical form. As we will see, for a heat engine to function there need to be two different environments available. The formulas below place restrictions on the efficiency of energy conversion, in terms of the different values of β of the two environments. We will derive these restrictions. First, however, it is useful to start to deal with the reciprocal of β rather than β itself. Recall that β is an intensive property: if two systems with different values of β are brought into contact, they will end up with a common value of β, somewhere between the original two values, and the overall entropy will rise. The same is true of 1/β, and indeed of any constant times 1/β. (Actually this statement is not true if one of the two values of β is positive and the other is negative; in this case the resulting value of β is intermediate but the resulting value of 1/β is not.) Note that 1/β can, by using the formulas in Chapter 11, be interpreted as a small change in energy divided by the change in entropy that causes it, to within the scale factor kB . Let us define the “absolute temperature” as T = 1 kB β (12.1) where kB = 1.381 × 10−23 Joules per Kelvin is Boltzmann’s constant. The probability distribution that comes from the use of the Principle of Maximum Entropy is, when written in terms of T , pi = e−α e−βEi −α −Ei /kB T =e e (12.2) (12.3) The interpretation of β in terms of temperature is consistent with the everyday properties of temperature, namely that two bodies at the same temperature do not exchange heat, and if two bodies at different temperatures come into contact one heats up and the other cools down so that their temperatures approach each other. In ordinary experience absolute temperature is positive, and the corresponding value of β is also. Because temperature is a more familiar concept than dual variables or Lagrange multipliers, from now on we will express our results in terms of temperature. Absolute temperature T is measured in Kelvins (sometimes incorrectly called degrees Kelvin), in honor of William Thomson (1824–1907), who proposed an absolute temperature scale in 1848.1 The Celsius scale, which is commonly used by the general public in most countries of the world, differs from the Kelvin scale by an additive constant, and the Fahrenheit scale, which is in common use in the United States, differs by both an additive constant and a multiplicative factor. Finally, to complete the roster of scales, William Rankine (1820–1872) proposed a scale which had 0 the same as the Kelvin scale, but the size of the degrees was the same as in the Fahrenheit scale. More than one temperature scale is needed because temperature is used for both scientific purposes (for which the Kelvin scale is well suited) and everyday experience. Naturally, the early scales were designed for use by the general public. Gabriel Fahrenheit (1686–1736) wanted a scale where the hottest and coldest weather in Europe would lie between 0 and 100. He realized that most people can deal most easily with numbers in that range. In 1742 Anders Celsius (1701–1744) decided that temperatures between 0 and 100 should cover the range where water is a liquid. In his initial Centigrade Scale, he represented the boiling point of water as 0 degrees and the freezing point as 100 degrees. Two years later it was suggested that these points be reversed.2 The result, named after Celsius in 1948, is now used throughout the world. For general interest, Table 12.1 shows a few temperatures of interest on the four scales, along with β. 12.2 Heat Engine The magnetic-dipole system we are considering is shown in Figure 12.1, where there are two environ­ ments at different temperatures, and the interaction of each with the system can be controlled by having the barriers either present or not (shown in the Figure as present). Although Figure 12.1 shows two dipoles in the system, the analysis here works with only one dipole, or with more than two, so long as there are many fewer dipoles in the system than in either environment. Now let us rewrite the formulas from Chapter 11 with the use of β replaced by temperature. Thus Equations 11.8 to 11.12 become 1 Thomson was a prolific scientist/engineer at Glasgow University in Scotland, with major contributions to electromagnetism, thermodynamics, and their industrial applications. He invented the name “Maxwell’s Demon.” In 1892 he was created Baron Kelvin of Largs for his work on the transatlantic cable. Kelvin is the name of the river that flows through the University. 2 According to some accounts the suggestion was made by Carolus Linnaeus (1707–1778), a colleague on the faculty of Uppsala University and a protege of Celsius’ uncle. Linnaeus is best known as the inventor of the scientific notation for plants and animals that is used to this day by botanists and zoologists. ◦ K Absolute Zero Outer Space (approx) Liquid Helium bp Liquid Nitrogen bp Water mp Room Temperature (approx) Water bp 0 2.7 4.22 77.34 273.15 290 373.15 ◦ C -273.15 -270 -268.93 -195.81 0.00 17 100.00 ◦ F -459.67 -455 -452.07 -320.46 32.00 62 212.00 R kB T = 0 4.9 7.6 139.2 491.67 520 671.67 1 β β (J−1 ) (J) 0 3.73 × 10−23 5.83 × 10−23 1.07 × 10−21 3.73 × 10−21 4.00 × 10−21 5.15 × 10−21 ∞ 2.68 × 1022 1.72 × 1022 9.36 × 1020 2.65 × 1020 2.50 × 1020 1.94 × 1020 Table 12.1: Various temperatures of interest (bp = boiling point, mp = melting point) ⊗ ⊗ ⊗ ··· ⊗ ⊗ ⊗ ⊗ ⊗ ⊗ ⊗ ⊗ ··· ⊗ ⊗ ⊗ ↑H↑ Figure 12.1: Dipole moment example. (Each dipole can be either up or down.) 1= � pi (12.4) pi Ei (12.5) i E= � i S = kB � � pi ln i −α −Ei /kB T pi = e α = ln e � � 1 pi � (12.6) (12.7) � e−Ei /kB T i S E = − kB kB T (12.8) The differential formulas from Chapter 11 for the case of the dipole model where each state has an energy proportional to H, Equations 11.30 to 11.36 become 0= � dpi (12.9) i dE = � i � Ei (H) dpi + E H � dH (12.10) � � E T dS = dE − dH H � � �� � � � � E 1 1 dα = dT − dH kB T T H � � �� � � � � Ei (H ) − E 1 1 dpi = pi dT − dH kB T T H � �� � �� � � � � � � � 1 1 1 E 2 dE = pi (Ei (H) − E) dT − dH + dH k T T H H B i � �� � �� � � � � � 1 1 1 2 T dS = pi (Ei (H) − E) dT − dH kB T T H i and the change in energy can be attributed to the effects of work dw and heat dq � � E dw = dH H dq = � (12.11) (12.12) (12.13) (12.14) (12.15) (12.16) Ei (H) dpi i = T dS 12.3 (12.17) Energy-Conversion Cycle This system can act as a heat engine if the interaction of the system with its environments, and the externally applied magnetic field, are both controlled appropriately. The idea is to make the system change in a way to be described, so that is goes through a succession of states and returns to the starting state. This represents one cycle, which can then be repeated many times. During one cycle heat is exchanged with the two environments, and work is exchanged between the system and the agent controlling the magnetic field. If the system, over a single cycle, gets more energy in the form of heat from the environments than it gives back to them, then energy must have been delivered to the agent controlling the magnetic field in the form of work. The cycle of the heat engine is shown below in Figure 12.2. Without loss of generality we can treat the case where H is positive. Assume that the left environment has a temperature T1 which is positive but less (i.e., a higher value of β) than the temperature T2 for the right environment (the two temperatures must be different for the device to work). This cycle is shown on the plane formed by axes corresponding to S and T of the system, and forms a rectangle, with corners marked a, b, c, and d, and sides corresponding to the values S1 , S2 , T1 , and T2 . Since the temperatures are assumed to be positive, the lower energy levels have a higher probability of being occupied. Therefore, the way we have defined the energies here, the energy E is negative. Thus as the field gets stronger, the energy gets more negative, which means that energy actually gets delivered from the system to the magnetic apparatus. Think of the magnetic field as increasing because a large permanent magnet is physically moved toward the system. The magnetic dipoles in the system exert a force of attraction on that magnet so as to draw it toward the system, and this force on the magnet as it is moved could be Figure 12.2: Temperature Cycle used to stretch a spring or raise a weight against gravity, thereby storing this energy. Energy that moves into the system (or out of the system) of a form like this, that can come from (or be added to) an external source of energy is work (or negative work). First consider the bottom leg of this cycle, during which the temperature of the system is increased from T1 to T2 without change in entropy. An operation without change in entropy is called adiabatic. By Equation 12.15 above, increasing T is accomplished by increasing H, while not permitting the system to interact with either of its two environments. (In other words, the barriers preventing the dipoles in the system from interacting with those in either of the two environments are in place.) The energy of the system goes down (to a more negative value) during this leg, so energy is being given to the external apparatus that produces the magnetic field, and the work done on the system is negative. Next, consider the right-hand leg of this cycle, during which the entropy is increased from S1 to S2 at constant temperature T2 . This step, at constant temperature, is called isothermal. According to Equa­ tion 12.15, this is accomplished by decreasing H, while the system is in contact with the right environment, which is assumed to be at temperature T2 . (In other words, the barrier on the left in Figure 12.1 is left in place but that on the right is withdrawn.) During this leg the change in energy E arises from heat, flowing in from the high-temperature environment, and work from the external magnetic apparatus. The heat is T2 (S2 − S1 ) and the work is positive since the decreasing H during this leg drives the energy toward 0. The next two legs are similar to the first two except the work and heat are opposite in direction, i.e., the heat is negative because energy flows from the system to the low-temperature environment. During the top leg the system is isolated from both environments, so the action is adiabatic. During the left-hand isothermal leg the system interacts with the low-temperature environment. After going around this cycle, the system is back where it started in terms of its energy, magnetic field, and entropy. The two environments are slightly changed but we assume that they are each so much larger than the system in terms of the number of dipoles present that they have not changed much. The net change is a slight loss of entropy for the high-temperature environment and a gain of an equal amount of entropy for the low-temperature environment. Because these are at different temperatures, the energy that is transferred when the heat flow happens is different—it is proportional to the temperature and therefore more energy leaves the high-temperature environment than goes into the low-temperature environment. The difference is a net negative work which shows up as energy at the magnetic apparatus. Thus heat from two environments is converted to work. The amount converted is nonzero only if the two environments are at different temperatures. Table 12.2 summarizes the heat engine cycle. Leg bottom right top left Total Start a b c d a End b c d a a Type adiabatic isothermal adiabatic isothermal complete cycle dS 0 positive 0 negative 0 dT positive 0 negative 0 0 H increases decreases decreases increases no change E decreases increases increases decreases no change Heat in 0 positive 0 negative positive Work in negative positive positive negative negative Table 12.2: Energy cycle For each cycle the energy lost by the high-temperature environment is T2 (S2 − S1 ) and the energy gained by the low-temperature environment is T1 (S2 − S1 ) and so the net energy converted is the difference (T2 − T1 )(S2 − S1 ). It would be desirable for a heat engine to convert as much of the heat lost by the high-temperature environment as possible to work. The machine here has efficiency � � work out T2 − T1 = (12.18) high-temperature heat in T2 This ratio is known as the Carnot efficiency, named after the French physicist Sadi Nicolas Léonard Carnot (1796 - 1832).3 He was the first to recognize that heat engines could not have perfect efficiency, and that the efficiency limit (which was subsequently named after him) applies to all types of reversible heat engines. The operations described above are reversible, i.e., the entire cycle can be run backwards, with the result that heat is pumped from the low-temperature environment to the one at high temperature. This action does not occur naturally, and indeed a similar analysis shows that work must be delivered by the magnetic apparatus to the magnetic dipoles for this to happen, so that more heat gets put into the high-temperature environment than is lost by the low-temperature environment. Heat engines run in this reverse fashion act as refrigerators or heat pumps. 3 For a biography check out http://www-groups.dcs.st-andrews.ac.uk/∼history/Mathematicians/Carnot Sadi.html