1. Second Law of Thermodynamics

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1 1. Second Law of hermodynamics he first law describes how the state of a system changes in response to work it performs and heat absorbed. However, the first law cannot explain certain facts about thermal systems. For example, it is a fact that it is impossible to devise an engine that extracts all the thermal energy from a thermal reservoir and turns that energy into work. Another example is the fact that when ice cubes are put into hot tea, the ice cubes melt and the tea cools. he first law requires only that the heat flow into the ice cubes match the heat flow out of the hot tea, and the heat flow could proceed in either direction. In summary: 1. Not all the thermal energy in a thermal system is available to do work. 2. hermal systems spontaneously change only in certain ways, and in particular, spontaneous heat flow always goes from a body at higher temperature to a body at lower temperature. hese facts are expressions of the Second Law of hermodynamics. 2. Engines and Refrigerators Many of the concepts of the second law were derived for a very practical application of turning thermal energy into work. Sadi Carnot, an engineer working during the Industrial Revolution, was the first person to give a correct statement of the second law in 1824 (even before the first law). A useful engine exhibits two key features: Engines must work in cycles for them to be useful. Cyclic engines must include more than one thermal reservoir. 2a. he Carnot Cycle Consider a process comprised of two isothermal legs and two adiabatic legs. If executed reversibly, this process describes a Carnot Cycle. he reversible cycle followed by this engine is of great importance in thermodynamics because it is the most efficient engine one can construct that operates between any two temperature extremes. By learning the efficiency of the Carnot cycle, we learn the best we can hope to accomplish in engine efficiency. 1

2 i. Leg 1-2 he leg from 1 to 2 is isothermal expansion at temperature 12 so u 12 = 0. Heat absorbed during this leg is: ii. Leg 2-3 Adiabatic, so q 23 = 0. and q 12 = w 12 (1) = 2 1 pdα = R 12 ln α 2 α 1 w 23 = u 23 (2) = c v ( 34 12) iii. Leg 3-4 he leg from 3 to 4 is isothermal compression at temperature 34 so iv. Leg 4-1 Adiabatic, so q 41 = 0. and q 34 = w 34 (3) = R 34 ln α 4 α 3 w 41 = u 41 (4) = c v ( 12 34) During the two adiabatic legs, the work cancels and dq = 0. he work and heat transfer over the cycle follow from the isothermal legs alone. We can relate the specific volumes to the changes in temperature along the adiabatic legs using Poisson s identity: so = ( α3 α 2 ) γ 1 (5) α 2 α 1 = α 3 α 4 (6) 2

3 he heat transferred during the isothermal legs yields dq = R 12 ln α 2 α 1 + R 34 ln α 4 α 3 (7) = R( )ln α 2 α 1 If 12 > 34 if α 2 > α 1 then pα = dq > 0 (8) he system performs net work which is converted from heat absorbed during the cycle. he carnot cycle is a paradigm of a heat engine. Heat must be absorbed at high temperature 12 during isothermal expansion, and rejected at low temperatures 34 during isothermal compression. More heat is absorbed at high temperature than is rejected at low temperature. he first law then implies that net heat absorbed during the cycle is balanced by net work performed by the system. If the preceding cycle is executed in reverse, the system constitutes a refrigerator. he net work and heat transfer over the cycle are proportional to the temperature difference between the heat source and heat sink. Not all of the heat absorbed by the system during expansion is converted into work, some of that heat is rejected during compression - therefore the efficiency of a heat engine is limited - even for a reversible cycle. v. Efficiency is a measure of what fraction of the heat flow from the hotter thermal reservoir (such as burning fuel) is converted into work. If the work done in one complete cycle is w, then we define efficiency η as the ratio of the work done to the total positive heat flow supplied: q h. If a Carnot engne absorbing q h units of heat from a hot reservoir at a temperature h and rejecting q c units of heat from the system to a cold reservoir at a temperature c has an efficiency η η = w q h (9) the first law states that du = dq pdα, for the two isothermal legs 0 = q h q c w, so the efficiency is 3

4 For the Carnot Engine: η = q h q c q h = 1 q c q h (10) q h = w 12 (11) q c = w 34 (12) he minus sign because we have defined q c as the heat flow from the system to the reservoir whereas in the first law the heat flow is from the thermal reservoir to the system. η = 1 q c q h (13) = 1 w 34 w 12 (14) = 1 + R 34ln α 4 α 3 R 12 ln α 2 α 1 (15) = 1 R 34ln α 3 α 4 R 12 ln α 2 α 1 (16) is: Because the ratio of the volumes is the same the Efficiency of any Carnot Cycle η = (17) his means that for a Carnot engine to be efficient, the temperatures of the two reservoirs should be quite different from one another. If c were to be zero, the efficiency would be 1, for this reason a perfectly efficient Carnot engine is not possible. 1. All Carnot cycles that operate between the same two temperatures have the same efficiency. 2. he Carnot engine is the most efficient engine possible that operates between any two given temperatures 4

5 2b. Entropy and the Second Law Consider two adiabatic with potential temperatures θ 1 and θ 2 on a p V diagram. In passing reversibly from one adiabatic to another along an isotherm, as in the Carnot cycle, heat is absorbed or rejected, where the amount of heat Q rev (the subscript rev indicates that the heat is exchanged reversibly) depends on the temperature of the isotherm. Moreover, because we have shown that in the Carnot Cycle q c / c = q h / h it follows that the ratio Q rev / is the same no matter which isotherm is chosen in passing from one adiabatic to another. herefore, the ratio Q rev / could be used as a measure of the difference between the two adiabats; Q rev / is called the difference in entropy (S) between the two adiabats. he second law of thermodynamics is inspired by the observation that the quantity q/ is independent of path under reversible conditions. his allows the introduction of the state variable entropy. We can look at this in another way: for a Carnot cycle, we have shown that q c /q h = c / h, this relation can be expressed as: q c + q h = 0 (18) c h Remember that q c is the heat flow from the carnot engine to the cold thermal reservoir, so q c is the heat flow to the engine at the cold temperature. his means that for a Carnot cycle q i i i = 0 where i labels the steps and we always regard the heat flows as the heat flow to the engine. Let s make this more general, consider the integral B A then: dq on a p V diagram, B A dq B B B du + pdα c v d = = A A + R dα A α = (c v ln B + R ln α B ) (c v ln A + R ln α A ) (19) his implies that the value of the integral depends only on the initial and final values on the variables, and not on the path along which the integral is evaluated. his implies that there is a state function s which, like the internal energy u, depends only on the state of the gas, and not on how it reached that state. his state function is called the entropy of the system. It is defined by: 5

6 s(b) s(a) = B A dq = c v ln B A + R ln α B α A (20) his immediately leads to the generalization that for a reversible cycle: dq = 0 (21) Remember that this result is valid over a cycle i.e. a closed path. his result is known as Clausius theorem. Another way to express this is as: ds = dq (22) Entropy Change for a Reversible Process 2c. Reversible and Irreversible Processes Reversible Process Idealized process for which the system can be restored to its initial state without leaving a net influence on the system or its environment. Natural Process Proceeds freely, where the system can be out of equilibrium and cannot be reversed entirely without leaving a net influence on either the system or its environment. he natural process is inherently irreversible. 2d. How Entropy Changes for Irreversible or Spontaneous Processes Irreversible processes (e.g. processes with friction) lead to increased inefficiency in any otherwise reversible engine. Efficiency for any system is: η = 1 q rejected q absorbed (23) Efficiency is reduced if if the heat flow to the system is reduced, or the algebraic sum of the heat flows into the system to decrease. But this is exactly the effect of irreversibilites that occur anywhere along the cycle. For example in a car, there is additional heat flow out due to the heating of the pistons and other parts of the engine. We can say that for an irreversible cycle 6

7 his is known as Clausius inequality. We can also combine the equations: dq 0 (24) his is perhaps the most common form of the Second Law of hermodynamics. Now consider the cycle in which leg 1-2 is irreversible and leg 2-1 is reversible dq 2 0 > = dq dq 2 Since the second term is s 1 s 2, then (25) s 2 s 1 > 2 1 dq (26) he difference in entropy between two points is greater than the integral of dq/ over an irreversible change ds dq (27) Generalized Entropy Change for a Reversible (equality holds) and Irreversible (inequality holds) Process How do we compute the increase in entropy when there is an irreversible transformation from thermodynamic state A to thermodynamic state B? We have seen that entropy is a function of state, so we can compute the change s(b)-s(a) by any means we like; that is we can use a reversible path to find the change in entropy as long as the path connects the two given states. i. he Entropy of an Isolated System Never Decreases In any reversible adiabatic process the change in entropy is zero. If there is no heat flow, but irreversibility, s > 0. (like a free gas expansion in an isolated container). Spontaneous processes in an isolated system increase the systems entropy. For this reason, for an isolated thermal system the state of maximum entropy is the state of stable equilibrium 7

8 2e. Restricted Forms of the Second Law For certain processes, the change of entropy implied by the second law is simplified. For an adiabatic process ds ad 0 (28) For a reversible adiabatic process ds = 0, this process is isentropic. For an isochoric process, wherein expansion work vanishes, the first law transforms into: ( ) d ds α = c v (29) his actually holds whether the process is reversible or irreversible. We can conclude that changes in entropy follow from: 1. Irreversible work (only increases s) 2. Heat transfer (can increase or decrease s) α 8

9 2f. Calculations of Entropy Change ds = dq ds > dq reversible (30) irreversible (31) HOWEVER, s is a state variable and only depends on endpoints. s irrev can be found by devising a series of reversible transformations. Reversible processes Reversible Adiabatic Process dq rev = 0, s = 0 dqrev Reversible Phase Change at constant and p s = 1 = qrev = l. Where l is the latent heat. We can also do it for extensive properties (depend on mass) S = lm m is the mass. Reversible Change of State for Ideal Gas dq rev = du + dw = c v d + pdα consequently, ds = dqrev = c v dln + Rdlnα s = c v ln( 2 / 1 )+Rln(α 2 /α 1 ) or for extensive properties S = mc v ln( 2 / 1 )+ nr ln(α 2 /α 1 ) where the first term is the temperature term and the second term is the volume term. Reversible Constant Pressure Heating dq rev = c p d αdp (second term cancels) consequently s = c p ln( 2 / 1 ) or S = mc p ln( 2 / 1 ) Irreversible processes Irreversible change of state of an ideal gas from p 1, 1, α 1 to p 2, 2, α 2. We can break this down into two processes 1. slow expansion to α 2 at constant 1 = s 1 2. heat to 2 at constant α 2 = s 2 S = S 1 + S 2 = nr ln(α 2 /α 1 ) + mc v ln( 2 / 1 ) Irreversible phase change (such as supercooled water turning into ice. It is irreversible because intermediate states consist of mixtures of ice and water that are not at equilibrium - as it would have to be at 0C. Also because a small amount of heat will not cause the ice at temperatures less that 0C to turn into water at less than 0C). We can break this down into three processes. 9

10 1. Isobaric heating of water S = mc p(water) ln( 2 / 1 ). 2. Phase change under constant p and. S = L f m 3. Isobaric cooling of ice S = mc p(ice) ln( 2 / 1 ) 2g. Adiabats as Isentropes If a process is adiabatic, dθ = 0 and ds 0. he entropy remains constant or it can increase through irreversible work (associated with frictional dissipation of kinetic energy). In the case of an air parcel, the conditions for adiabatic behavior are closely related to those for reversibility. Adiabatic behavior excludes turbulent mixing and irreversible expansion work, consequently the conditions for adiabatic behavior are equal to conditions of isentropic behavior. Under these circumstances potential temperature surfaces θ = constant coincide with isentropic surfaces s = constant. i. Diabatic conditions which imply a change of potential temperature proportional to the heat absorbed by an air parcel. using the potential temperature definition using the first law dlnθ dln = κdlnp (32) so since ds dq/ dln κdlnp = dq c p dlnθ = dq c p (33) (34) dlnθ ds c p (35) dlnθ = ds c p (36) last equality because it involves only state variables and the equality must hold irrespective of whether or not the process is reversible. 10

11 his means that the change in potential temperature is directly related to the change in entropy. Integrating we obtain the relationship between entropy and potential temperature s = c p lnθ + constant (37) ransformations in which entropy (and therefore potential temperature) is constant are called isentropic. herefore, adiabats are often called isentropes in atmospheric sciences. Potential temperature surfaces, θ = ct, coincide with isentropic surfaces, s = ct. An air parcel coincident initially with a certain isentropic surface remains on that surface. Because those surfaces tend to be quasi-horizonal, adiabatic behavior implies no net vertical motion. However, under diabatic conditions, an air parcel moves across isentropic surfaces according to the heat exchanged with its environment. his heat exchange results in net vertical motion. 2h. he Fundamental Relations Substituting the second law into the two forms of the first law leads to: For a reversible process these lead to du ds pdα (38) dh ds + αdp (39) (40) du = ds pdα (41) dh = ds + αdp (42) (43) It is convenient to introduce a new state variable the Gibbs function (g) defined by: g = h s = u + pv s (44) In terms of this variable the fundamental relations become: 11

12 dg = sd + αdp (45) 12

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