Application of thermodynamic extremum principles C. Fernández-Pineda and S. Velasco Citation: American Journal of Physics

Application of thermodynamic extremum principles

C. Fernández-Pineda and S. Velasco

Citation: American Journal of Physics 69, 1160 (2001); doi: 10.1119/1.1397456 View online: http://dx.doi.org/10.1119/1.1397456

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Application of thermodynamic extremum principles

C. Ferna´ndez-Pineda^a)

Departamento de Fı´sica Aplicada I, Facultad de Fı´sicas, Universidad Complutense, 28040 Madrid, Spain S. Velasco

Departamento de Fı´sica Aplicada, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, Spain

共Received 18 December 2000; accepted 30 May 2001兲

A simple system is used to illustrate the application of different extremum principles in thermodynamics. The system consists of an ideal gas contained in an adiabatically isolated cylinder interacting with a constant-pressure work device through an adiabatic movable piston. A kinetic model is also used to analyze the time evolution of the system toward the final equilibrium state.

© 2001 American Association of Physics Teachers.

关DOI: 10.1119/1.1397456兴

I. INTRODUCTION

The thermodynamic extremum principles were formulated by Gibbs in essentially two versions.¹

I. For the equilibrium of any isolated system it is neces- sary and sufficient that in all possible variations of the state of the system which do not alter its energy, the variation of its entropy shall either vanish or be negative共entropy maxi- mum principle兲:

共␦^S兲U,V,n⭐0. 共1兲

II. For the equilibrium of any isolated system it is neces- sary and sufficient that in all possible variations of the state of the system which do not alter its entropy, the variation of its energy shall either vanish or be positive共internal energy minimum principle兲:

共␦^U兲S,V,n⭓0. 共2兲

Clearly, these theorems refer to hydrostatic closed systems where the volume V and the mole number n remain constant.

These constraints, constant V and n, were not originally in these statements, but were later added as equations of condi- tion by Gibbs.¹ Gibbs also proved the equivalence between the statements by showing that a violation of one leads to a violation of the other. A discussion of the above extremum principles, including proofs of their equivalence, can be found in most thermodynamics textbooks.^2–5 A less known third extremum principle, not considered by Gibbs, can be formulated.

III. For the equilibrium of any isolated system it is neces- sary and sufficient that in all possible variations of the state of the system which do not alter its entropy and its energy, the variation of its volume shall either vanish or be positive 共volume minimum principle兲:

共␦^V兲S,U,n⭓0. 共3兲

Kazes and Cutler⁶have demonstrated the validity of this vol- ume minimum principle共see, also, Refs. 7 and 8兲.

The thermodynamic extremum principles can only be fully understood through the concept of a composite system, first introduced by Carathe´odory.⁹A composite system is a set of two or more subsystems separated by walls avoiding the transfer of work and/or heat and/or matter. The subsystems must be simple, i.e., homogeneous, isotropic, and without surface, electric, magnetic, or gravitational effects. The walls

provide internal constraints. Then, assuming the additivity principle for the entropy, it is possible to associate an entropy value with the equilibrium states of a composite system. The relaxation of any internal constraint, keeping fixed the exter- nal ones 共conservation laws and environmental conditions兲, leads the composite system, by means of a spontaneous evo- lution through nonequilibrium states, from an initial equilib- rium state to a less restrictive final equilibrium state, i.e., to a final equilibrium state defined by a number of independent variables smaller than in the initial state. Both the initial and the final states have a well-defined entropy. Therefore, in a composite system, the entropy change between two equilib- rium states linked by a spontaneous process is also well de- fined. In these spontaneous processes the entropy of an iso- lated composite system must always increase 共a particular case of the weak Clausius evolution principle¹⁰兲.

At this point, one should be aware of the difference be- tween evolutionary and variational changes. An evolutionary change brings the system from an initial equilibrium state to a less restrictive final equilibrium state when an internal con- straint is released, and energy or matter can flow between subsystems during the process. However, the Gibbs state- ments refer to variational 共or virtual兲 changes. Variational changes compare states of a system and can never involve energy or matter flows. As Bailyn⁵has pointed out, a varia- tional change does not mean that the system interacts with the environment; it means only that a comparison is to be made between the variables of one state with those of other possible states. Such variational changes are used to illustrate the extremum principles.

Although the extremum principles are equivalent, we re- mark that it is not always possible to use them indepen- dently; i.e., they are related. For instance, a previous calcu- lation of the entropy value of the final equilibrium state by means of the entropy maximum principle is necessary for using the internal energy minimum principle. This is shown in Fig. 1 over the plane V⫽Veq⫽constant. One can see that, given the initial state, one can reach different final equilib- rium states if one fixes U_eq⫽Ui for the entropy maximum principle, or S_eq*_⫽Si for the internal energy minimum prin- ciple.

The aim of this work is to clarify, using a simple example, the rather abstract ideas underlying the extremum principles in thermodynamics. The example has the pedagogical value of being mathematically tractable since the system consid-

共11兲, November 2001

ered needs only one variable for describing its equilibrium states. The work is structured as follows. In Sec. II the sys- tem is described from a thermodynamic viewpoint. In Sec.

III we apply the extremum principles for the entropy共I兲, the internal energy共II兲, and the volume 共III兲 in order to identify the different equilibrium states. This allows us to obtain the final equilibrium state and to analyze its extremum character.

Numerical results are reported for a particular case, illustrat- ing the behavior of the system with different processes. In Sec. IV we present a simple kinetic model for the system.

This model predicts the same final equilibrium state reported in Sec. III and allows one to analyze the time evolution of the system toward this equilibrium state.

II. BAZAROV’S PROBLEM

Let us consider the system proposed by Bazarov,¹¹consist- ing of one mol of an ideal gas contained in an adiabatically isolated cylinder with a movable, frictionless, adiabatic pis- ton under constant external pressure P_w, as illustrated in Fig.

2. The following expressions are assumed for the pressure, P_g, the internal energy, U_g, and the entropy, S_g, of the gas:

P_g⫽RT_g

V_g , 共4兲

U_g⫽Ug,0⫹cvT_g, 共5兲

S_g⫽Sg,0⫹cvln T_g⫹R ln Vg, 共6兲 where T_g is the temperature of the gas, V_g its volume, c_vits molar heat capacity at constant volume, and U_g,0and S_g,0are constants. Furthermore, we assume that the environment acts

as a constant-pressure work device, i.e., an adiabatic system whose pressure, P_w, and entropy have constant values.¹²On the other hand, since the internal energy of the work device, U_w, refers to the gravitational potential energy associated with the hanging mass in Fig. 2, one has

dU_w⫽m_wg

A Adh_w⫽⫺PwdV_w, 共7兲 where m_wg is the weight of the hanging mass, h_w is its height, A is the area of the piston, and Vw is the volume of the right side in Fig. 2. Therefore, the internal energy, U_w, and the entropy, S_w, of the work device are given by

U_w⫽Uw,0⫺PwV_w, 共8兲

S_w⫽Sw,0, 共9兲

where U_w,0and S_w,0are constants.

Bazarov proposed to determine the equilibrium conditions for the gas and to show that under such conditions the en- tropy is a maximum. The solution of this problem was also given by Bazarov.¹³ However, as we shall see below, the system considered共gas plus work device兲 has only one inde- pendent variable, while Bazarov’s work used two indepen- dent variables.

III. APPLICATION OF THE EXTREMUM PRINCIPLES

Let us assume that the gas is found in an initial equilib- rium state with temperature T_g,i and volume V_g,i, the piston being in a fixed position. If P_g,i⫽Pw, by removing the con- straint fixing the piston, the gas evolves toward a final equi- librium state with a pressure

P_g,eq⫽RT_g,eq

V_g,eq ⫽Pw. 共10兲

Our aim is to analyze this final equilibrium state from the viewpoint of the extremum principles for the total entropy, internal energy, and volume.

A. Entropy maximum principle

From Eqs. 共6兲 and 共9兲, the entropy of the total system is given by

S⫽Sg⫹Sw⫽S0⫹cvln T_g⫹R ln Vg, 共11兲 where S₀⫽Sg,0⫹Sw,0 is a constant. Furthermore, since the internal energy and the volume of the total system remain constant one has

U_g⫹Uw⫽constant, 共12兲

V_g⫹Vw⫽constant. 共13兲

Substituting Eqs. 共5兲 and 共8兲 into Eq. 共12兲, and taking into account Eq.共13兲, one obtains

c_vT_g⫹PwV_g⫽A, 共14兲

where A is a constant that can be calculated from the initial equilibrium state: A⫽cvT_g,i⫹PwV_g,i. Equation 共14兲 shows that the equilibrium states of the composite system can be described by means of only one internal variable, i.e., in the maximization process for the entropy 共11兲 under external constraints共12兲 and 共13兲 there is only one independent vari-

Fig. 1. Constrained states with U⫽Ueq⫽Ui⫽constant (䊊). Constrained states with S⫽Seq*⫽Si⫽constant (⫻). Constrained states with S⫽Seq

⫽constant (〫). The equivalence between the entropy maximum principle and the internal energy minimum principle is only supported for the pro- cesses denoted by共䊊兲 and by 共〫兲.

Fig. 2. The system under study.

1161 Am. J. Phys., Vol. 69, No. 11, November 2001 C. Ferna´ndez-Pineda and S. Velasco 1161

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able. Let us take the gas volume V_g as this variable. Then, from Eq. 共14兲 one has

T_g⫽A⫺PwV_g

c_v , 共15兲

which substituted into Eq.共11兲 gives

S_U,V⫽S0⫹cvln

冉

冊

In order to investigate the behavior of S_U,V near the equilib- rium state, we shall denote␦^Vg⬅Vg⫺Vg,eq, and expand Eq.

共16兲 in a Taylor series around the equilibrium state, 共␦^S兲U,V⬅SU,V共Vg兲⫺SU,V共Vg,eq兲

⫽

冉

冊

␦^Vg⫹1

2

冉

冊

共␦^Vg兲²⫹¯ , 共17兲 where, from Eqs.共15兲 and 共16兲,

冉

冊

⫽⫺ c_vP_w

A⫺PwV_g,eq⫹ R

V_g,eq⫽⫺ P_w T_g,eq⫹ R

V_g,eq,

冉

冊

⫽⫺ c_vP_w²

共A⫺PwV_g,eq兲²⫺ R V_g,eq²

⫽⫺ P_w² c_vT_g,eq² ⫺ R

V_g,eq² . 共19兲

Condition共10兲 assures that the first derivative 共18兲 is zero in equilibrium, consistent with the extremum principle. Further- more, substituting 共10兲 into the second derivative 共19兲 one obtains

冉

冊

⫽⫺ ␥^R

V_g,eq² , 共20兲

where we have used the Mayer relation c_p⫺cv⫽R, cpbeing the molar heat capacity at constant pressure, and␥⫽cp/c_v is the adiabatic coefficient of the gas. The second derivative 共20兲 is clearly negative, consistent with the extremum char- acter共maximum兲 for the entropy.

From Eqs.共15兲 and 共10兲 one obtains T_g,eq⫽A⫺PwV_g,eq

c_v ⫽

冉

冊

Substitution of Eq.共21兲 into Eq. 共10兲 yields V_g,eq⫽RT_g,eq

P_w ⫽

冉

冊

Equations 共21兲 and 共22兲 give, respectively, the temperature and the volume of the gas in the final equilibrium state in terms of the pressure and the temperature of the gas in the initial equilibrium state and the equilibrium condition 共10兲, i.e., the external pressure P_w. Results共21兲 and 共22兲 can also be obtained by applying the first law to the gas for an adia- batic process under a constant external pressure P_w and by assuming that the initial and final states of the process are equilibrium states.

B. Internal energy minimum principle

In order to apply the extremum principle for the internal energy, it is necessary to know at least one state of the set of equilibrium states where the minimization process is done.

This is the final equilibrium state obtained from the entropy maximum principle and is given by Eqs.共21兲 and 共22兲. Then we can proceed in the following way. From Eqs.共5兲 and 共8兲, the internal energy of the total system is given by

U⫽Ug⫹Uw⫽U0⫹cvT_g⫺PwV_w, 共23兲 where U₀⫽Ug,0⫹Uw,0is a constant. Now the external con- straints fix the total entropy and volume. The closure relation for the volume is given by Eq. 共13兲 while the conservation law for the entropy follows directly from Eq.共11兲,

T_gV_g^R/cv⫽B, 共24兲

where B is a constant that can be calculated from the final equilibrium state: B⫽Tg,eqV_g,eq^R/cv, where T_g,eq and V_g,eq are given by Eqs. 共21兲 and 共22兲, respectively. Equation 共24兲 shows that in the minimization process for the internal en- ergy 共23兲 under external constraints 共13兲 and 共24兲, there is only one independent variable. Taking the gas volume V_g as this variable, from Eqs.共13兲, 共23兲, and 共24兲, one has

U_S,V⫽U0*_⫹ ^cvB

V_g^R/cv⫹PwV_g, 共25兲

where U₀* is a constant. By expanding Eq. 共25兲 in a Taylor series around the equilibrium state, one obtains

共␦^U兲S,V⬅US,V共Vg兲⫺US,V共Vg,eq兲

⫽

冉

冊

␦^Vg⫹1

2

冉

冊

共␦^Vg兲²⫹¯ , 共26兲 where, from Eqs.共10兲, 共24兲, and 共25兲,

冉

冊

⫽⫺ RB

V_g,eq^␥ ⫹Pw⫽⫺RT_g,eq

V_g,eq ⫹Pw, 共27兲

冉

冊

⫽RB␥

V_g,eq^␥⫹1⫽R␥^Tg,eq

V_g,eq² . 共28兲

From the preceding discussion, the first derivative 共27兲 is clearly zero in equilibrium, while the second derivative 共28兲 is positive, consistent with the existence of a minimum.

C. Volume minimum principle

As in the above case, the application of the extremum principle for the volume requires one known state from the set of equilibrium states where the minimization process is done. Again, this is the final equilibrium state obtained from the entropy maximum principle. Now the function to be con- sidered is the total volume

V⫽Vg⫹Vw, 共29兲

while the external constraints fix the total entropy and inter- nal energy. The constraint for the entropy gives Eq.共24兲 and, taking into account Eq. 共23兲, the constraint 共12兲 for the in- ternal energy leads to

c_cT_g⫺PwV_w⫽C, 共30兲

where C is a constant. Substituting Eqs. 共24兲 and 共30兲 into Eq. 共29兲, one obtains

V_S,U⫽⫺ C

P_w⫹Vg⫹ c_vB

P_wV_g^R/cv, 共31兲

where B is given by Eq. 共24兲. By expanding Eq. 共31兲 in a Taylor series around the equilibrium state, one obtains

共␦^V兲S,U⬅VS,U共Vg兲⫺VS,U共Vg,eq兲

⫽

冉

冊

␦^Vg⫹1

2

冉

冊

共␦^Vg兲²⫹¯ , 共32兲 where, from Eqs.共24兲 and 共31兲,

冉

冊

⫽1⫺ RB

P_wV_g,eq^␥ ⫽1⫺ RT_g,eq

P_wV_g,eq, 共33兲

冉

冊

⫽ RB␥

P_wV_g,eq^␥⫹1⫽R␥^Tg,eq

P_wV_g,eq² . 共34兲

The first derivative共33兲 is clearly zero in equilibrium, while the second derivative 共34兲 is positive, consistent with the existence of a minimum.

In summary, for the composite system considered 共ideal gas plus work device兲, the extremum principles for the en- tropy, internal energy, and volume consist in finding the ex- trema of three one-dimensional functions. By taking the gas volume V_g as the independent variable, these functions are:

S_U,V⫺S0⫽cvln

冉

冊

U_S,V⫺U0*_⫽^cvB

V_g^R/cv⫹PwV_g, 共36兲

V_S,U⫹ C

P_w⫽Vg⫹ c_vB

P_wV_g^R/cv, 共37兲

with

A⫽cvT_g,i⫹PwV_g,i⫽

冉

冊

共38兲 and

B⫽Tg,eqV_g,eq^R/cv⫽TgV_g^R/cv, 共39兲 where P_g,i and T_g,i are the pressure and the temperature of the gas in the initial equilibrium state, P_w is the external pressure, and T_g,eqand V_g,eqare given by Eqs.共21兲 and 共22兲, respectively.

In Fig. 3 we have plotted Eqs. 共35兲–共37兲 for the case of one mol of a monoatomic ideal gas (c_v⫽3R/2) with Pg,i

⫽2 bar and Tg,i⫽300 K, and a constant-pressure work de- vice with P_w⫽1 bar. In this case, Eqs. 共21兲 and 共22兲 give T_g,eq⫽240 K and Vg,eq⫽0.02 m³, while Eqs. 共38兲 and 共39兲 give A⫽4988.4 J and B⫽17.66 m²K. One can see that the entropy function 共35兲 has a maximum at Vg⫽Vg,eq

⫽0.02 m³, while the internal energy function 共36兲 and the volume function共37兲 present a minimum for the same value of the gas volume.

In Fig. 4 we have plotted in the (T_g,V_g) plane the gas states through which the maximization process for the en-

tropy is made, Eq.共38兲, and through which the minimization processes for the internal energy and for the volume are made, Eq. 共39兲. Both functions share a common point, the final equilibrium state (T_g,eq,V_g,eq), and the straight line 共38兲 is tangent to the curve 共39兲 at this point. We remark that these are the gas states, but not the states of the composite system共gas plus work device兲, along the different extremum processes. We remember that the thermodynamic description of the composite system is made in the space (T_g,V_g,V_w).

Therefore, in this space, the maximization process for the entropy is made along the curve defined by Eqs. 共13兲 and 共38兲, the minimization process for the internal energy is

Fig. 3. Behavior of the functions SU,V⫺S0关Eq. 共35兲兴, US,V⫺U0*关Eq. 共36兲兴, and VS,U⫹C/Pw关Eq. 共37兲兴 for the system of Fig. 3 with 1 mol of a mono- atomic ideal gas at initial conditions of 2 bar and 300 K, and an external pressure of 1 bar.

Fig. 4. States of the gas through the process of maximization of the total entropy共solid line兲 for the system of Fig. 3 with 1 mol of a monoatomic ideal gas at initial conditions of 2 bar and 300 K and an external pressure of 1 bar, and through the processes of the minimization of the total internal energy and the total volume with Sg⫽Sg,eq共dashed line兲.

1163 Am. J. Phys., Vol. 69, No. 11, November 2001 C. Ferna´ndez-Pineda and S. Velasco 1163

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made along the curve defined by Eqs.共13兲 and 共39兲, and the minimization process for the volume is made along the curve defined by Eqs. 共31兲 and 共39兲.

IV. A KINETIC MODEL

In the previous section we have solved, from a purely thermodynamic viewpoint, the problem of predicting the fi- nal equilibrium state of the composite system. In the present section we solve the same problem from a microscopic view- point by using a simple kinetic model that can help students to visualize the time evolution of the temperature and vol- ume of the gas toward the final equilibrium state given by Eqs. 共21兲 and 共22兲. The present model is based on the one proposed by Crosignani et al.¹⁴for the problem of two ideal gases separated by an internal adiabatic movable piston. A macroscopic approach to the time evolution of the piston in the one-cylinder problem has been reported by Gruber.¹⁵

Let us consider the system of Fig. 2. There are N_A, where N_Ais Avogadro’s number, identical point particles of mass m inside an adiabatic cylinder with an adiabatic movable piston of mass M and area A. A constant force Fw⫽PwA acts on the right face of the piston. We denote by x the piston posi- tion, so that the gas volume is given by V_g⫽Ax. We assume that the collisions between gas particles and between gas particles and the piston are elastic. Furthermore, we assume that the gas particles reach a Maxwellian molecular velocity distribution on a time scale negligible in comparison with that of the piston motion. Therefore, at any instant the tem- perature T_g(t) is well defined, and the pressure and the in- ternal energy of the gas are given, respectively, by Eqs. 共4兲 and共5兲.

The dynamics of the piston is described by

Md²x

dt²⫽Fg⫺Fw⫽共Pg⫺Pw兲A, 共40兲 where F_gis the force exerted on the piston by the gas. On the other hand, the energy conservation law reads

dK dt ⫹dU_g

dt ⫹dU_w dt ⫽Mdx

dt d²x dt²⫹cv

dT_g

dt ⫹PwAdx dt⫽0,

共41兲 where K⫽(M/2)(dx/dt)²is the kinetic energy of the piston, and we have used Eqs. 共5兲 and 共8兲 and taken into account that dV_w⫽⫺dVg⫽⫺A dx. Now, following Bauman and Cockerham’s kinetic model, which explicitly takes into ac- count the influence of the finite piston velocity on the mo- mentum change of the gas molecules,^16,17 the gas pressure P_g is given by

P_g⫽mN_A

V_g

冉

冊

⫽mN_A

Ax

冉

^冑

冊

where the average values 具^v2典 ^and 具v典 for the molecular velocity are obtained from Maxwell’s velocity distribution law, and k_B is Boltzmann’s constant. Substituting Eq. 共42兲 into Eq.共40兲, and Eq. 共40兲 into Eq. 共41兲, the complete set of time evolution equations is

Md²x

dt²⫽⫺PwA⫹RT_g

x ⫺

冑

^冑

冉

冊

⫹M

x

冉

冊

c_vdT_g

dt ⫽⫺RT_g x

dx

dt⫹

冑

冑

x

冉

冊

⫺M

x

冉

冊

whereM⫽mNAis the molar mass of the gas. Equation共43兲, up to the first order in the piston velocity, is similar to the one reported by Gruber.¹⁵

Equations 共43兲 and 共44兲 allow one to obtain the equilib- rium values of x and T_g, once the corresponding initial val- ues are known. By setting both d²x/dt²⫽0 and dx/dt⫽0 in Eq.共43兲, which corresponds to the final rest position, one has

P_w⫽RT_g,eq Axeq

⫽Pg,eq, 共45兲

independent of the mass of the piston. The result 共45兲 is in agreement with the mechanical equilibrium condition 共10兲.

Furthermore, assuming that the piston is initially at rest 关(dx/dt)i⫽0兴, integration of the energy conservation law 共41兲 allows one to obtain

c_vT_g,eq⫹PwAxeq⫽cvT_g,i⫹PwAxi, 共46兲 from which, taking into account Eqs. 共4兲 and 共45兲, one ob- tains

T_g,eq⫽

冉

冊

in agreement with Eq.共21兲.

Introducing the dimensionless variables ␰⫽x/xi, ␪

⫽Tg/T_g,i, and ␶⫽t/ta, with t_a⫽(Mxi

2/RT_g,i)^1/2

⫽(MRTg,i)^1/2/( P_g,iA), Eqs. 共43兲 and 共44兲 can be rewritten as

␰^¨⫽⫺␣⫹␪

␰^⫺␦

冑

␰^⫹

␲␦² 8

␰^˙2

␰ ^{, 共48兲}

␪^˙⫽⫺共␥⫺1兲␪␰^˙

␰^⫹共␥⫺1兲␦

冑

␰ ^⫺

␲␦²

8 共␥⫺1兲␰^˙3

␰ ^, 共49兲 where ␣⫽Pw/ P_g,i, ␦⫽(8M/␲^{M )1/2,} ␥⫽cp/c_v, and the dot stands for differentiation with respect to ␶. From Eqs.

共47兲 and 共45兲, the equilibrium values of␪ ^and␰^are

␪eq⫽c_v⫹R␣

c_p , 共50兲

␰eq⫽␪eq

␣ ^⫽

c_v⫹R␣

c_p␣ ^{, 共51兲}

which are independent of the mass of the piston.

Figure 5 shows the numerical solution of Eqs. 共48兲 and 共49兲 for the case of 1 mol of gas Ar 共cv⫽3R/2, cp⫽5R/2, M⫽40⫻10^⫺3kg/mol兲, M⫽10 kg, and Pg,i⫽2Pw. In this case, ␣⫽0.5, ␦⫽0.1, ␪eq⫽0.8, and ␰eq⫽1.6. We note that we have chosen a piston mass so that the assumption ␦Ⰶ1

underlying the kinetic model is verified.¹⁴ Since our equa- tions were derived for 1 mol of gas a large value of M is required. One can see that both the dimensionless gas tem- perature and the dimensionless piston position exhibit damped oscillations toward the equilibrium position with very similar relaxation times. The parameter ␦ governs this relaxation time and one can easily check that it decreases as

␦ increases, i.e., as the piston becomes less massive and the molar mass of the gas increases. In order to give a numerical meaning to the ␶values involved 共see Fig. 5兲, suppose that P_g,i⫽2 bar and Tg,i⫽300 K. Then, considering a cylinder with radius r⫽0.1 m and cross-sectional area A⫽3.14

⫻10^⫺2m², one obtains t_a⫽0.025 s. In this case, a value of

␶⫽200 共the time scale in Fig. 5兲 corresponds to t⫽ta␶

⫽5 s.

V. SUMMARY

In this paper we have applied three different thermody- namic extremum principles to analyze the equilibrium state in an isolated composite system consisting of an ideal gas interacting with a constant-pressure work device through an adiabatic movable piston. It is worth noting, however, that the system considered here differs from the so-called adia- batic piston problem 共APP兲 that is a composite system con- sisting of two gases separated by an internal adiabatic mov- able wall.14,15,18 –20 In both cases, thermodynamics establishes the equality of the final pressures. However, by using only the laws of thermostatistics, while in the first case the final gas temperature and volume can be obtained from the initial conditions, in the second case, the final tempera- tures and volumes cannot be determined.共The calculation of the final volume and temperature in the APP is done in Ref.

14 using a microscopic model.兲 Mathematically, the differ- ence between these systems lies in the fact that in the first case there is only one independent internal variable, while in the second case there are two independent internal variables.

We have also obtained the equilibrium state for the system considered by means of a simple kinetic model. This model does not take into account microscopic fluctuations in the gas pressure. The role played by these fluctuations is an intrigu- ing question.²¹ It is worth mentioning that this role in the APP has been very recently studied by Gruberg and

Frachebourg²²and, in the mesoscopic regime, by Crosignani and Di Porto.²³It has been argued that the stochastic motion of the piston, induced by the pressure fluctuations, drives the gas toward a final equilibrium state with the same pressure and the same temperature in both sides,^22,24,25 so that the mechanical equilibrium state must be considered as a quasi- equilibrium state. However, the time scale for this thermali- zation process is very long 共unobservable, in practice兲 for macroscopic pistons.

ACKNOWLEDGMENTS

S.V. thanks the Comisio´n Interministerial de Ciencia y Tecnologı´a共CI-CYT兲 of Spain under Grant No. PB 98-0261 and the Junta de CyL-FSE of Spain under Grant No.

SA097/01 for financial support.

a兲Electronic mail: Fdezpine@eucmos.sim.ucm.es

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23B. Crosignani and P. Di Porto, ‘‘On the validity of the second law of thermodynamics in the mesoscopic realm,’’ Europhys. Lett. 53, 290–296 共2001兲.

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Physica A 268, 412– 423共1999兲.

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Fig. 5. Plot of ␪共␶兲 共a兲 and ␰共␶兲 共b兲 for ␪(0)⫽1, ␰⁽⁰⁾⫽1,␰^{˙ (0)}⫽0,␣

⫽Pw/ Pg,i⫽0.5, and ␦⫽(8M/␲M )^1/2⫽0.1, with ␪⫽Tg/Tg,i, ␰⫽x/xi, and␶⫽t(RTg,i/ M x_i²)^1/2.

1165 Am. J. Phys., Vol. 69, No. 11, November 2001 C. Ferna´ndez-Pineda and S. Velasco 1165

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