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Revista Brasileira de Ensino de Física, vol. 42, e20190127 (2020)

www.scielo.br/rbef

DOI: http://dx.doi.org/10.1590/1806-9126-RBEF-2019-0127
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Licença Creative Commons

Thermodynamic Potentials and Natural Variables

M. Amaku1,2, F. A. B. Coutinho*1, L. N. Oliveira3

1Universidade de São Paulo, Faculdade de Medicina, São Paulo, SP, Brasil
2Universidade de São Paulo, Faculdade de Medicina Veterinária e Zootecnia, São Paulo, SP, Brasil
3Universidade de São Paulo, Instituto de Física de São Carlos, São Carlos, SP, Brasil

Received on May 30, 2019. Revised on September 13, 2018. Accepted on October 4, 2019.

Most books on Thermodynamics explain what thermodynamic potentials are and how conveniently they describe

the properties of physical systems. Certain books add that, to be useful, the thermodynamic potentials must be expressed in their “natural variables”. Here we show that, given a set of physical variables, an appropriate

thermodynamic potential can always be defined, which contains all the thermodynamic information about the

system. We adopt various perspectives to discuss this point, which to the best of our knowledge has not been

clearly presented in the literature. Keywords: Thermodynamic Potentials, natural variables, Legendre transforms.

1. Introduction

same statement cannot be applied to the temperature. In real fluids, even in simple ones, the proportionality

Basic concepts are most easily understood when we dis-

cuss simple systems. Consider an ideal gas in a cylinder.

The cylinder is closed, its walls are conducting, and a

fluid at fixed temperature

contains a fixed number

nonetheless be varied, by means of a plunger. We push

the plunger and do work on the gas. Heat flows out to maintain the gas at the temperature T. If, instead, we

slowly pull out the plunger, heats flows inwards.

In experiments, the pressure

can be controlled, and the fluid temperature

be controlled. The variables and

dent. It has been known for centuries that this system approximately obeys an equation of state, which binds

the three variables:

to

T

is washed out, and the Internal Energy is more

conveniently expressed as a function of the entropy and

volume: U = U(S, V ).
T

surrounds it. The cylinder

Experimentalists frown at this dependence, for en-

tropies are more difficult to measure than temperatures.

Moreover, they may have to let the plunger move back

and forth and adjust the cylinder pressure to that of the

environment. They would therefore prefer the variables

P and T, instead of S and V .
N

of particles. Its volume can

V
P

applied to the plunger

can also

are not indepen-
At this point, it would seem natural to say “Very well,

let us make the experimentalist happy. Determine the

T
P

,

  • T
  • V

entropy and volume as functions of

T

and

, all we have to do, then,

(T, P) for and . The

T, P)], which expresses as

P

. Since

U

is

known as a function of

is to substitute S(T, P) and result will be T, P   , V

a function of T and P.”

S

and

V

  • V
  • S
  • V

U

[

S

  • (
  • )
  • (

U
PV = N kBT,

(1)

But then we would be at odds with good textbooks, which prefer to define another extensive quantity, on equal footing with the internal energy: the Gibbs Free Energy G(T, P), a function of the experimentalist’s fa-

vorite variables. The Internal Energy and the Gibbs Free

Energy are thermodynamic potentials. As we shall see, a

where kB is Boltzmann’s constant.

One might think that Eq. (1) is all we need to describe the ideal gas. The equation of state, nonetheless, tells us

nothing about other important properties of the system.

The Internal Energy

U

rises or drops as the temperature simple expression relates

Free Energy must be expressed as a function of and P, just like the Free Energy must be expressed as a function of and . The temperature and the

G

(

T, P) to
U
(S, V ). The Gibbs

changes. We might also be interested in the entropy

S

,

  • G
  • T

and so forth.

U

The ideal gas is a mathematical concept: a collection

of particles moving independently from each other and

bouncing elastically off the cylinder walls. The simple dy-

  • S
  • V
  • T

pressure

P

are called the natural variables of G. The

natural variables of U are S and V .

namics makes

U

directly proportional to the temperature

Once again, our inner voice interrupts the train of

thoughts to argue in favor of the experimentalist: “This

and number of particles. The proportionality to

N

, a vari-

able that—except as explicitly indicated—we will keep

fixed, is an inherent property of the internal energy. The

is all very well, but

G

is obtained from

U

. Therefore, in

order to compute G(T, P), we will first have to express

*Endereço de correspondência: [email protected].

the entropy and volume as functions of temperature and

Copyright by Sociedade Brasileira de Física. Printed in Brazil.

  • e20190127-2
  • Thermodynamic Potentials and Natural Variables

pressure to compute

U

[

S

(

T, P

)

, V

(

T, P)]. Our friend, the

and the pressure

P = −

ꢀꢀꢀꢀ

experimentalist, wants the Internal Energy as a function

∂U(S, V )
∂V
∂U ∂V

≡ −

.

(4)

of

T

and

P

. So give her

U

[

S

(

T, P

)

, V
(T, P)]. Why waste

S

time computing G(T, P)? Who needs the Gibbs Free

Energy?”

Here we have adopted the traditional notation [2, 4], which explicitly indicates the fixed quantity defining

the partial derivative. Since we work with fixed number

of particles, to simplify the notation we have avoided

Textbooks on Thermodynamics define the thermody-

namic potentials and associate them with their natural

variables. A few texts offer indirect answers to the two

adding a second, redundant subscript

derivatives.

N

to the partial

questions we have posed. To explain why

S

and

P

are the

natural variables of the Enthalpy, Schroeder shows that

changes in have simple physical interpretation when

the pressure is held fixed [1]. To explain why and are

  • In view of Eq. (3)
  • ,

T

and

S

are said to be conjugate and are

conjugates. In a conjugate pair, one of the variables ( or

P) is intensive, independent of the number of particles,

H

  • variables. Likewise, in view of Eq. (4)
  • ,

  • P
  • V

  • T
  • P

T

the natural variables of the Gibbs Free Energy, Callen

shows that, for a system in contact with a pressure and

a temperature reservoirs, the equilibrium value of any of

while the other (S or

V

) grows in proportion to the

number of particles and is, hence, extensive.

The dependence on extensive variables makes

its internal parameters will minimize

G

at the reservoir

U

some-

pressure and temperature [2]. Callen also explains that

information is lost when a thermodynamic potential is

expressed not as a function of its natural variables, but

as a function of its derivative with respect to those vari-

ables [2]. To prop up the argument, the author presents a

geometrical depiction of this problem and of its solution

by means of Lagrange Transformations. This explanation

pleases experienced teachers. To the same extent, it dis-

appoints the students, who often regard the illustration

as an abstract solution to an abstruse conundrum.

The purpose of this paper is to answer the two questions directly, in a physical setting, that is, to explain why it is better to work with G(T, P)

what special among the thermodynamic potentials. That

the Internal Energy is a (single-valued) convex function

of all its natural variables,

S

and

V

, was first pointed

  • and are
  • out by Gibbs. Given the convexity and that

  • S
  • V

extensive, it is a simple matter to derive the Second Law

of Thermodynamics, as recalled by Wightman, in a re-

markably clear, succinct account of Gibbs’s reasoning [5].

The other thermodynamic potentials cannot be directly

related to the Second Law because they depend on one or more intensive variables; while they are convex functions

of the extensive variables, they are concave function of

the intensive ones [2].

Nonetheless, practical considerations often focus one’s

interest on intensive variables. One must then turn to than with

U

[

S(T, P), V (T, P)]. We will show that

  • U[T, P] ≡ U
  • [S(T, P), V (T, P)] conveys less information

thermodynamic potentials other than

U
(S, V ). We have than U(S, V ), or G(T, P). In order to preserve informa-

tion, we have to express each thermodynamic potential

as a function of its natural variables. Conversely, the vari-

ables of interest uniquely determine the thermodynamic

potential. In our example, the experimentalist would like

to work with

not for U(T, P).

already discussed one of them, the Gibbs Free Energy

G

(

T, P). Since we have four variables (

S

,

V

,

T

, and

P

),

there must be more than two thermodynamic potentials,

and can ask for the number of alternatives.

As illustrated by Eq. (2), the differential of a thermo-

dynamic potential is a linear combination comprising the differentials of its natural variables; the linear coefficients

are the conjugates to the natural variables. Out of the

two conjugate pairs (S, T) and (V, P), we can construct

T

and

P

. She should then ask for

G
(T, P),

2. Four thermodynamic potentials

four pairs of natural variables: (i)

S

and

V

, (ii)

S

and

P

,

As explained in Sec. 1, four physical variables are associated with the gas in the cylinder: S, and

When these variables are increased or reduced, the ther-

modynamic potentials and change. The changes dU

in the Internal Energy are especially simple to describe,

since the First Law of Thermodynamics states that any

dU is a linear combination of the changes in its natural

variables [3],

(iii) and , and (iv) and

  • T
  • V
  • T
  • P

. We have, therefore, four

T

,

P

,

V

.

thermodynamic potentials. Two of them were introduced

in Sec. 1. The other two are the Helmoltz free energy

F(T, V ) and the enthalpy H(S, P).

  • U
  • G

2.1. Expressions for the four thermodynamical potentials

We now describe the schematic procedure determining each of the thermodynamic potentials in Table 1. The

procedure is schematic because it does not explain how

the resulting quantities can be expressed as functions of the pertinent natural variables, a task discussed in

Sec. 2.2.

dU = T d S − P d V,

(2) with linear coefficients T and P.
Given U(S, V ), Eq. (2) yields the temperature

ꢀꢀ

∂U(S, V )
∂S
∂U ∂S

ꢀꢀ

Consider the Helmholtz Free Energy, the thermody-

T =

≡

,

(3)

V

namic potential whose natural variables are

T

and

V

. Our

  • Amaku et al.
  • e20190127-3

analysis starts with the First Law of Thermodynamics,

Eq. (2), which expresses changes in the Internal Energy

Substitution of the right-hand side of Eq. (2) for dU

on the right-hand side of Eq. (11) then shows that

as a linear combination of the changes in

T

and

V

. We

dG = −SdT + V dP.

(13)

want to substitute the variable

T

for

S

. To this end, we

subtract d(T S) from the right-hand side of Eq. (2), to

As we have already pointed out, these manipulations

show that

are schematic. To make the equalities for H, and

practical, we must express the right-hand sides of Eqs. (6) (9), and (12) as functions of the natural variables (T, V ),

F

,

G

,

dF = dU − d(T S),

(5)

from which we obtain an expression for the Helmholtz

Free Energy:

(

S, P), and (T, P), respectively. Section 2.2 explains how

this can be done.

F = U − T S .

Moreover, from Eqs. (2) and (5), it follows that

dF = −SdT − P d V,

(6)

2.2. Expression of the four thermodynamical potentials as functions of their natural variables.

(7)

Consider, as an example, the expression for the Helmholtz

Free Energy derived in Sec. 2.1, Eq. (6). The first term

on the right-hand side, U, is known as a function of

the entropy and volume. Since we are interested in the

which expresses changes in the Free Energy as a linear combination of the changes in its natural variables,

Tand V .

Free Energy, we want to express

U

, and also the entropy,
Next, we want to calculate the Enthalpy,

H

=

H
(S, P).

which is a factor in the last term on the right-hand side,

We want, therefore, to substitute a term proportional to

dP for the term proportional to dV on the right-hand

side of Eq. (2). We then write

as functions of

our problem:

T

and

V

. The following sequence solves

  • ꢁ
  • ꢂ

∂U ∂S dH = dU + d(P V ),

(8)

Step 1 From T =

, find T = T(S, V );

Step 2 Invert the expressVion obtained in Step 1 to find

S = S(T, V );

which shows that

H = U + P V.

(9)
Step 3 Substitute S(T, V ) for S on the right-hand side

of Eq. (6) to find F(T, V ).

Equalities such as Eq. (8) have simple physical inter-

pretations. An Enthalpy change ∆ at constant pressure

is the sum of the associated internal-energy change ∆U

with the work that must be done on the surroundings to vacate the volume ∆

H

As we shall see in Sec. 4.2, no information is lost in this

procedure. Equation (6), which defines the Helmholtz Free Energy, is a Legendre Transformation, a procedure designed to ensure that the resulting potential

P

∆

V
V

. One can likewise interpret all the other thermodynamics potentials, as explained by

Schroeder [1].

  • [
  • F(T, V )] contain the same information as the original

one [U(S, V )].

From the practical viewpoint, it is more important to

express the changes in the Entropy as a linear combi-

Analogous algebra reduces Eq. (9) to an expression for

H

(

S, P), and Eq. (12) to an expression for

G
(T, P). For

nation of the changes in its natural variables,

S

and

P

.

a more detailed presentation, see Zia et al. [6].

With this goal in mind, we substitute the right-hand side of Eq. (2) for dU on the right-hand side of Eq. (8), which

yields the expression

The left-hand column of Table 1 collects the central

results inSec. 2.1. For each thermodynamic potential

U

,

F

,

H

, and

G

, the right-hand column presents the expres-

sions for the two conjugate variables directly obtained

from Eqs. (2), (7), (10), and (13), respectively.

dH = T d S + V dP.

(10)

Experimentalists find Eq. (10) very convenient. Work-

ing at fixed pressure, they only have to measure the heat

3. Multiplicity of the thermodynamic potentials

dQ

=

T d S that is exchanged with the environment to

determine the change in Enthalpy.

Finally, to determine the Gibbs Free Energy

G
(T, P),

Equation (2) covers systems with only two degrees of

freedom. It suits the gas in our cylinder well, which can

be described by two independent intensive variables: the

we add

d

(

PV ) to and subtract

d
(T S) from the right-hand

side of Eq. (2):

temperature

T

and pressure

most general situation.

P

. This, however, is not the

dG = dU − d(T S) + d(P V ),

which shows that
(11) (12)

Consider, for instance, instead of the gas in the cylinder,

a stretchable wire. Its volume can be varied and hence

G = U − T S + P V.

defines an additional degree of freedom. Let

X

=

L − L

0

  • e20190127-4
  • Thermodynamic Potentials and Natural Variables

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    Physical Chemistry II “The mistress of the world and her shadow” Chemistry 402 L. G. Sobotka Department of Chemistry Washington University, St Louis, MO, 63130 January 3, 2012 Contents IIntroduction 7 1 Physical Chemistry II - 402 -Thermodynamics (mostly) 8 1.1Who,when,where.............................................. 8 1.2CourseContent/Logistics.......................................... 8 1.3Grading.................................................... 8 1.3.1 Exams................................................. 8 1.3.2 Quizzes................................................ 8 1.3.3 ProblemSets............................................. 8 1.3.4 Grading................................................ 8 2Constants 9 3 The Structure of Physical Science 10 3.1ClassicalMechanics.............................................. 10 3.2QuantumMechanics............................................. 11 3.3StatisticalMechanics............................................. 11 3.4Thermodynamics............................................... 12 3.5Kinetics.................................................... 13 4RequisiteMath 15 4.1 Exact differentials.............................................. 15 4.2Euler’sReciprocityrelation......................................... 15 4.2.1 Example................................................ 16 4.3Euler’sCyclicrelation............................................ 16 4.3.1 Example................................................ 16 4.4Integratingfactors.............................................. 17 4.5LegendreTransformations.........................................
  • Exact and Inexact Differentials in the Early Development of Mechanics

    Exact and Inexact Differentials in the Early Development of Mechanics

    Revista Brasileira de Ensino de Física, vol. 42, e20190192 (2020) Articles www.scielo.br/rbef cb DOI: http://dx.doi.org/10.1590/1806-9126-RBEF-2019-0192 Licença Creative Commons Exact and inexact differentials in the early development of mechanics and thermodynamics Mário J. de Oliveira*1 1Universidade de São Paulo, Instituto de Física, São Paulo, SP, Brasil Received on July 31, 2019. Revised on October 8, 2019. Accepted on October 13, 2019. We give an account and a critical analysis of the use of exact and inexact differentials in the early development of mechanics and thermodynamics, and the emergence of differential calculus and how it was applied to solve some mechanical problems, such as those related to the cycloidal pendulum. The Lagrange equations of motions are presented in the form they were originally obtained in terms of differentials from the principle of virtual work. The derivation of the conservation of energy in differential form as obtained originally by Clausius from the equivalence of heat and work is also examined. Keywords: differential, differential calculus, analytical mechanics, thermodynamics. 1. Introduction variable x. If another variable y depends on the indepen- dent variable x, then the resulting increment dy of y is It is usual to formulate the basic equations of thermo- its differential. The quotient of these two differentials, dynamics in terms of differentials. The conservation of dy/dx, was interpreted geometrically by Leibniz as the energy is written as ratio of the ordinate y of a point on a curve and the length of the subtangent associated to this point.
  • Thermodynamic Potential of Free Energy for Thermo-Elastic-Plastic Body

    Thermodynamic Potential of Free Energy for Thermo-Elastic-Plastic Body

    Continuum Mech. Thermodyn. (2018) 30:221–232 https://doi.org/10.1007/s00161-017-0597-3 ORIGINAL ARTICLE Z. Sloderbach´ · J. Paja˛k Thermodynamic potential of free energy for thermo-elastic-plastic body Received: 22 January 2017 / Accepted: 11 September 2017 / Published online: 22 September 2017 © The Author(s) 2017. This article is an open access publication Abstract The procedure of derivation of thermodynamic potential of free energy (Helmholtz free energy) for a thermo-elastic-plastic body is presented. This procedure concerns a special thermodynamic model of a thermo-elastic-plastic body with isotropic hardening characteristics. The classical thermodynamics of irre- versible processes for material characterized by macroscopic internal parameters is used in the derivation. Thermodynamic potential of free energy may be used for practical determination of the level of stored energy accumulated in material during plastic processing applied, e.g., for industry components and other machinery parts received by plastic deformation processing. In this paper the stored energy for the simple stretching of austenitic steel will be presented. Keywords Free energy · Thermo-elastic-plastic body · Enthalpy · Exergy · Stored energy of plastic deformations · Mechanical energy dissipation · Thermodynamic reference state 1 Introduction The derivation of thermodynamic potential of free energy for a model of the thermo-elastic-plastic body with the isotropic hardening is presented. The additive form of the thermodynamic potential is assumed, see, e.g., [1–4]. Such assumption causes that the influence of plastic strains on thermo-elastic properties of the bodies is not taken into account, meaning the lack of “elastic–plastic” coupling effects, see Sloderbach´ [4–10].
  • W. M. White Geochemistry Appendix II Summary of Important Equations

    W. M. White Geochemistry Appendix II Summary of Important Equations

    W. M. White Geochemistry Appendix II Summary of Important Equations Equations of State: Ideal GasLaw: PV = NRT Coefficient of Thermal Expansion: ∂ α≡α ≡ 1 V V ∂T Compressibility: ∂V β≡β≡± 1 V ∂P Van der Waals Equation: RT a P= - 2 V ± b V The Laws of Thermdynamics: First Law: ∆U = Q + W 1 written in differential form: dU = dQ + dW 2 work done on the system and heat added to the system are positive. The first law states the equivalence of heat and work and the conservation of energy. Second Law: dQrev = TdS 3 Two ways of stating the second law are Every system left to itself will, on average, change to a condition of maximum probability and Heat cannot be extracted from a body and turned entirely into work. Third Law: lim S= 0 T → 0 3 This follows from the facts that S = R ln Ω and Ω =1 at T = 0 for a perfectly crystalline pure substance. Primary Variables of Thermodynamics The leading thermodynamic properties of a fluid are determined by the relations which exist between the volume, pressure, termperature, energy and entropy of a given mass of fluid in a state of thermodynamic equilibrium - J. W. Gibbs The primary variables of thermodynamics are P, V, T, U, and S. Other thermodynamic functions can be stated in terms of these variables. For various combination of these variables there are 1 W. M. White Geochemistry Appendix II Equation Summary characteristics functions. The characteristic function for S and V is one of the primary variables: U. Thus dU = TdS + PdV 5 Other Important Thermodynamic Functions What then is the use of thermodynamic equations? They are useful precisely because some quantities are easier to measure than others.
  • Basic Thermodynamics

    Basic Thermodynamics

    Basic Thermodynamics Handout 1 First Law of Thermodynamics Basically a statement of conservation of energy which includes heat as a form of energy: dU = ¯dQ + ¯dW; where ¯dQ is heat absorbed by the system, and ¯dW is work done on the system. U is a function of state called internal energy. Zeroth Law of Thermodynamics Suppose A, B and C are distinct thermodynamic systems. If A is in thermal equilibrium with B, and B is in thermal equilibrium with C, then A is in thermal equilibrium with C. Systems in thermodynamic equilibrium can be described in terms of physical variables that do not change with time. The Zeroth Law implies that there exists some physical property which systems that are in thermal equilibrium have in common, regardless of their size, shape, substance, etc. We call this property temperature.A thermometer is a device that can be used to assign a numerical value to temperature through measurements of a physical property. Constant-volume gas thermometer. The mercury reservoir is raised or lowered to adjust the difference in height h between the two mer- cury columns M and M 0 so that the volume of gas in the bulb + capillary is constant. Absolute temperature (perfect gas scale) lim ! (pV ) T (K) = 273:16 p 0 T ; limp!0(pV )TP where TP stands for the triple point of water. Right: Readings of a constant- volume gas thermometer for the temperature of condensing steam as a function of the pressure p3 at the triple point of water. Curves for different gases are shown.