The first law of thermodynamics postulates the balance of energy which states that the rate of the total energy of the body equals the power due to mechanical and thermal loads. This can be recast in the equation
$$\begin{aligned} {\dot{\mathcal {E}}} + {\dot{\mathcal {K}}} = {{\mathcal {P}^\text {mech}}}+ {{\mathcal {P}^\text {therm}}}\end{aligned}$$
(6)
where the internal energy is given by
\(\mathcal {E}\) and the kinetic energy by
\(\mathcal {K}\); the mechanical power is given by
\({{\mathcal {P}^\text {mech}}}\) and the thermal power by
\({{\mathcal {P}^\text {therm}}}\), cf. [
1]. The same axiom is given, equivalently, after integrating (
6) over time by
$$\begin{aligned} \mathcal {E}+\mathcal {K}= \mathcal {W}+ \mathcal {Q}+ c_\text {int} \end{aligned}$$
(7)
with the mechanical work
\(\mathcal {W}\), the thermal work
\(\mathcal {Q}\), and some integration constant
\(c_\text {int}\). The internal energy of the full body
\(\mathcal {E}\) consists of the total free energy
\(\int _\omega \Psi \ \mathrm {d}v\) and the heat
\(\int _\omega h^\text {in} \ \mathrm {d}v\), i.e., energy contributions which are neither related to external work nor mass transport. The heat is thus the amount of energy stored in the thermal movement of the atoms and measured by
\(h^\text {in} = \rho \theta {\bar{s}} \) with the absolute temperature
\(\theta \) and the mass-specific entropy
\({\bar{s}}=s/\rho \). The entropy can accordingly be interpreted as temperature-specific energy which, according to the 2nd law of thermodynamics, indicates the part of the internal energy that is not directly accessible by mechanical processes. The internal energy is then given by
$$\begin{aligned} \mathcal {E}= \int _\omega \rho \, \bar{\Psi } \ \mathrm {d}v + \int _\omega \rho \,\theta {\bar{s}} \ \mathrm {d}v = \int _\Omega \rho \, \bar{\Psi } J \ \mathrm {d}V + \int _\Omega \rho \, \theta {\bar{s}} J \ \mathrm {d}V. \end{aligned}$$
(8)
Here,
\(\bar{\Psi }\) denotes the mass-specific free energy density and
\(\rho \) the mass density. The material domain in the current configuration is termed as
\(\omega \), whereas the material domain in the reference configuration is indicated by
\(\Omega \). The transformation of the volume element in the current configuration to its equivalence in the reference configuration is performed by
\(\ \mathrm {d}v = J \ \mathrm {d}V\) with
\(J:= \mathrm {det}\varvec{F}\) and the deformation gradient
\(\varvec{F}:=\varvec{I}+ \partial \varvec{u}/\partial \varvec{X}\) with the identity tensor of second order
\(\varvec{I}\), see [
1]. The spatial coordinate in the current configuration is denoted as
\(\varvec{x}\) and in the reference configuration as
\(\varvec{x}(t=0)=\varvec{x}^{(0)}=:\varvec{X}\). The free energy can be expressed by
$$\begin{aligned} \int _\omega \Psi \ \mathrm {d}v = \mathcal {E}- \int _\omega \theta s \ \mathrm {d}v, \end{aligned}$$
(9)
while the kinetic energy is given by
$$\begin{aligned} \mathcal {K}= \int _\omega \frac{1}{2} \rho \, ||{\dot{\varvec{u}}}||^2 \ \mathrm {d}v \end{aligned}$$
(10)
with the velocity
\({\dot{\varvec{u}}}\). External forces can be divided into surface and body forces, indicated by the traction vector
\(\varvec{t}^\star \) at the boundary of the body
\(\partial \omega \) and the mass-specific body force
\(\bar{\varvec{b}}^\star \). Then, the mechanical work
\(\mathcal {W}\) is given by
$$\begin{aligned} \mathcal {W}= \int _{\partial \omega }\varvec{t}^\star \cdot \varvec{u}\ \mathrm {d}a + \int _\omega \rho \,\bar{\varvec{b}}^\star \cdot \varvec{u}\ \mathrm {d}v, \end{aligned}$$
(11)
whereas the mechanical power reads
$$\begin{aligned} {{\mathcal {P}^\text {mech}}}= \int _{\partial \omega }\varvec{t}^\star \cdot {\dot{\varvec{u}}} \ \mathrm {d}a + \int _\omega \rho \,\bar{\varvec{b}}^\star \cdot {\dot{\varvec{u}}} \ \mathrm {d}v = \int _\omega \varvec{\sigma }:\varvec{D}\ \mathrm {d}v + \int _\omega \rho \, \frac{\mathrm {D}}{\mathrm {D}t} ||{\dot{\varvec{u}}}||^2 \ \mathrm {d}v \ , \end{aligned}$$
(12)
see [
1]. Here,
\(\varvec{\sigma }\) denotes the Cauchy stress; the stretch rate tensor
\(\varvec{D}:=(\varvec{L}+\varvec{L}^T)/2\) is the symmetric part of the spatial velocity gradient
\(\varvec{L}:=\partial \varvec{v}/\partial \varvec{x}\). It is worth mentioning that we use pure Neumann boundary conditions, i.e.,
\(\partial \omega =\Gamma _\sigma \) and
\(\Gamma _u=\emptyset \) where
\(\Gamma _\sigma \) denotes the boundary with prescribed tractions and
\(\Gamma _u\) denotes the boundary with prescribed displacements. However, the complete derivation which follows holds true when also Dirichlet boundary conditions are used. We skip the straightforward inclusion of Dirichlet boundary conditions here for a more convenient presentation. The material time derivative is defined as
$$\begin{aligned} \frac{\mathrm {D}}{\mathrm {D}t} := \frac{\partial }{\partial t} + \varvec{v}\cdot \frac{\partial }{\partial \varvec{x}} \ . \end{aligned}$$
(13)
Thermal power is transferred into the system by the heat flux vector
\(\varvec{q}\) and generated within the system by the external heat source
\(\rho \bar{h}\). Let us define that the rate of the total energy of the body is increased when the heat flux vector is pointing into the body. Then, the thermal power is given by
$$\begin{aligned} {{\mathcal {P}^\text {therm}}}= & {} -\int _{\partial \omega }\varvec{q}\cdot \varvec{n}\ \mathrm {d}a + \int _\omega \rho \,\bar{h} \ \mathrm {d}v = - \int _\omega \nabla _{\varvec{x}}\cdot \varvec{q}\ \mathrm {d}v + \int _\omega \rho \,\bar{h} \ \mathrm {d}v \nonumber \\= & {} - \int _\Omega \nabla _{\varvec{X}}\cdot \varvec{q}^{(0)}\ \mathrm {d}V + \int _\Omega \rho ^{(0)}\,\bar{h} \ \mathrm {d}V \end{aligned}$$
(14)
where
\(\nabla _{\varvec{x}}\cdot \bullet \equiv \mathrm {div}\bullet \) indicates the divergence in the current configuration. The superscript
\(\bullet ^{(0)}\) refers to quantities evaluated in the reference configuration and
\(\rho ^{(0)}= J \, \rho \). The divergence in the reference configuration is indicated by
\(\nabla _{\varvec{X}}\cdot \bullet \equiv \mathrm {DIV}\bullet \). Thus, the amount of thermal exchange of the body with its surrounding
\(\mathcal {Q}\) is given by time integration of
\({{\mathcal {P}^\text {therm}}}\) which yields
$$\begin{aligned} \mathcal {Q}= -\int _\Omega \int \nabla _{\varvec{X}}\cdot \varvec{q}^{(0)}\ \mathrm {d}t \ \mathrm {d}V + \int _\Omega \int \rho ^{(0)}\,\bar{h} \ \mathrm {d}t \ \mathrm {d}V \ . \end{aligned}$$
(15)
Inserting the definitions for the internal energy
\(\mathcal {E}\), the kinetic energy
\(\mathcal {K}\), for the mechanical power
\({{\mathcal {P}^\text {mech}}}\) and the thermal power
\({{\mathcal {P}^\text {therm}}}\) into the balance of energy (
6), we obtain the local form in the current configuration
$$\begin{aligned} \rho \,\dot{\bar{\Psi }} + \rho \, \dot{(\theta {\overline{s}})} = \varvec{\sigma }:\varvec{D}- \nabla _{\varvec{x}}\cdot \varvec{q}+ \rho \,\bar{h} \end{aligned}$$
(16)
and in the reference configuration
$$\begin{aligned} \rho ^{(0)}\, \dot{\bar{\Psi }} + \rho ^{(0)}\,\dot{(\theta {\overline{s}})} = \varvec{P}:{\dot{\varvec{F}}} - \nabla _{\varvec{X}}\cdot \varvec{q}^{(0)}+ \rho ^{(0)}\,\bar{h} \ , \end{aligned}$$
(17)
see [
1], which have to hold for arbitrary processes. Here, the 1
\(^\text {st}\) Piola–Kirchhoff stress tensor has been used which is related to the Cauchy stress tensor by
\(\varvec{P}= {\varvec{\sigma }\cdot \mathrm {cof}\varvec{F}}\) with the cofactor tensor
\(\mathrm {cof}\varvec{F}:= J \, {\varvec{F}^{-T}}\) which transforms vectorial area elements from the reference configuration, termed as
\(\mathrm {d}\varvec{a}^{(0)}\), to the current configuration, termed as
\(\mathrm {d}\varvec{a}\), via
\(\mathrm {d}\varvec{a}=\mathrm {cof}\varvec{F}\cdot \mathrm {d}\varvec{a}^{(0)}\). Analogously, the heat flux vectors in reference and current configuration are related by
\(\varvec{q}^{(0)}= \varvec{q}\cdot \mathrm {cof}\varvec{F}\). It is worth mentioning that (
16) and (
17) give the energy balance for thermo-mechanically coupled problems. In case of other coupled problems, i.e., for chemical or electrical coupling, the energy balance has to be modified accordingly. For instance, the formulation of the free energy and the interaction to the surrounding in terms of work or power have to be adapted.