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In classical thermodynamics heat and work have to be represented by inexact differentials $\delta Q$ and $\delta W$, but once you extend the classical thermodynamics space of variables with the variable … A good discussion of why heat and work are exact differentials $dQ(t)$ and $dW(t)$ when you include time is given in the well-known textbook by Prigogine & Kondepudi: «Modern thermodynamics: from heat …

composite system with a given energy, but without a thermodynamic temperature (e.g. a composite system made of two thermally isolated solids: one hot and the other cold) Using the fundamental equation of thermodynamics …

The fundamental quantity in thermodynamics is entropy, which is a function of $n$-variables $S=S(x_1, x_2,...,x_n)$. … Using the definition of average $$\langle A \rangle = \int A P \mathrm{d}x_1 \mathrm{d}x_2 \cdots \mathrm{d}x_n$$ the demonstration of the central result of linear nonequilibrium thermodynamics $$\ …

Using entropy changes where the variation of entropy is the sum of production and flows $$\frac{dS}{dt} = \frac{d_iS}{dt} + \frac{d_eS}{dt}$$ a steady or stationary state is one for which entropy is …

Which is the heat for a moving system if $ Q_0 $ is the heat for a system at rest? If you ask Planck, Einstein, von Laué, Pauli, or Tolman the heat $ Q $ for the moving system is given by $$ Q = \fr …

Mechanical work $dW_\mathrm{mech} = -pdV$ is due to a volume change for a pressure $p$. But other kind of thermodynamic works exist: Chemical work $dW_\mathrm{chem} = \mu dN$ involves change in comp …

Heat is not a property of a system. Heat is a process function. Temperature is a property of a system because is a state function. For instance, the state of a simple gas is given by temperature, pres …

A process is reversible if and only if there is not production of entropy. If you perform a process quasi-statically, you are minimizing the production of entropy (e.g. by adding infinitesimal weight …

The thermodynamic definition of temperature is $$T \equiv \left( \frac{\partial S}{\partial U}\right)^{-1} $$ where $S$ is the thermodynamic entropy of the system and $U$ its internal energy. The th …

Supercritical fluids are well described by real and ideal gas laws. A common equation of state for both liquids and solids is $$V_m = C_1 + C_2 T + C_3 T^2 - C_4 p - C_5 p T$$ where $V_m$ is molar …

The internal energy of an ideal gas only depends on temperature. This result is not restricted to free expansions, but is completely general. One arrives to this conclusion by using the Helmholtz equa …

Heat is a process quantity, not a system quantity, and there is no indicator that you can attach to the object, but you could use a thermometer to record the distribution of temperatures of the object …

By definition a reversible process in an isolated system cannot increase entropy. If entropy is increased during a process in an isolated system then the process is irreversible, by definition.

If we mean equilibrium statistical mechanics and classical thermodynamics, then statistical mechanics complements thermodynamics by providing explicit values to some coefficients that thermodynamics cannot … quantum thermodynamics is a proper nonlinear extension of quantum mechanics What is Quantum Thermodynamics? …

The classical thermodynamics version $\Delta S \ge 0$ for isolated systems is not fundamental. First it doesn't apply to open systems and has to be replaced by $\Delta_i S \ge 0$. … Precisely that is the reason why thermodynamics was invented to deal with such observations and complement Newtonian mechanics. …