1 Basic Concepts and Balances
1.1. Thermal energy and the first law of thermodynamics
Let us recall that the first law of thermodynamics is a law of energy conservation. It introduces the “internal energy” U, which represents the sum of the energies (kinetic and potential) of the system.
Assuming that there is no mass exchange and that we are considering a closed system that is subject to a thermodynamic transformation between two states, initial (1) and final (2), the variation in internal energy, U(2) – U(1), is the sum of the following two terms:
- – the macroscopic works performed, W1→2; generally this is the work of the pressure forces;
- – the energy exchange between the system and its outside: Q1→2.
This is reflected by: ![]()
We can therefore deduce a formal definition of the thermal energy (or heat) that is exchanged by the system between the initial state defined by (1) and the final state defined by (2): ![]()
For the thermodynamic systems studied, the work involved is generally due to pressure forces. The work is then given by the integral of these forces:
For the systems encountered in heat transfer analyses, volumes are generally constant (isochoric systems). The pressure forces ...