Energy internal, definition

Definition.—An ideal gas is a fluid which obeys Boyle s law, and the internal energy of which is independent of the volume ... 

Consider a process at constant pressure for which the change in internal energy is AU and the change in volume is A V. It then follows from the definition of enthalpy in Eq. 9 that the change in enthalpy is... 

It has been seen thus far that the first law, when applied to thermodynamic processes, identifies the existence of a property called the internal energy. It may in other words be stated that analysis of the first law leads to the definition of a derived property known as internal energy. Similarly, the second law, when applied to such processes, leads to the definition of a new property, known as the entropy. Here again it may in other words be said that analysis of the second law leads to the definition of another derived property, the entropy. If the first law is said to be the law of internal energy, then the second law may be called the law of entropy. The three Es, namely energy, equilibrium and entropy, are centrally important in the study of thermodynamics. It is sometimes stated that classical thermodynamics is dominated by the second law. 

Bohr theory the first theory of atomic structure which involved definite internal energy levels for electrons. 

The formal definition of the electronic chemical hardness is that it is the derivative of the electronic chemical potential (i.e., the internal energy) with respect to the number of valence electrons (Atkins, 1991). The electronic chemical potential itself is the change in total energy of a molecule with a change of the number of valence electrons. Since the elastic moduli depend on valence electron densities, it might be expected that they would also depend on chemical hardness densities (energy/volume). This is indeed the case. 

Following from Equation (3.3), we say that internal energy is a state function. A more formal definition of state function is, A thermodynamic property (such as internal energy) that depends only on the present state of the system, and is independent of its previous history . In other words, a state function depends only on those variables that define the current state of the system, such as how much material is present, whether it is a solid, liquid or gas, etc. 

Substitution of this simple relationship into the definition of internal energy in Equation (3.5) yields... 

Fig. 2.4. Definition of appearance energy and visualization of changes in internal energy distributions, F(E), of relevant species upon electron ionization and subsequent fragmentation. The energy scale is shown compressed for the ions.

The energy increment that is passed on has been called the enthalpy excess, but might better be called the thermal or internal energy excess, for the products at the iso-choric adiabatic explosion temperature have, by definition, exactly the same total internal energy as the original undetonated explosive... 

A closely related quantity to the internal energy is the enthalpy, H. It, too, has SI units of joules and is defined as the internal energy plus the pressure-volume product, PV. As in most cases, we are concerned with changes in internal energy and enthalpy from one state to another, so that the definition of enthalpy for infinitesimal changes in state is... 

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