Internal energy

The energy of the system

The energy is an abstract concept created to quantify the interactions between the various components of a medium. It is very difficult to define and can apprehend itself only in concrete cases. Nevertheless, the concept energy is essential tools for the comprehension of the physical and chemical phenomena.

Let us consider a thermodynamic Système made up of molecules or atoms. This object has a total energy which can be broken up into two parts:

• kinetic Energy corresponding to the movement of the object as to the movements of the particles which constitute it,

• potential Energy due to the interactions of the object with the external via fields, gravitational medium, electric or magnetic but so due to the interactions between the molecules, ions, atoms, electrons, cores, Nucléon S… which constitute the system.

One sees that there exist two levels of reality for the energy of the system :

• a Macroscopic level , sensitive to our directions i.e. on our human scale, corresponding to the macroscopic kinetic energy of the system moving in a given reference frame: $E\; (c.macro)\; ~$ and with potential energies macroscopic of the system placed in fields of gravitation, electric or magnetic: $\backslash \; sum\; E\; (p.macro)\; ~$ .

• a Microscopic level inaccessible to our directions, corresponding to the microscopic kinetic energies which one can compare to the thermal Agitation particles: $\backslash \; sum\; E\; (c.micro)\; ~$ and with all the potential energies of microscopic interactions that one can assimilate, inter alia, with energies of Chemical bond and with energies of interactions between the Nucléon S (nuclear energies): $\backslash \; sum\; E\; (p.micro)\; ~$ .

Internal energy

the sum of microscopic energies constitutes energy interns U of the system, i.e. its clean energy .

$U\; =\; \backslash \; sum\; E\; (c.micro)\; +\; \backslash \; sum\; E\; (p.micro)\; ~$

The total energy of the system can be written:

• $E\; (total)\; =\; (E\; (c.macro)\; +\; \backslash \; sum\; E\; (p.macro))\; +\; (\backslash \; sum\; E\; (c.micro)\; +\; \backslash \; sum\; E\; (p.micro))\; ~$

• $E\; (total)\; =\; (E\; (c.macro)\; +\; \backslash \; sum\; E\; (p.macro))\; +\; U~$

Being given complexity of interactions to level microscopic, energy interns U is not calculable and it is what explains only the majority of the functions of state of the system, which depend on it (excluded the Entropie S), is not known absolutely. One can only calculate their variation. Internal energy is a function of state system. Its variation depends only on the final state and the initial state of balances and not on the nature of the transformation. Its Differential of the east an exact total differential .

Application to the chemical systems

In the case of a Chemical reaction, the reactional system will be at rest on a macroscopic scale (the engine is not moving in the fields of gravitation, electric and magnetic). Its macroscopic energy thus remains constant.

• $E\; (total)\; =\; U\; +\; constante~$

The variation of energy of the system during the chemical reaction is thus equal to the variation of its internal energy:

• $\backslash \; Delta\; E\; (total)\; =\; \backslash \; Delta\; U~$

The First principle of thermodynamics indicates that there is conservation of energy and in this case if the internal energy of the system varies it is that there is energy exchange with the external medium either in the form of work W or in the form of heat Q. One supposes obviously that the system is closed and thus that there is no exchange of matter.

One can write:

• $\backslash \; Delta\; U\; =\; (W\; +\; Q)\; ~$

This expression is used to summarize the statement of the First principle of thermodynamics .

If the system is isolated i.e. if there is no exchange with the external medium,

• $\backslash \; Delta\; U\; =\; 0~$ : internal energy remains constant.

If the transformation is cyclic, the system returns in its initial state and as internal energy is a function of state,

• $\backslash \; Delta\; U\; =\; (W\; +\; Q)\; =\; 0~$ : internal energy remains constant and $(W\; =\; -\; Q)\; ~$

If volume V is constant (isochoric Transformation) and if concerned work is only with the compressive forces, then work is null. From where:

• $\backslash \; Delta\; U\; =\; Q\_V~$

Under these conditions concerned heat becomes equal to the variation of the function of state U and does not depend any more of the followed way. This property is at the base of the Calorimétrie to constant volume practiced in a calorimetric Bombe .

Differential forms of internal energy

The principal differential forms of the Thermodynamique concerning energy interns are:

$\backslash \; qquad\; of\; =\; \backslash \; delta\; Q\; +\; \backslash \; delta\; W$

$\backslash \; qquad\; of\; =\; TdS\; -\; pdV$

or

$\backslash \; qquad\; of\; =\; C\_VdT\; +\; (LP)\; FD$

See too

• isochoric $C\_V$ Heat capacity.
• $P$ Pressure
• $V$ Volume
• $T$ Temperature
• $S$ Entropy
• $H$ Enthalpy
• Function of state, variable of state, equation of state

Simple: Internal energy

Random links:

Championship of the world of Formula 1 2004 | Buscate | Narrowly monitored trains | Boca del Río | Commutation (linguistic) | Rory_McGrath

© 2007-2008 speedlook.com; article text available under the terms of GFDL, from fr.wikipedia.org