Energy can exist in numerous forms such
as thermal, mechanical, kinetic, potential, electrical, magnetic,
chemical, and nuclear, and their sum constitutes the total energy E
(or e on a unit mass basis) of a system. The forms of energy related
to the molecular structure of a system and the degree of the
molecular activity are referred to as the microscopic energy.
The sum of all microscopic forms of
energy is called the internal energy of a system, and is denoted by U
(or u on a unit mass basis). The international unit of energy is
joule (J) or kilojoule (1 kJ = 1000 J). In the English system, the
unit of energy is the British thermal unit (Btu), which is defined as
the energy needed to raise the temperature of 1 lbm of water at 60°F
by 1°F.
The magnitudes of kJ and Btu are almost
identical (1 Btu = 1.055056 kJ). Another well-known unit of energy is
the calorie (1 cal = 4.1868 J), which is defined as the energy needed
to raise the temperature of 1 gram of water at 14.5°C by 1°C.
Internal energy may be viewed as the
sum of the kinetic and potential energies of the molecules. The
portion of the internal energy of a system associated with the
kinetic energy of the molecules is called sensible energy or sensible
heat.
The average velocity and the degree of
activity of the molecules are proportional to the temperature. Thus,
at higher temperatures the molecules will possess higher kinetic
energy, and as a result, the system will have a higher internal
energy.
The internal energy is also associated
with the intermolecular forces between the molecules of a system.
These are the forces that bind the molecules to each other, and, as
one would expect, they are strongest in solids and weakest in gases.
If sufficient energy is added to the
molecules of a solid or liquid, they will overcome these molecular
forces and simply break away, turning the system to a gas. This is a
phase change process and because of this added energy, a system in
the gas phase is at a higher internal energy level than it is in the
solid or the liquid phase. The internal energy associated with the
phase of a system is called latent energy or latent heat.
The changes mentioned above can occur
without a change in the chemical composition of a system. Most heat
transfer problems fall into this category, and one does not need to
pay any attention to the forces binding the atoms in a molecule
together.
The internal energy associated with the
atomic bonds in a molecule is called chemical (or bond) energy,
whereas the internal energy associated with the bonds within the
nucleus of the atom itself is called nuclear energy. The chemical and
nuclear energies are absorbed or released during chemical or nuclear
reactions, respectively.
In the analysis of systems that involve
fluid flow, we frequently encounter the combination of properties u
and Pv. For the sake of simplicity and convenience, this combination
is defined as enthalpy h.
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