سه شنبه ۲۷ شهریور ۰۳ | ۱۷:۵۵ ۸ بازديد
odynamics is simply an expression of the conservation of energy
principle, and it asserts that energy is a thermodynamic property. The second law of ther-
modynamics asserts that energy has quality as well as quantity, and actual processes occur
in the direction of decreasing quality of energy. For example, a cup of hot coffee left on a
table eventually cools, but a cup of cool coffee in the same room never gets hot by itself. The
high-temperature energy of the coffee is degraded (transformed into a less useful form at a
lower temperature) once it is transferred to the surrounding air.
Heat and Other Forms of Energy
Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electri-
cal, 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 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 g of water at 14.5 by 1°C.
Internal energy may be v
principle, and it asserts that energy is a thermodynamic property. The second law of ther-
modynamics asserts that energy has quality as well as quantity, and actual processes occur
in the direction of decreasing quality of energy. For example, a cup of hot coffee left on a
table eventually cools, but a cup of cool coffee in the same room never gets hot by itself. The
high-temperature energy of the coffee is degraded (transformed into a less useful form at a
lower temperature) once it is transferred to the surrounding air.
Heat and Other Forms of Energy
Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electri-
cal, 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 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 g of water at 14.5 by 1°C.
Internal energy may be v
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