Test 1 Flashcards
Conservation of energy principle
states that during an interaction, energy can change from one form to another but the total amount of energy remains constant. That is, energy cannot be created or destroyed (see first law of thermodynamics).
First law of thermodynamics
is simply a statement of the conservation of energy principle, and it asserts that total energy is a thermodynamic property. Joule’s experiments indicate the following: For all adiabatic processes between two specified states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process. It may be expressed as follows: Energy can be neither created nor destroyed; it can only change forms. The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process.
Second law of thermodynamics
(increase of entropy principle) is expressed as the entropy of an isolated system during a process always increases or, in the limiting case of a reversible process, remains constant. In other words, the entropy of an isolated system never decreases. It also asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.
Classical thermodynamics
is the macroscopic approach to the study of thermodynamics that does not require knowledge of the behavior of individual particles.
Statistical thermodynamics
is an approach to thermodynamics more elaborate than classical thermodynamics, based on the average behavior of large groups of individual particles.
Dimensions
are any physical characterizations of a quantity
Units
are the arbitrary magnitudes assigned to the dimensions.
Primary or fundamental dimensions
such as mass m, length L, time t, and temperature T, are the basis for the derivation of secondary dimensions.
Secondary or derived dimensions
such as velocity V, energy E, and volume V, are expressed in terms of the primary dimensions
English system
which is also known as the United States Customary System (USCS), has the respective units the pound-mass (lbm), foot (ft), and second (s). The pound symbol lb is actually the abbreviation of libra, which was the ancient Roman unit of weight.
Metric SI
(from Le Système International d’ Unités), which is also known as the International System, is based on six fundamental dimensions. Their units, adopted in 1954 at the Tenth General Conference of Weights and Measures, are: meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, degree Kelvin (K) for temperature, candela (cd) for luminous intensity (amount of light), and mole (mol) for the amount of matter.
Joule
(J) is a unit of energy and has the unit “newton-meter (N·m).”
Btu (British thermal unit)
is the energy unit in the English system needed to raise the temperature of 1 lbm of water at 68°F by 1°F.
Calorie
(cal) is the amount of energy in the metric system needed to raise the temperature of 1 g of water at 15°C by 1°C.
Watt
(W) is a unit that is equivalent to joule per second (J/s).
Secondary units
are expressed in terms of the primary units.
Unity conversion ratios
are ratios of units that are based on the definitions of the units in question that are identically equal to 1, are unitless, and can be inserted into any calculation to properly convert units.
Surroundings
are everything outside the system boundaries.
Boundary
is the real or imaginary surface that separates the system from its surroundings. The boundary of a system can be fixed or movable.
Closed system
consists of a fixed amount of mass (control mass), and no mass can cross its boundary. But energy, in the form of heat or work, can cross the boundary.
Isolated system
is a closed system in which energy is not allowed to cross the boundary.
Open system
is any arbitrary region in space through which mass and energy can pass across the boundary.
Control volume
(also see open system) is any arbitrary region in space through which mass and energy can pass across the boundary. Most control volumes have fixed boundaries and thus do not involve any moving boundaries. A control volume may also involve heat and work interactions just as a closed system, in addition to mass interaction.
Property
is any characteristic of a system. Some familiar properties are pressure P, temperature T, volume V, and mass m. The list can be extended to include less familiar ones such as viscosity, thermal conductivity, modulus of elasticity, thermal expansion coefficient, electric resistivity, and even velocity and elevation.
Extensive properties
are those whose values depend on the size—or extent—of the system. Mass m, volume V, and total energy E are some examples of extensive properties. Extensive properties of a nonreacting ideal- or real-gas mixture are obtained by just adding the contributions of each component of the mixture.
ecific properties
are extensive properties per unit mass. Some examples of specific properties are specific volume (v = V/m) and specific total energy (e = E/m).
Continuum
is a view of mass as continuous, homogeneous matter with no holes. Matter is made up of atoms that are widely spaced in the gas phase. Yet it is very convenient to disregard the atomic nature of a substance. The continuum idealization allows us to treat properties as point functions, and to assume the properties to vary continually in space with no jump discontinuities. This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules. This is the case in practically all problems, except some specialized ones.
Rarefied gas flow theory
applies to a substance in which the mean free path of its molecules is large compared to the characteristic length of the systems such that the impact of individual molecules should be considered, and the substance cannot be modeled as a continuum.
