Thermodynamic Definitions
System: A collection of material, or region in space, set aside for thermodynamic
investigation. The system is chosen by the investigator.
Boundary: Surface or surfaces, real or imaginary, by which either a closed or open system is
delineated from the surroundings for analysis.
Surrounding(Environment): Material, or regions in space, outside the boundaries of the
system(s) but which may interact and influence system behavior. Usually limited to
interactions with the system.
Closed system: A system which does not exchange mass with its surroundings.
Open system (Control Volume): A system which exchanges mass with its surroundings.
Control surface: The surface surrounding an open system or control volume.
Isolated system: A system which does not interact with the environment. Changes of
conditions in the environment have no influence on the behavior of isolated systems.
Adiabatic system: A system which has no thermal interaction between system and
surroundings. In practice, a thick layer of insulation material approximates an adiabatic wall.
The adiabatic system may experience interactions other than thermal.
Property: A property is a macroscopic characteristic of the system (mass, volume, energy,
pressure, temperature, specific volume). A certain quantity is a property if, and only if, its
change between two states is independent of the process (history).
Thermodynamic property: A particular property from among the larger set (see "property")
which, on the basis of experiment and experience, has been found to be important for
thermodynamic analysis.
Extensive property: One whose numerical value depends on the extent of the system. In a
homogeneous system, it is proportional to the mass of the system, e.g. volume, area, length,
energy.
Intensive property: One whose numerical value is independent of the extent of the system,
e.g. density, pressure, stress, temperature.
Temperature: The property of a system which determines whether or not it is in thermal
equilibrium with another system.
Energy: A quantity which has the capacity for producing a change of state in a closed system.
Work: A form of energy transfer. Work occurs when a force acts over a displacement. Work
is said to be done by a system if the sole effect external to the system can be made equivalent to
raising a weight.
Heat Transfer: Another form of energy transfer. The energy transferred between systems, or
between a system and its surrounding, due to a temperature difference.
Dimension: An independent quantity which describes a system, a property of a system, or is
necessary in the analysis of a system. e.g. time, length, area, volume, mass, temperature, force,
energy.
Unit: The name of the arbitrary chosen element, used in assigning a numerical value to a
dimension, and adopted as a standard for reporting measurements and conducting analysis.
SI and British units used in this course. Conversion factors: See inside of book cover.
Thermodynamic state: The condition or circumstances of a system as described by values of
its thermodynamic properties. When values are known for a sufficient number of properties to
complete the description, the state of the system is said to be "known". The minimum set of
independent thermodynamic properties sufficient to describe the state of the system is
sometimes called the thermodynamic coordinates.
Equilibrium: A state of affairs usually characterized by stability or the absence of tendency
for spontaneous change. Systems which, upon isolation, are observed to exhibit no change in
properties (and therefore state) are said to be in equilibrium.
Thermodynamic equilibrium: The condition of a system such that all influences (thermal,
mechanical, chemical, etc.) which have the potential for altering its state are in balance.
Thermal equilibrium: A condition said to exist within a system or between a system and its
surroundings, or between two systems, when, despite intimate thermal contact, no change in
properties is observed.
Mechanical equilibrium:
No net force exists on or in the system acting from the
surroundings or from within the system. Furthermore, no work transfers are occuring within
subsystems and all parts of the system are at rest relative to eachother and the surroundings.
Typically mechanical equilibrium of a system is reached prior to thermal equilibrium after a
process has occurred.
Process: This describes the continuous series of state changes undergone by a system due to
interactions between the system and another system, between the system and its surroundings,
or due to an initial lack of equilibrium in the system which caused it to change spontaneously.
Path: The locus of states through which a system passes when going from an initial state to a
final state. For some systems, the thermodynamic state is described by only two independent
properties. Hence, the path is a trace on a plane.
Quasistatic (or quasi-equilibrium) process: A process conducted in such a manner (i.e.
slowly, without friction) that the system is at all times only infinitesimally removed from a
state of thermodynamic equilibrium.
Adiabatic process: A process in which no heat is exchanged with the environment.
Cycle: A process, or series of processes, in which the initial and final states are identical.
Laws: Experimental observations on systems that have been formulated into mathematical
statements.
Zeroth law of thermodynamics: When each of two subsystems are in thermal equilibrium
with a third subsystem (i.e. thermometer), they are in thermal equilibrium with each other.
First law of thermodynamics: The value of the net work done by or on a closed system
undergoing an adiabatic process between two given states depends solely on the end states and
not on the details of the adiabatic process. (Energy conservation statement) The change in the
total energy of a closed system during a process is equal to the heat transfer to the system
minus the work done by the system.
Second law of thermodynamics: (Clausius statement) It is impossible for any system to
operate in such a way that the sole result would be an energy transfer by heat from a cooler to a
hotter body. (Kelvin-Planck statement) It is impossible for any system to operate in a
thermodynamic cycle and deliver a net amount of work to its surroundings while receiving
energy by heat transfer from a single thermal reservoir.
Macroscopic thermodynamics: (classical thermodynamics) The continuum approximation
holds so that the behavior of the system can be represented using observations on the system as
a whole. The properties of a system are defined at a point rather than being averaged over the
system. This course will be concerned only with classical thermodynamics.
Microscopic thermodynamics: (statistical thermodynamics) The continuum approximation
breaks down. Statistical methods are necessary to predict an average or macroscopic behavior
of the system.
Continuum approximation (Knudsen number): Macroscopic averages suffice over
microscopic behavior. The mean free path is small with respect to the characteristic size of the
system of interest, Kn = /Lchar << 1.
Mean free path (: The average distance a molecule moves within a gas at a constant speed
along a straight line between successive collisions.