Thermodynamics I terms and definitions

Thermodynamics

The study of energy, energy transformations, and relationships among properties of matter.

System

The specific quantity of matter or region of space selected for analysis.

Surroundings

Everything external to the system.

Boundary

The real or imaginary surface separating a system from its surroundings.

Closed system

A system containing a fixed quantity of matter across whose boundary mass does not pass.

Control volume

A specified region in space through which mass may cross the boundary.

Control mass

Another term for a closed system containing a fixed amount of matter.

Open system

Another term for a control volume.

Control surface

The boundary surrounding a control volume.

Macroscopic approach

An approach that describes a system using measurable bulk properties rather than individual molecules.

Microscopic approach

An approach that considers molecular or particle-level behavior.

Property

A measurable characteristic of a system that describes its state.

State

The condition of a system as described by its properties.

Process

A change from one state to another.

Path

The sequence of intermediate states through which a system passes during a process.

Thermodynamic equilibrium

A condition in which the system has no tendency to undergo spontaneous change when isolated from its surroundings.

Thermal equilibrium

Equilibrium in which temperature is uniform and there is no driving force for heat transfer within the system.

Mechanical equilibrium

Equilibrium in which there are no unbalanced pressure forces within the system.

Chemical equilibrium

Equilibrium in which there is no net tendency for chemical reactions or changes in chemical composition.

Intensive property

A property that does not depend on the amount of matter present.

Extensive property

A property whose value depends on the amount of matter in the system.

Specific property

An extensive property expressed per unit mass, making it an intensive quantity.

Specific volume

Volume occupied per unit mass; v = V/m.

Density

Mass per unit volume; rho = m/V.

Pressure

Normal force exerted per unit area.

Absolute pressure

Pressure measured relative to a perfect vacuum.

Gauge pressure

Pressure measured relative to local atmospheric pressure.

Vacuum pressure

The amount by which atmospheric pressure exceeds a pressure below atmospheric pressure.

Atmospheric pressure

The pressure exerted by Earth's atmosphere.

Temperature

A property used to indicate thermal state and determine the direction of heat transfer.

Kelvin scale

The absolute temperature scale used in SI, with zero at absolute zero.

Rankine scale

The absolute temperature scale corresponding to Fahrenheit units.

Celsius scale

A temperature scale related to Kelvin by T(K) = T(C) + 273.15.

Fahrenheit scale

A temperature scale related to Rankine by T(R) = T(F) + 459.67.

Absolute zero

The theoretical lowest possible temperature on an absolute temperature scale.

SI system

The International System of Units based primarily on meters, kilograms, seconds, and kelvins.

English Engineering units

A unit system commonly using feet, pounds-force, seconds, and degrees Rankine or Fahrenheit.

Thermodynamic modeling

The process of simplifying a real physical system into a model that can be analyzed using thermodynamic principles.

State postulate

The principle that for a simple compressible system in equilibrium, two independent intensive properties are sufficient to fix the state.

Independent properties

Properties that can be varied independently to establish a system's state.

Energy

The capacity associated with a system to produce effects such as work or heat transfer.

Kinetic energy

The energy associated with motion; KE = mV^2/2.

Potential energy

The energy associated with elevation in a gravitational field; PE = mgz.

Internal energy

The energy contained within a system associated with microscopic forms of energy.

Total energy

The sum of internal, kinetic, and potential energies; E = U + KE + PE.

Specific internal energy

Internal energy per unit mass, usually represented by u.

Work

Energy transfer across a system boundary associated with a force acting through a displacement or equivalent mechanism.

Heat

Energy transfer across a system boundary caused solely by a temperature difference.

Energy transfer

Movement of energy across a system boundary by heat, work, or mass flow.

Power

The rate at which energy is transferred or work is done.

Heat sign convention

Heat transfer into the system is taken as positive.

Work sign convention

Work done by the system on its surroundings is taken as positive.

Expansion work

Work associated with movement of a system boundary during expansion or compression.

Boundary work

Work associated with movement of a system boundary.

Quasiequilibrium process

A process that proceeds through a continuous sequence of states that are approximately in equilibrium.

Polytropic process

A process commonly modeled by pV^n = constant.

Energy balance

The accounting statement that energy cannot be created or destroyed, only transferred or transformed.

First Law of Thermodynamics

The principle of conservation of energy applied to thermodynamic systems.

Closed-system energy balance

The energy change of a closed system equals net energy transferred to it by heat and work.

Cyclic process

A process in which the system returns to its initial state.

Cycle energy balance

For a complete cycle, the net energy change of the system is zero, so net heat transfer equals net work transfer.

Power cycle

A thermodynamic cycle whose net effect is production of work.

Refrigeration cycle

A cycle designed to transfer heat from a low-temperature region to a higher-temperature region using work input.

Heat pump cycle

A cycle designed primarily to deliver heat to a high-temperature region using work input.

Thermal efficiency

The ratio of net work output to heat supplied for a power cycle.

Coefficient of performance (COP)

A measure of performance for refrigerators or heat pumps, defined as desired heat transfer divided by work input.

Steady-state operation

Operation in which properties and flow conditions at fixed locations do not change with time.

Transient operation

Operation in which system conditions change with time.

Energy storage

The accumulation of energy in a system or storage device for later use.

Pure substance

A substance with uniform and unchanging chemical composition.

Phase

A physically distinct, homogeneous form of a substance such as liquid, vapor, or solid.

Saturated liquid

A liquid at the state where vaporization is about to begin.

Saturated vapor

A vapor at the state where condensation is about to begin.

Two-phase liquid-vapor mixture

A state containing both saturated liquid and saturated vapor.

Superheated vapor

A vapor at a temperature higher than the saturation temperature at a specified pressure.

Compressed liquid

A liquid at a temperature below its saturation temperature at a specified pressure.

Saturation temperature

The temperature at which phase change occurs at a specified pressure.

Saturation pressure

The pressure corresponding to phase change at a specified temperature.

Critical point

The state at which saturated liquid and saturated vapor become indistinguishable.

Critical temperature

The temperature above which a vapor cannot be liquefied solely by increasing pressure.

Critical pressure

The pressure at the critical point.

Triple point

The unique state at which solid, liquid, and vapor phases coexist in equilibrium.

Quality

The mass fraction of vapor in a saturated liquid-vapor mixture.

Quality equation

The relation x = m_vapor/m_total for a two-phase mixture.

Property table

A tabulated source of thermodynamic properties at specified states.

Saturation table

A property table containing properties at saturated liquid and saturated vapor states.

Superheated vapor table

A table used to obtain properties of vapor states beyond the saturation region.

Compressed-liquid table

A table used to obtain properties of liquid states outside the saturation region.

Enthalpy

A thermodynamic property defined by h = u + pv.

Specific enthalpy

Enthalpy per unit mass; h = u + pv.

Specific heat at constant volume

The change in specific internal energy with temperature at constant volume, commonly denoted cv.

Specific heat at constant pressure

The change in specific enthalpy with temperature at constant pressure, commonly denoted cp.

Incompressible substance model

A model in which specific volume is treated as approximately constant.

Compressibility factor

The dimensionless quantity Z = pv/(RT), used to measure deviation from ideal-gas behavior.

Reduced pressure

Pressure divided by critical pressure.

Reduced temperature

Absolute temperature divided by critical temperature.

Universal gas constant

A constant relating pressure, volume, amount of substance, and temperature for ideal gases.

Specific gas constant

The gas constant per unit mass for a particular substance.

Ideal gas

A gas modeled by pv = RT, with behavior approximating negligible intermolecular effects.

Ideal-gas equation of state

The relation pv = RT for a fixed amount of gas on a specific basis.