# Glossary

Absolute pressure

Absolute pressure is a pressure that is measured relative to an ideal reference, or absolute vacuum. It is the sum of the atmospheric pressure and the gauge pressure. Absolute pressure is ALWAYS a positive number.

Absolute temperature

Absolute temperature, also called thermodynamic temperature, is the temperature expressed on the Kelvin or Rankine scale. Absolute zero (0 K or 0 R) is the minimum possible temperature. Matter at absolute zero is in the state of lowest possible (minimum) energy.

An adiabatic process is a process, in which heat transfer does NOT occur between a system and its surroundings.

Boundary

Boundary or system boundary refers to the surface that separates the system and its surroundings.

Boundary work

Boundary work refers to the work done by a substance at the system boundary due to the expansion or compression of the substance.

Chemical equilibrium

Chemical equilibrium is a state in which the forward and backward reactions proceed at the same rate, causing no net change of the concentrations in either the reactants or the products. A system free from chemical reactions is in chemical equilibrium.

Clausius statement

It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body (heat sink) to a higher-temperature body (heat source).

Closed system

A closed system is a system of a fixed mass. Mass transfer cannot happen between a closed system and its surroundings.

Constant-pressure specific heat

Constant-pressure specific heat is a property of a substance. It equals to the amount of energy required to raise the temperature of one unit mass (i.e., 1 kg) of the substance by one degree in an isobaric process.

Constant-volume specific heat

Constant-volume specific heat is a property of a substance. It equals to the amount of energy required to raise the temperature of one unit mass of the substance by one degree in an isochoric process.

Continuum

Continuum refers to a continuous homogeneous matter distributed throughout a system.

Control volume

Control volume is also called open system. It is a selected region in space, which allows mass and energy to transfer across the boundary between the system and its surroundings.

Cycle

A cycle consists of a series of processes. The final state of a cycle is always identical to its initial state.

Density

Density is the mass per unit volume of a body or a system.

Enthalpy

Enthalpy is a thermodynamic property. It is defined as the sum of the internal energy of a system and the flow work associated with the flowing fluid.

Entropy

Entropy is a thermodynamic property, which quantifies the degree of disorder of a system resulting from the dispersal of energy and matter in a process.

Equation of state

An equation of state is an expression that relates pressure, temperature and specific volume of a gas.

Equilibrium

Equilibrium refers to a uniform condition throughout a system.

Equilibrium state

An equilibrium state refers to a state of a system in equilibrium.

Extensive property

An extensive property refers to a thermodynamic property of a system, whose magnitude depends on the mass of the system. Examples of extensive properties include mass, volume, internal energy, enthalpy, and entropy.

Fusion line

Fusion line is a curve that represents the transition between the solid and liquid phases of a substance.

Heat

Heat is a form of energy. Heat transfer will take place between two objects if they are at different temperatures.

Heat engine

Heat engine is a device that produces work by absorbing heat from a high-temperature body (heat source) and rejecting the waste heat to a low-temperature body (heat sink).

Heat sink

A heat engine, refrigerator or heat pump must operate between a high-temperature body and a low-temperature body. The low-temperature body is called heat sink.

Heat source

A heat engine, refrigerator or heat pump must operate between a high-temperature body and a low temperature body. The high-temperature body is called heat source.

Ideal gas

An ideal gas is a gas that obeys the ideal gas equation of state, Pv=RT.

Intensive property

An intensive property is a thermodynamic property that does not depend on the mass of a system. Examples of intensive properties include pressure, temperature, density, specific volume, specific internal energy, specific enthalpy, and specific entropy.

Internal energy

Internal energy is a form of thermal energy. From a macroscopic level, it is strongly associated with the temperature of a system. From a microscopic level, it is associated with the motions and structure of the molecules of a system.

Irreversibilities

Irreversibilities refer to factors that render a process irreversible.

Isentropic process

An isentropic process refers to a process that is reversible and adiabatic. The entropy remains constant in an isentropic process.

Isobaric process

An isobaric process refers to a process whose pressure remains constant.

Isochoric process

An isochoric process refers to a process of constant specific volume.

Isolated system

An isolated system cannot exchange mass or energy with its surroundings.

Isothermal process

An isothermal process refers to a process whose temperature remains constant.

Kelvin-Planck Statement

It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.

Mechanical equilibrium

Mechanical equilibrium refers to an equilibrium condition, in which the pressure of a system has no tendency to change over time.

Open system

Open system is also called control volume. It is a selected region in space, which allows mass and energy to transfer across the boundary between the system and its surroundings.

Phase diagram

Phase diagram is a graphical representation of a substance's state (solid, liquid or vapour) under different conditions of temperature and pressure.

Phase equilibrium

Phase equilibrium is an equilibrium condition. For a system consisting of a mixture of multiple phases, if the composition of the mixture remains constant over time, the system is in phase equilibrium.

Pressure

Pressure is the normal force exerted by an object on a surface per unit area of the surface.

Process

A process refers to the change in a system from one state to another state.

Pure substance

A pure substance refers to a matter that has a homogeneous and definite chemical composition. A pure substance may exist in a single phase or as a multi-phase mixture.

Quasi-equilibrium process

A quasi-equilibrium process refers to a process, in which all states are equilibrium states.

Real gas

A real gas refers to a gas, whose pressure, temperate and specific volume behaviour cannot be represented by the ideal gas equation of state.

Reversible process

A reversible process refers to a process that can be reversed without leaving any changes in either the system or its surroundings. In a reversible process, both the system and its surroundings can always return to their original states.

Single phase

Single phase refers to the solid, liquid or vapour (or gaseous) phase that a substance exists at a certain condition.

Specific boundary work

Specific boundary work is the boundary work done by one unit mass of a substance.

Specific enthalpy

Specific enthalpy is the enthalpy per unit mass of a system.

Specific entropy

Specific entropy is the entropy per unit mass of a system.

Specific heat

Specific heat, also called heat capacity, is a thermodynamic property to quantify the energy storage capacity of a substance. It is defined as the amount of heat required to raise the temperature of one unit mass of a substance by one degree.

Specific internal energy

Specific internal energy is the internal energy per unit mass of a system.

Specific property

A specific property is the corresponding extensive property per unit mass. Examples of specific properties include specific volume, specific internal energy, specific enthalpy, and specific entropy. Specific properties are intensive properties.

Specific volume

Specific volume is the volume per unit mass of a system. It is the reciprocal of density.

State

A state refers to a specific condition of a system that is described by a unique set of thermodynamic properties, such as pressure, temperature, specific volume, specific enthalpy, and so on.

A steady flow through a control volume refers to a flow, in which the properties, such as the mass and energy of the control volume remain unchanged over time.

Sublimation line

The sublimation line is the curve that represents the transition between the solid and vapour phases of a substance in a phase diagram.

Surroundings

In thermodynamic analysis, the universe is divided into two parts: a system and its surroundings. Surroundings refer to the rest of the universe outside of the system.

System

In thermodynamic analysis, the universe is divided into two parts: a system and its surroundings. A system refers to a selected quantity of matter or a region in space.

Temperature

Temperature is a measurable thermodynamic property that indicates the hotness or coldness of a body.

Thermal equilibrium

Thermal equilibrium is an equilibrium condition. A system in thermal equilibrium has a uniform temperature everywhere.

Thermodynamics

Thermodynamics is a branch of science. It originally focused on the scientific theories of heat-work conversion, and the operations and efficiency improvement of heat engines. Nowadays, the applications of thermodynamics have extended to all fields related to energy conversion and conservation.

Transient flow

A transient flow refers to a flow through a control volume, in which the properties, such as the mass and energy of the control volume change over time.

Triple point

The triple point refers to a unique state of a substance, at which the three phases, solid, liquid and vapour, coexist in equilibrium.

Vapourization line

The vapourization line refers to the curve that represents the transition between the liquid and vapour phases of a substance in a phase diagram.

Work

Work is a form of energy that is transferred to or from a body by applying a force on that body along a displacement.