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Thermodynamics is the branch of physics that deals deals with large scale reactions of a system that can be observed and measures and the relationship between properties such as energy, heat, pressure, temperaturework, etc. on a system.  It is a fundamental part of physics and is used to describe and understand the behavior of many different physical systems, ranging from tiny particles to large-scale processes like weather patterns and the behavior of stars.

Thermodynamic Index

The laws of thermodynamics describe the behavior of energy in different systems.  The first law states that energy cannot be created or destroyed, only converted from one form to another.  The second law states that the total entropy (a measure of the degree of disorder in a system) in a closed system will always increase over time, and that energy will always tend to flow from hotter to cooler objects.  The third law states that it is impossible to reach absolute zero (the lowest possible temperature).

Thermodynamics has many practical applications, including the design of engines, refrigeration systems, and other devices that convert energy from one form to another.  It is also important in chemistry, where it is used to study the behavior of chemical reactions and the properties of materials at different temperatures and pressures.


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Nomenclature & Symbols

System Types

  • Boundary  -  The real or imaginary surface that separates the system ftom the surroundings and can be fixed or movable.
  • Closed System  -  Exchanges only energy with its surroundings.  No mass can cross the system.
  • Isolated System  -  Keeps the energy and matter within the system and everything else out.  No transfer in or out.
  • Open System  -  Freely exchanges energy and matter with its surroundings.  Both mass and energy can cross the boundary.


Properties of a System

The identifiable and observable characteristics of a system by which it can be specified.


Properties of a System

The identifiable and observable characteristics of a system by which it can be specified.


Thermodynamic Processes

Any process that involves heat energy moving within a system or between systems.

  • Adiabatic Process  -  No heat is transferred to or from the system.
  • Cyclic Process  -  The system at the end is the same as the state at the beginning.  The system does not change over a complete cycle.
  • Irreversible Process  -  Cannot return both the system and the surroundings to their original conditions.
  • Isenthalpic Process  -  A process of constant enthalpy.
  • Isochoric Process  -  The system’s volume does not change.
  • Isobaric Process  -  The system’s pressure does not change.
  • Isothermal Process  -  The system’s temperature remains constant.
  • Non-quasi-static Process  -  Most of the processes happening around us can be termed non-quasi-static process.
  • Quasi-static Process  -  Slow enough for the system to maintain internal thermodynamic equilibrium.
  • Reversible Process  -  Can be made to retrace its path by differential changes in the environment.  It leaves no change in either the system or surroundings.


Thermodynamics Glossary


  • Adiabatic Compression  -  The external work done is equal to the increased internal energy of the air in the system. Here the heat is neither subtracted or added from the surrounding air to the system air.  As there is an increase in the temperature of the system the pressure of the system tends to be more than the volume.
  • Adiabatic Expansion  -  The ideal behaviour of a system where the temperature keeps on increasing and pressure remains constant.  It refers generally to a closed system.
  • Adiabatic Heat Drop  -  The heat energy released and theoretically capable of transformation into mechanical work during the adiabatic expansion of unit weight of steam or other vapour of gas.
  • Adiabatic Process  -  An adiabatic process describes a process that remains aloof from its surroundings.  It is a process in which no heat transfer occurs between a system and its surroundings.  Here, the temperature of the system can vary in order to avoid any heat transfer.
  • Air  -  A gas consisting principally of a mixture ot 78% nitrogen, 22% oxygen, 1% argon, and the other 4% minor gasses like approximately 0.4% carbon dioxide, 0.08% neon, 0.005% helium, 0.0001% krypton, 0.00005% hydrogen, and trace amounts of xenon gas.  The composition can vary according to local conditions.
  • Amagat's Law  -  States that the volume of a mixture is equal to the sum of the partial volume of its components.
  • Antoine Equation  -  The relationship between vapor pressure and the temperature of pure substances.
  • Atmospheric air  -  The air in the atmosphere, which normally contains some water vapor.



  • Celsius  -  A unit of temperature most commonly used through out the world.
  • Chemical Energy  -  Is when two substances are combined or mixed together (atoms and molecules interacting) to produce a chemical reaction.
  • Clean Steam  -  Steam that is generated from pure water, free from contaminants and impurities.
  • Closed System  -  Exchanges only energy with its surroundings, not matter.
  • Combustion  -  A reaction called rapid oxidation or burning produced with the right combination of a fuel, oxygen, and heat.
  • Combustion Efficiency  -  The amoount of heat released during combustion divided by the heating value of the fuel.
  • Complete Combustion  -  A combustion process in which all the carbon in the fuel burns to \(CO_2\), all the hydrogen burns to \(H_2O\), and all the sulfure (if any) burns to \(SO_2\).
  • Compression Ratio  -  The ratio of the maximum volume to the minimum volume in a cylinder.
  • Compressor  -  A device that forces air or gas into a smaller area increasing the volume and creating a usable force of energy.
  • Condensation  -  A physical change of a substance from a gas phase to a liquid phase.  The molecules are fast moving and far apare. 
  • Condenser  -  An outdoor part of the AC or heat pump that releases or collects heat.
  • Conductor  -  A material through which heat passes.
  • Critical Point  -  The point at which the saturated liquid and saturated vapor states are identical.
  • Critical Pressure  -  The ctitical pressure of a vapor is the pressure required to liquefy it at the critical temperature and is the highest pressure on the temperature-pressure graph for standard vapor.
  • Critical Properties  -  The properties of a fluid at a location where Mach number is unity.
  • Critical Ratio  -  The ratio of the stagnation to static properties when the Mach number is unity.
  • Critical State of a Substance  -  The state at which liquid and vapor coexist in equilibrium.  At critical state, latent heat of vaporization becomes zero.
  • Critical Temperature  -  The temperature of the vapor above which no pressure, however high, will produce liquefaction.
  • Critical Velocity  -  The velocity above which fluid flow is turbulent.
  • Critical Volume  -  The volume of substance at the critical point.


  • Dead State  -  When a system is in thermodynamic equilibrium with the envitonment.
  • Deficiency of Air  -  When the amount of air is less than the stoichiometer amount.
  • Density  -  The ratio of the amount of matter in an object compared to its volume.
  • Deposition  -  A physical change of a substance from a gas phase to a solid phase and does not pass through the transitional liquid phase.  This is exactly the opposite of sublimation.
  • Dry Air  -  Air that contains no water vapor.
  • Dry Bulb Temperature  -  The actual temperature of air.
  • Dry steam  -  Does not contain water held in suspension.  It is similar to superheated steam.
  • Dynamic Temperature  -  The kinetic energy per unit mass divided by the constant pressure specific heat and corresponds to the temperature rise during the stagnation process.


  • Efficiency  -  Is expressed in percentage and always less than 100%.
  • Elastic Modulus  -  See Young's Modulus
  • Emissivity  -  The surface depends on the material effectiveness in emitting energy as thermal radiation and varies between 0.0 and 1.0.

  • Enthalpy  -  Measures the sum of internal energy changes in heat under constant pressure of the system.
  • Enthalpy of Combustion  - The enthropy of reaction during a steady-flow combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a specific preasure and temperature and represents the amount of heat released.
  • Enthalpy Departure  -  The difference between the enthalpy of a real gas and the enthalpy of the gas at an ideal state and it represents the variation of the enthalpy of a gas with pressure at a fixed temperatiure.
  • Enthalpy Departure Factor  -  The difference between the entropy of a real gas at a given pressure and temperature and the entropy of the gas at an ideal gas state at the same pressure and temperature.
  • Enthalpy of Formation  -  The enthalpy of a substance at a specific state due to its chemical composition.
  • Enthalpy of Reaction  -  The difference between the enthalpy of the products at a specific state and the enthalpy of the reactants at the same state for a complete reaction.
  • Entropy  -  Measures the unavailable energy in a heat system.
  • Entropy Change of a Closed System  -  The entropy transfer accompanting heat transfer and the entropy generation within the system boundaries.
  • Equilibrium  -  In a equilibrium state there are no unbalanced potentials, or driving forces within the system.
  • Evaporation  -  The condition that occurs when heat is absorbed by the surface of a liquid and it changes to vapor.
  • Evaporative Condenser  -  Combines the principles of force circulation convection currents with the ability of a vaporizing liquid to absorb heat.


  • Fahrenheit  -  A unit of temperature used in the United States and a few other countries.
  • Fire Point  -  Lowest temperature at which a combusible fluid will burst into flame in the presence of an foreign ignition source.
  • First Law of Thermodynamics  -  The total amount of energy in the universe is constant and that it can neither be created or destroyed.
  • Flash point  -  The temperature at which combustion is initiated.
  • Fouling  -  Accumulation of unwanted deposits on heat transfer surfaces.  This creates an additional barrier for heat movement, reduces heat transfer efficiency.
  • Freezing  -  A physical change of a substance from a liquid phase to a solid phase.


  • Gas  -  Able to be compressed to fit a confined space and expanded when released.
  • Gas Compressibility Factor  -  A factor independent of the quantity of gas and determined by the character of the gas, the temperature, and pressure.
  • Gauge Pressure  -  the difference between the absolute pressure and the local atmospheric pressure.
  • Geothermal Heat Pump  -  Uses the ground as the heat source.


  • Heat  -  A form of energy that causes physical change in what is being heated.  The lack of heat is cold.
  • Heat-driven System  -  Refrigerator systems whose energy input is based on heat transfer from an external source.
  • Heat Exchanger  -  A device used to transer heat from one medium to another at different temperatures.  The heat transfer can be air or a liquid such as water or oil.
  • thermal conductivity 1Heat Flux  -  The heat flow rate transfer through a given surface, per unit surface.
  • Heat Transfer  -  The exertion of power that is created by heat, or the increase in temperature.  It is the transfer of heat from one system to another.  There are four ways to transfer heat: conduction, convection, mixing, and radiation.
  • Heat Transfer by Conduction  -  It is the flow of energy between two objects, or within one object, where there is a temperature differential.
  • Heat Transfer by Convection  -  Energy transferred by random molecular motion as well as energy transferred by the bulk motion of the fluid.
  • Heat Transfer Rate  -  The amount of hear transfered per unit of time per fluid or material.
  • Henry's Law  -  One of the gas laws, at a constant temperature, the volume of a gas which will dissolve into a solution is directly porportional to the partial pressure of that gas above the solution in equilibrium with the liquid.


  • Ideal Gas  -  Defined as one in which all collisions between atoms or molecules are perfectly elastic and in which there are no intermolecular attractive forces.
  • Ideal Gas Law  -  Used to predict pressure, temperature & volume changes in ideal gasses.
  • Ignition Temperature  -  The minimum temperature to which a fuel must be brought to start the combustion.
  • Inert Gas  -  A gas which does not normally combine chemically with the base metal or filler metal.
  • Internal Energy  -  The total of all energies associated with the motion of the molecules in the system.
  • Intensive Properties  -  See intrinsic properties.
  • Intrinsic Properties  -  Those properties which are independent of the mass of the system, such as density, pressure, and temperature.
  • Isolated System  -  Keeps the energy and matter within the system and everything else out.  No transfer in or out.
  • Isothermal Process  -  An isothermal process is a process that happens when there are no variations (constant) in the temperature of the system, but other parameters such as volume and pressure, can be changed accordingly.



  • Kelvin  -  A unit of temperature normally used for scientific calculations.
  • Kinetic Energy  -  The energy in moving objects or mass.  If it moves, it has kinetic energy.


  • Latent Energy  -  The internal energy associated with the phase of a system.
  • Latent Heat  -  The energy absorbed or released by a substance during a constant temperature or phase change from a solid to liquid, liquid to gas or vise versa.
  • Linear Thermal Expansion  -  A porportional change in the origional length and change in temperature due to the heating or cooling of an object.


  • Mass Fraction  -  The ratio of the mass of one component in a mixture to the total mass of the minture.
  • Matter  -  When you look around, everything you see or may not see in the universe is made up of matter.  If it has a mass and takes up space, it is matter.
  • Molecular Weight  -  The average mass of one molecule of a substance, 1⁄12 (0.083) the mass of carbon 12 atom.


  • Newton's Law of Cooling  -  The law states that the rate of heat loss of a body is directly porportional to the difference in the temperature between the body and its surrounding provided the temperature difference is small and the nature of radiating surface remains the same.


  • Open System  -  Freely exchanges energy and matter with its surroundings.
  • Open, Closed, and Isolated Systems  -  A thermodynamics system is a specific quanity of matter with a defined boundary and everything outside the boundary is the surroundings.  The system boundary can either be stationary or moving.  There are three types of thermodynamics systems: open, closed, and isolated.


  • Plant Steam  -  The steam generated in an industrial or commercial facility for various applications within the plant.



  • Radiant Heat  -  Heat communicated by radiation and transmitted by electromagnetic waves.
  • Rankine  -  Aunit of temperature used in the US engineering field. 0 is set at absolute zero.


  • Saturation Temperature  -  Temperature of the liquid surface corresponding to the pressure of the vapour in contact with in.
  • Saturated Phase  -  Any phase of a substance existing under saturated conditions, wherein two or more phases of a pure substance can exist togeather in equilibrium.
  • Saturated Vapor  -  A vapor at a temperature of the boiling point on the verge of condensing.
  • Second Law of Thermodynamics  -  States that heat cannot pass from a cold body to a hot one by a self-acting process without the help of an external influence.
  • Semi-perfect Heat  -  One which follows the ideal gas relations with specific heats being functions of temperature.
  • Sensible Heat  -  The heat added to a substance which increases its temperature but not the phase.
  • Shrinkage  -  The contraction in volume as a liquid or metal cools or freezes from its normal temperature.
  • Solubility  -  The maximum amount of solid that can be dissolved in a liquid at a specific temperature.
  • Specific Enthalpy  -  The enthalpy per unit mass.  It is also the total specific internal energy in a system and the product of pressure and specific volume.
  • Specific Heat  -  The amount of heat required to raise the temperature of a material 1 degree.
  • Specific Heat Capacity  -  The amount of energy required to increase the temperature of the substance by 1°C.
  • Steam  -  The invisible vapor (gas) when water is heated to its boiling point and passes from a liquid to a gaseous state.
  • Steam density  -  Has a higher density than water vapor, the higher the pressure the higher the steam density.
  • Steam Flow Regimes  -  Because steam can behave as a liquid and as a gas and everything inbetween, the way steam flows through a pipe can change through long straight runs.
  • Steam Quality  -  The porportion of saturated steam (vapor) in a saturated condensate (liquid) / steam (vapor) mixture.
  • Steam Users  -  Steam is used for many things in piping design.  In the oil and gas industry, steam is used to heat production tanks, keep flow lines from freezing and to increase oil production in formations that are very viscious.
  • Sublimation  -  A physical change of a substance from a solid phase to a gas phase and does not pass through the transitional liquid phase.
  • Subcooled Liquid  -  A liquid existing at a temperature lower than its saturated temperature, or at a pressure higher than its saturated pressure.
  • Superheat  -  Addition of heat to a fluid after it has completely vaporized.  The temperature increases but pressure does not.
  • Superheated Vapor  -  When vapor has absorbed more heat than is needed to vaporize.  It will not condense when small amounts of heat is removed.


  • Temperature  -  The amount of heat or cold, but it is neither heat or cold.  Temperature is expressed as a number that is related to energy and porportional to a type of energy, but it is not energy.
  • Temperature Differential  -  The difference between two specific temperature points of a volume at a given time in a system.
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      -  Caused by the change in temperature.
  • Thermal Analysis  -  A method of studying transformations in metal by measuring the temperature at which thermal arrests occure.
  • Thermal Contraction  -  The decrease in a linear dimension and volume of a material with a change of temperature.
  • Thermal Cracking  -  A fluid degradation that occurs when a fluid  is heated above the maximum bulk temperature.  Consequently, the fluid's viscosity, flash point, fire point and auto-ignition temperature reduce significantly.
  • Thermal Efficiency  -  The fraction of heat that is converted to work or desired output divided by required input.
  • Thermal Energy  -  The exertion of power that is created by heat, or the increase in temperature.
  • Thermal Expansion Coefficient  -  The percentage change in the length of the material per degree of temperature change, heated solid or liquid.
  • Thermal Fatique  -  Failure resulting from rapid cycles of alternating heating and cooling.
  • Thermal Insulation  -  Any material that is used as a barrier to heat transfer.
  • Thermal Insulator  -  Does not conduct heat readily and is used for either heat conservation or personnel protection.
  • Thermal Resistance  -  Measures the temperature difference by which an object or material resists a heat flow.

  • Thermal Stability  -  The ability of a fluid to resist degradation under high-temperature operating conditions.
  • Thermal Stress  -  Results from non uniform distribution of temperature.
  • Total Energy  -  The sum of numerous forms of energy such as chemical, electrical, magnetic, mechanical, nuclear, potential, thermal, and etc.




  • Waste Heat  -  Energy that must be dissipated to the atmosphere from a process such as the heat transfer from condensing steam in the condenser of a system power plant.
  • Work  -  The overcoming of resistance through space and is the measure of force x distance.




  • Zeroth Law of Thermodynamics  -  When two thermal systems are in equilibrium and they with a third, then all are equal to each other, meaning  \(A=B\)  and  \(B=C\)  then  \(A=C\)


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Density of an Ideal Gas
Dry Steam

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