Thermodynamics Definitions

 Important Definitions

1- Intensive properties:

Are those independent of the size of system such as(Temp., Pressure and density).

2- Extensive properties:

Are those dependent on the size of system such as (Mass, Volume and Energy). Stored Internal Energy

3- Specific properties:

Extensive properties per unit mass Mass & Energy conservation: Einestien Rule: E =MC2  where C is the speed of light =3 ×108 m/s

4- Mass flow rate:

amount of mass flowing across-section per unit time (ṁ).

5- State:

a condition of a system at which all the properties have fixed values.

6- Equilibrium:

implies a state of balance.

7- Thermal Equilibrium:

when temperature is the same throughout the entire system.

8- Mechanical Equilibrium:

there is no change in pressure throughout the entire system.

9- Phase Equilibrium:

the mass of each phase reached an equilibrium value and stayed there.

10- Chemical Equilibrium:

when chemical composition does not change with time.

11- The state postulate:

The state of a simple compressible system is completely specified by two independent intensive properties.


12- Process:

any change that a system undergoes from one equilibrium state to another.

13- Path:

A series of states through which a system passes through a process.

14- Quasi-equilibrium:

When a process proceed in such a manner that the system remains close to an equilibrium state at all times.

15- Isothermal:

a process during which the temperature is constant.

16- Isobaric:

a process during which the pressure is constant.


17- Isochoric:

a process during which the volume is constant. Thermodynamics can be approached in either of two, usually distinct, ways:

1- Statistical thermodynamics or microscopic or molecular approach

2- Classical thermodynamics or macroscopic approach


Newton’s Laws

1st law: 

If the net forces are balanced ( i.e. if the vector sum is zero ), the object will not accelerate

2nd law: 

The force is proportional to the rate of change of  Momentum

3rd law: 

If one object exerts a force on another, then the second object exerts an equal but opposite force on the first.


Energy Classification

1- Stored energy which is contained within the system boundaries. [potential energy ,kinetic energy, internal energy etc. ]

2- Energy in transition which crosses the system boundaries. [Heat, work , electrical energy etc. ]


Transferred Energy

Heat (Q) Driving potential : temperature difference

Work - mechanical ( Wm ) Driving potential : unbalance in mechanical forces

Work - electrical ( We ) Driving potential : voltage difference Intrinsic or internal energy


- Kinetic ( Uk ) Associated with absolute temperature

- Potential ( Up ) Associated with intermolecular or interatomic forces


- Chemical ( Uch ) Associated with changes in molecular structure


- Nuclear ( UNu ) Associated with changes in atomic structure

The internal energy U = Uk + Up + Uch + UNu

The energy of the system E = PE + KE + U

Total Energy E = 0.5mv2 + mgz+U

Van Der Waals EOS:


Compressed Liquid (sub-cooled): Liquid under conditions that’s  not about to vaporize.

Saturated Liquid: Liquid which is about to vaporize.

Saturated vapour: Vapour which is about to condense.

Superheated vapour: Vapour which is not about to condense. 

Saturation Temperature: the temperature at which a pure substance changes phase at a given pressure.

Latent Heat: the amount of heat released or absorbed during a phase change.

Compressibility factor: a measure of deviation from ideal gas behavior.

Z=1 for ideal gas Z < or > 1 for real gas

Z factor is approximately the same for all gases at the same reduced temperature and pressure(The principle of corresponding states).


Van Der Waals EOS:

Specific Heat: The energy required to raise the temperature of a unit mass of a substance by one degree. (kj ⁄kg.oC)

Heat: is the form of energy that transfers between two systems due to temperature difference (Q KJ).

Adiabatic Process: a process during which there is no heat transfer (Q=0).

Work: is the energy transfer associated with a force acting through a distance (KJ).

Path function: a property which magnitude depends on the path followed during a process as well as end states such as heat & work.

Point function: a property which depends on the state only and not on how the system reached it like volume.

V : Potential Difference

I : Current

Steady flow process: a one the fluid flows through a control volume steadily (No change with time).

Turbine: Work is positive as it’s done by the fluid.

Compressors, Pumps &Fans: Work is negative as it’s from another source and is supplied to the fluid.

Throttling valves: small devices cause a pressure drop which is accompanied with a temperature drop 

Joule-Thomson co-efficient: determine the magnitude of that temperature drop.

Isenthalpic action h1=h2.

Kelvin-Planck statement:

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

Or, ‘‘No heat engine can have a 100 % thermal efficiency’’

Heat Engine:

A device that converts heat to work.

1. Receives heat from a source.

2. Converts part of it to work.

3. Delivers the waste heat to a sink.

4. Operate on a cycle.

Such as , Gas Turbine and Car Engine.


Steam Power Plant:

Works on a thermodynamic cycle like 

(External combustion Engine).

Clausius statement:

‘’It’s 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 to a higher temperature one’’.

Energy efficiency rating EER:

The amount of heat removed from the cooled area in BTU for 1 wh of electricity consumed.


Devices that transfer heat from low temperature medium to a high temperature one.

Heat Pump: the same purpose of refrigerators but the reverse objective.

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Thanks for reading - 
Naitik Patel
Industrial Guide

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