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Important Terms Used In Thermodynamics

What is Thermodynamics?

The word thermodynamics comes from the two Greek words, “thermo” which means heat, and “dynamics” which means power, as in general known as heat power. This post is about important terms used in thermodynamics.

Thermodynamics is the science of energy with deals with the transfer of energy from one place to another and from one form to another form. It mainly deals with heat and work energy and their effect on the properties of substances.

According to the dictionary the branch of physical science deals with the relations between heat and other forms of energy (such as mechanical, electrical, or chemical energy), and, by extension, the relationships between all forms of energy.

The thermodynamic energy was developed to increase the efficiency of early steam engines. Thermodynamics works principally based on three laws of thermodynamics which are universally valid when applied to systems.

Terms Used in Thermodynamics

The following are the important terms used in thermodynamics:


It is the amount of matter contained in a given body and it does not vary with the change in its position on the earth’s surface. Mass is measured in Kg.


Force is defined as an agent, which produces or destroys the motion. The formula for force is Force=mass X acceleration (F=m Χ a). and the unit is Kg-m/s² or N (Newton)


Pressure is defined as the normal force per unit area. The formula for pressure is P=F/A.

  • N / m² or N / mm² or KN/ m² or KN / mm²
  • In general, the pressure is expressed in terms of ”BAR”.
  • One bar = 1 Χ 105 N / m².
  • Sometimes the pressure is also expressed in terms of ”PASCAL”.
  • One Pascal = 1 N / m².


The volume of the gas is defined as the space, which the gas occupies. It is written in V. It is expressed in m³. Where 1m³=1000 litres.

Specific Volume

The specific volume of a substance is its volume per unit mass. It is written as Vs. And it is expressed in m³/ kg. Where specific volume = volume/mass.


The density of a substance is its mass per unit volume. Density is written as ò. And it is expressed in kg / m³. 


The temperature of a substance is the degree of hotness or coldness of a body. Temperature is written as T. It is measured using a device called “THERMOMETER“.

The conditions of temperature and pressure at 0ºC and 760mm of Hg respectively are termed as N.T.P conditions. the conditions of temperature and pressure at 15º C and 760mm of Hg respectively are termed as S.T.P conditions.

Two scales are used for measuring the temperature,

  1. Centigrade Scale
  2. Fahrenheit Scale

The unit of temperature is expressed in terms of ºC (degree centigrade) or K (Kelvin). The relationship between the centigrade scale and the Kelvin scale is given by K=ºC+273. Similarly, the relationship between the Rankine scale and the Fahrenheit scale is given by R= ºF + 460. The relationship between the centigrade and Fahrenheit scale is given by C / 5 = F- 32 / 9.


Work is defined as the product of force (F) & distance moved (X) in the direction of applied force. The formula for work is W = F x X and it is expressed in terms of N-m or Joule, i.e., 1 N-m = 1 J (Joule).


Energy is defined as the capacity to do work. There are two types of energy Stored energy (S) & Transit energy (T). Energy is written as E. And it is expressed in terms of N-m or Joule, i.e., 1 N-m= 1 J (Joule).

1. Stored energy (S)

IT is the energy possessed by a system within its boundaries. Ex:

  1. potential energy (P.E = mgh),
  2. kinetic energy (K.E  =  ½  mv²),
  3. internal energy (U).


  1. Potential energy is the energy possessed by a body or system by its position.
  2. Kinetic energy is the energy possessed by a body or a system by its motion.
  3. Internal energy is the energy possessed by a body or a system due to its molecular arrangement or motion of molecules.

Always the total energy is the sum of PE‚ KE & Internal energy. Formula Total Energy = P.E+ K.E+U

2. Transit energy or energy in transition (T)

It is the energy possessed by the system, which is capable of crossing the system boundaries. Ex: heat, work & electrical energy.


Heat is defined as the energy transferred across the boundary of a system because of the temperature difference between the system & surroundings. The formula for heat is written as Q. And it is expressed in terms, of Joule.

Heat is taken as +ve if it flows into the system from the surroundings and it is taken as -ve if it flows from the system to the surroundings.


power is defined as the rate of doing work or the ratio of work done to the time taken. The formula of power is P= W/ t. And it is expressed in terms of Joule /sec or watt, i.e., 1 Joule/ sec = 1 watt.

Where Power = work done / Time taken.


It is defined as the sum of the internal energy (U) & product pressure & volume (PV). The formula is H = U+PV. It is expressed in terms of N-m or Joule.


It is the thermodynamic property of a working substance, which increases with the addition of heat and decreases with the removal of heat. Entropy, in general, is expressed as a function of pressure & temperature.  It is expressed in terms of KJ / ºK.

formula δQ = T ds

Qhere δQ = heat added or rejected. T = absolute temperature ds = Change in entropy.

Thermodynamic System and Surrounding

A thermodynamic system represents a finite portion of the universe. It refers to the section in which we note the observations. The surrounding is the rest of the universe.

Based on how matter and energy enter and exit the system, we can categorize the system into two major types. They are as follows:

Open System

These are the systems in which both matter and energy exchange occur.

A great example would be boiling water on the stove. The container functions as an open system, receiving heat energy from the outside and emitting water vapors.

Closed System

Systems that only interchange energy, not matter, with their surroundings are referred to as closed systems. A sealed water bottle that is kept in the refrigerator always has the same amount of water in it. However, it loses energy to the surroundings, causing the temperature of the water inside to drop.

The isolated system is a different kind of system. In this case, the system and its surroundings cannot exchange either matter or energy. A thermos flask is a perfect example of one of these systems.

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About Saif M

Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at www.theengineerspost.com

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