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  • A combination of two words, (THERM = HEAT & DYNAMICS = MOTION).
  • Thermodynamics is the branch of physics which deals with energy change involving all types of chemical and physical changes.
  • In thermodynamics, we basically deal with the macroscopic bodies or systems i.e., an assembly of a larger number of molecules so that it has certain values of pressure, volume, temperature.
  • Heat and Work are the two modes of energy interaction of one body with another.

Important Thermodynamic Terms


  • It is the part of the universe which is under observation.
  • It must be a macroscopic i.e., consisting of an extremely large number of particles so that it has some values (temp., pressure etc.).
  • This system may be a solid, liquid and gas or a combination of any of these.


  • Rest part of the universe which interacts with the system.
  • In general, surrounding is very large so any change in properties taking place can be neglected.


  • Anything which separates the system from its surrounding.
  • It may be imaginary, rigid or non- rigid, conductor of heat (diathermic) or non-conductor (adiabatic), permeable (mass transfer possibility) or non-permeable.

Read More- Thermodynamics: Objective Questions Answers


  • Depending upon the types of the boundary it is of following types-


A system having the capability to transfer both matter and heat.


This can transfer energy, heat but not mass.


It can neither transfer heat nor matter. A universe is an isolated system.

Property of a System

  • A measurable quantity.



  • Values remain same on the division of a system.
  • Examples are- pressure, temperature, density, vapor density, specific heat, surface tension, viscosity, molar mass, melting point etc.


  • Values change on the division of the system.
  • Examples are- volume, mass, internal energy, heat capacity, moles, entropy, enthalpy etc.


  • Extensive property defined per mole or gram changes to an intensive property.
  • Ratio of two extensive is intensive.
  • Extensive properties are additive but intensive are not.


  • The state of a thermodynamic system is basically described by its macroscopic property, for example, state of a gas is described by pressure, temperature, volume etc.


  • Quantities like pressure, volume, entropy etc. which help us to study the behavior of a thermodynamic system.
  • Its value depends on the present state and is independent of the past state and path followed to arrive at that state.

Path Function

  • When state function moves from point A to B, it follows a path to reach B from A. So, the variables or state functions which not only depend on ‘A’ (initial state) and ‘B’ (final state) but also the path followed.


  • The path along which change of state takes place or change of thermodynamic variables with time.


  • When a reaction is carried or a process having the potential to proceed without any external agency’s assistance.

Cyclic Process

  • A system after going through a number of different thermodynamic processes returns to its original state is called a cyclic process.
  • Change in all state functions must be zero (∆U = 0, etc.)

Reversible Process

  • Reversible processes are carried out at infinitesimally slow.
  • If the initial state can be achieved at any state by reversing the process.
  • It takes infinite time and infinite steps to show any appreciable changes.
  • It is a hypothetical process.
  • It exists in all state of equilibrium at each state.
  • In the case of an ideal gas, the ideal gas (PV = nRT) equation can be applied throughout the process.
  • Friction, viscosity etc. should not be present for the process to be reversible.

Irreversible Process

  • If the initial state of the process can be restored without any changes in the surrounding or system.
  • It takes finite time and finite steps to show any appreciable changes.
  • It exists in the state of equilibrium at initial and final states.
  • The ideal gas equation can be applied in the initial and final state.

Quasi-Static Process

  • A process which is extremely slow.
  • Quasi means ‘Almost’, so almost a static process when goes from one state to another.
  • It should always be in equilibrium and there may be the loss of energy.


  • A process may be reversible or irreversible for a system but it is always reversible for the surroundings.
  • All reversible process is quasi-static but all quasi-static process are not reversible because energy loss may be possible in quasi-static but not in reversible.

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