Laws of Thermodynamics

Thermodynamics is the branch of physics which deals with heat and temperature and their relation with energy and work. Speaking broadly, thermodynamics is concerned with the transfer of energy from one system to another, and from one form to another. The behaviour of the above quantities is determined by the four laws of thermodynamics. The four laws of thermodynamics describe all the minute changes which happen when the energy system of a body changes, in addition to explaining the ability of an energy system to perform some work on its surroundings. It was only in the 19th century that the studies on thermodynamics amplified; for optimising the performance of the steam engines. There are eight founding schools of thermodynamics.

List of scientists contributing to thermodynamics


Before we begin our discussion on the four laws of thermodynamics, we shall acquaint ourselves with some terms.

Thermodynamic System: An assembly of a very large number of particles having a certain value of pressure (P), volume (V), and temperature (T).

Surroundings: Every thing outside the system which can have a direct effect on the system.

Reversible Process: An idealized process in which the system is in equilibrium at every point.

Cyclic Process: When a system returns to its starting point.

Extensive State Variables: The variables which indicate the size of the system. These variables are internal energy (U), volume (V), and the total mass (M).

Intensive State Variables: The variables which do not indicate the size of the system; for example, pressure (P), temperature (T), and density (ρ).

Entropy: The degree of disorder in the system. It is maximum for gases and least for the solids.


System and surroundings

Law I: The Zeroth Law of Thermodynamics

The zeroth law states that if A is in thermal equilibrium with B, and B is in thermal equilibrium with C, then A and C are also in thermal equilibrium with each other.

zeroth law

  • The zeroth law helps us to assign the temperatures to different bodies. The bodies in thermal equilibrium with each other will have the same temperature.
  • When two bodies of different temperatures are placed in contact, heat starts flowing from the body with high temperature to the body with low temperature till their temperatures become equal.
  • The stage when the temperatures of the bodies become equal is called thermal equilibrium. Thus, the direction of the flow of heat is governed by the temperature.

LAW II: The First Law of Thermodynamics 

The first law states that the heat supplied (Q) to a system is partially used in increasing the internal energy (∆U) of the system and partially used in expanding the gas (∆W); i.e., Q=∆U+W


First law of thermodynamics

The sign conventions are of utmost importance while following the first law of thermodynamics:

Sign convention

  • This law is also known as the law of conservation of energy.
  • The flow of heat describes the transfer of energy.
  • The internal energy of a system is related to the temperature as following, Etotal = KEsystem + PEsystem + Usystem.
  • The energy transfer to a system or from a system is called work.
  • It can be rightly said that a machine cannot perpetually work if an appropriate amount of energy is not given as an input.

Thermodynamic Processes:

  • Adiabatic: Constant heat
  • Isothermal: Constant temperature
  • Isochoric: Constant volume
  • Isobaric: Constant pressure

Thermodynamic processes

Limitations of the first law of thermodynamics:

  • It does not indicate the direction of heat change.
  • It does not give any idea about the extent of heat change.
  • It gives no information about the source of heat.

LAW III: The Second Law of Thermodynamics

The second law of thermodynamics states that it is not possible to have a process in which the entropy of an isolated system is decreased.

Second law of thermodynamics


  • According to Clausius, no process is possible, whose result is the transfer of heat from a colder object to a hotter object.
  • In terms of entropy, it either stays the same or increases but never decreases. However, a decrease of entropy may occur in the systems which are not isolated, provided they increase the entropy of the environment by the same amount.
  • The entropy of a system remains unchanged for any process that goes from an initial state to the final state; regardless of the fact whether a process is reversible or irreversible.
  • According to the second law, heat transferred (∆Q) is the product of temperature (of the system and source or destination of heat) with the increment (ds) of the system’s entropy (S); ∆Q=TdS.

Second law of thermodynamics

Law IV: The Third Law of Thermodynamics

The third law states that the entropy of a system approaches minimum when the temperature of the system approaches absolute zero (−273.15°C, 0 K).

  • The value of entropy at zero K is zero. Nonetheless, there is some amount of residual energy in the system in some cases.
  • When the temperature of the system is zero, a system will have minimum thermal energy. This statement is true if the perfect crystal has only one state and that too with minimum energy.

Third law of thermodynamics


  • S= entropy
  • k= Boltzman constant

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