The laws of thermodynamics are fundamental laws which describe the behaviour of heat and work in a thermodynamic system. These laws forbid phenomena such as perpetual motion machines, a hypothetical machine the development of which was pursued during the industrial revolution.


:Heat added to a system
:Internal energy of a system
:Energy expended through work

Zeroth Law

“If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.”

The zeroth law was developed by Ralph Fowler in 1935 and despite being developed latter than the other three laws it was named the zeroth law as it was a logical predecessor.

This law indicates that two objects can reach thermal equilibrium (i.e. have the same temperature) regardless their respective mass or internal energy per mass as they have different specific heats. Therefore it is implied by this law that there is a measurable property called temperature on which heat transfer depends.

In practice this law means that while a large block of steel may have more internal energy than a small droplet of water resting on it, no energy can be transferred to the droplet if both bodies are at the same temperature.

First Law

“Energy can neither be created nor destroyed. It can only change forms.”

The first law of thermodynamics was initially derived from the works of Rudolf Clausius and William Rankine in 1850. It describes the conservation of energy in a chemical or physical process and may be written as follows:

Here it is important to note that as per IUPAC conventions, energy transfers to a system are positive while energy transfers out of a system are negative. Therefore for a system in which heat is supplied to the system and performs work on its surroundings will be positive while will be negative.

This law is observed in many engineering applications such as pumping a fluid where electrical energy is partially converted into potential and kinetic energy in the fluid (governed by the efficiency of the pump) and the rest covered into heat and sound.

Second Law

“Entropy of an isolated system never decreases.”

The second law of thermodynamics essentially states that over time an isolated system will progress towards thermodynamic equilibrium, the state of greatest entropy. In more practical terms the second law of thermodynamics states that heat cannot be transferred from a colder body to a hotter one.

In engineering the second law of thermodynamics is utilized in the design of heat engines such as the steam engine and internal combustion engine. Application of the second law of thermodynamics allows the maximum attainable efficiencies of these engines to be calculated. As the law dictates that heat cannot be transferred from a colder body to a hotter one, the heat source can only be exploited until it cools to the temperature of the heat sink (often ambient).

Third Law

“As temperature approaches absolute zero, the entropy of a system approaches a constant minimum.”

The third law of thermodynamics essentially states that as the temperature of a system reduces to absolute zero the system will reach its minimum energy state. As stated by Guggenheim in 1967 a consequence of the third law "it is impossible by any procedure, no matter how idealized, to reduce the temperature of any system to zero temperature in a finite number of finite operations".