Second Law of Thermodynamics: Explanation, Statement & Application

Second Law of Thermodynamics: There are four laws of thermodynamics, the Zeroth Law of thermodynamics, the First Law of thermodynamics, the Second Law of thermodynamics, Third Law of thermodynamics. Among the four laws of thermodynamics, you would often have seen the example of the second law in daily life. You would surely have experienced this if we bring a hot cup of tea into contact with a cold glass of water, we observe that the hot cup of tea cools down and the cold object heats up until it attains equilibrium. This is exactly what happens in the second law of thermodynamics. If there is any spontaneous process occurring in the universe then its entropy increases till it attains equilibrium. The concept of the second law of thermodynamics was stated in the 19th century by the Scottish physicist William Thomson (Lord Kelvin) and the German physicist Rudolf Clausius. 

Second Law of Thermodynamics

The First law of thermodynamics relates to the principle of the law of conservation. The second law of thermodynamics establishes entropy as the physical property of a thermodynamic system. This is a physical law that concerns heat and energy. The Second Law of Thermodynamics states that any spontaneous process will result in an increase in entropy of the entire universe in an isolated system which will further increase over time or remains constant. The change in entropy will never decrease. Mathematically, it can be written as:

ΔSuniv > 0

Where, 

ΔSuniv represents the change in entropy of the universe.

Entropy 

Entropy is the measure of randomness and disorder in the system. Entropy depends upon many factors, such as-

• Internal Energy
• Molecular size and its complexity
• Temperature.
• Physical states of Substance.

Two statements of Second Law of Thermodynamics

There are two statements of the second law of thermodynamics that were given by Kelvin-Plank and Clausius.

Kelvin-Plank Statement:  No process is possible whose sole result would be the absorption of heat from a reservoir and the complete conversion of the heat into work.

Clausius Statement: No process is possible whose sole result would be the transfer of heat from a colder object to a hotter object.

Application of the Second Law of Thermodynamics

The application of the Second law of thermodynamics are-

1. The second law of thermodynamics states that heat always moves from a hot body to a cold body. All heat engine cycles, including Otto, Diesel, etc., as well as all working fluids employed in the engines, are covered under this rule.
2. Refrigerators
3. Heat Pumps