The Kelvin-Planck Statement of the Second Law of Thermodynamics

The Kelvin-Planck statement of the second law of thermodynamics explains the working of a heat engine in a cyclic process. The first law of thermodynamics has limitations; for instance, it could not explain why some processes in a closed system are reversed. The second law of thermodynamics explains these limitations. In this regard, the Kelvin-Planck statement explains the concept of work done and the efficiency of a heat engine. Read on for detailed notes on the Kelvin-Planck statement of the 2nd law of thermodynamics.

The Second Law of Thermodynamics

The second law deals with the efficiency and work done on heat engines. It gives the limitation for the coefficient of a mechanical refrigerator. 

In simple terms, the second law of thermodynamics states that the total efficiency of a heat engine working in a cycling process can never reach unity. Therefore, the law implies that heat released to the cold reservoir by a heat engine can never be made zero.

For a mechanical refrigerator, the second law states that the coefficient of performance can never be infinite. Thus, the total external work (W) done on the refrigerator will never be zero.

Kelvin-Planck Statement of the 2nd Law of Thermodynamics

The Kelvin-Planck statement denies the possibility of a perfect heat engine, upholding the statement of the second law of thermodynamics. The statement is as follows:

“No process is possible whose result is the absorption of heat from a reservoir and the complete conversion of the heat into work.“

According to this statement, it is impossible to create a heat engine in a thermodynamic cycle that generates power during the heat exchange with one heat reservoir. In other words, the total amount of external work done on these engines cannot be converted into useful work. 

Heat engines need to exchange heat with two heat reservoirs. The two reservoirs include a heat source and a heat sink. During the working of the heat engine, a significant part of total thermal energy is released into the thermal sink.

Clausius Statement of the 2nd Law of Thermodynamics

“Heat can not flow from low temperature to high temperature without the application of external work. Also, energy will not flow spontaneously from a low-temperature object to a higher-temperature object.“

Heat Engine

• A heat engine is a mechanical device used to undergo a cyclic process. This process converts the heat produced in the system to work done by the system. 
• The heat engine consists of a working substance called the system of the engine. It works as the engine’s fuel. For example, in a diesel engine, fuel vapour and air molecules are the working substances.
• The working substance also undergoes a cycle of processes. In some of these processes, the heat engine absorbs heat Q1 from an external source. The external source is called a reservoir and has a temperature of T1. T1 is usually very high. 
• In the following cycle, the working substance releases an amount of heat, Q2, to an external source at a temperature of T2. The value of T2 is comparatively lower than T1. 
• The total work done by the heat engine (W) in the system is transferred to the environment. 

Applications of the Kelvin-Planck Statement

The Kelvin-Planck statement of the 2nd law of thermodynamics is important and has many applications in our day-to-day lives. Here are some real-life examples of this statement:

• Most automobiles that we see every day are based on the Kelvin-Planck statement and use Carnot’s engine for operations. 
• Different automobiles use different fuels to operate their engines. For example, a car may use petrol, and a ship or truck may use diesel. However, the fundamental law of heat engines does not change. 
• Many examples of electronics are based on the second law of thermodynamics, such as refrigerators, air-conditioners, and other appliances.
• The gas compressor, such as fans, also uses Kelvin-Planck’s statement.
• Heat transfer, most commonly seen in coolers and radiators, is the most important application of the second law of thermodynamics. Heat is transferred between two mediums by the following three processes: 
  1. Conduction
  2. Convection
  3. Radiation
• The importance of the Kelvin-Planck statement is also seen in the power plant industry. Thermal, nuclear, hydroelectric, and other types of power plants are designed on the principles of the second law of thermodynamics. 

Conclusion

A heat engine is defined as a closed system that goes through a cyclic process and converts heat produced by the mechanics into the work done by the engine. The Kelvin-Planck statement of the 2nd law of thermodynamics explains the efficiency of heat engines. 

The Kelvin-Planck statement is important as it has a wide range of applications in our day-to-day life. The concept is used to design various appliances and machines with heat engines. The above article gives comprehensive notes on the Kelvin-Planck statement.