## Second Law of Thermodynamics – Class 11 | Chapter – 12 | Physics Short Notes Series PDF for NEET & JEE

Second Law of Thermodynamics: The second law of thermodynamics is a fundamental principle in physics that describes the direction in which physical processes occur. It states that the total entropy of a closed system can never decrease over time and can only remain constant or increase. In other words, the second law of thermodynamics expresses the concept that natural processes tend towards a state of increasing disorder, randomness, and chaos.

The second law of thermodynamics can be stated in several different ways:

• Heat flows spontaneously from hot to cold objects, and never in the opposite direction, unless an external energy source is applied.
• It is impossible to convert heat completely into work in a cyclic process, without any other changes occurring.
• Any isolated system will tend towards a state of maximum disorder, or maximum entropy.

The second law of thermodynamics has important implications for energy conversion processes and the efficiency of energy systems. It sets limits on the maximum efficiency that can be achieved in converting energy from one form to another and helps to explain why energy cannot be completely recycled. The second law of thermodynamics also has implications for the evolution of biological systems, the formation of stars and galaxies, and the eventual fate of the universe.

## Second law of thermodynamics: Equation

The second law of thermodynamics cannot be expressed in a simple mathematical equation, but it can be quantified using the concept of entropy. The change in entropy of a system, ΔS, is defined as the heat transferred to or from the system, Q, divided by the temperature, T, at which the heat transfer occurs:

ΔS = Q/T

This equation expresses the fact that heat flows from hot to cold objects and that the transfer of heat always results in an increase in entropy. The second law of thermodynamics can be restated in terms of this equation by saying that the total entropy of a closed system can never decrease and can only remain constant or increase over time.

Another important equation that expresses the second law of thermodynamics is the Clausius inequality:

∮δQ/T ≤ 0

This equation expresses the fact that the integral of δQ/T around a closed path in a thermodynamic system is always less than or equal to zero, indicating that the net heat transfer in a cyclic process cannot be used to perform work without any other changes occurring. The Clausius inequality can also be restated in terms of the concept of entropy, by saying that the total entropy of a closed system can never decrease over time.

## Second Law of Thermodynamics: Statements

The second law of thermodynamics can be expressed in several different statements, including:

• Kelvin-Planck statement: It is impossible to construct a heat engine that operates in a complete cycle and produces no effect other than the extraction of heat from a hot reservoir and the performance of an equivalent amount of work. In other words, it is impossible to convert all of the heat energy input into work without any heat energy being lost to the environment.
• Clausius statement: It is impossible to construct a device that operates in a complete cycle and produces no effect other than the transfer of heat from a cold body to a hot body. In other words, heat cannot spontaneously flow from a colder body to a hotter body without external work being done.
• Entropy statement: The entropy of any isolated system always tends to increase over time, approaching a maximum value at equilibrium. In other words, natural processes tend towards a state of increasing disorder, randomness, and chaos.
• Gibbs statement: For a system at constant temperature and pressure, the change in the Gibbs free energy (ΔG) during a process must be negative for the process to be spontaneous. In other words, a process can only occur spontaneously if it results in a decrease in free energy.

All of these statements express the fundamental concept that natural processes tend towards a state of increasing disorder and that it is impossible to completely convert all of the energy in a system into useful work. The second law of thermodynamics has important implications for energy conversion processes, the efficiency of energy systems, and the limits on the maximum efficiency that can be achieved. It also has implications for the evolution of biological systems, the formation of stars and galaxies, and the eventual fate of the universe.

 JOIN OUR TELEGRAM CHANNELS Biology Quiz & Notes Physics Quiz & Notes Chemistry Quiz & Notes