A cornot engine whose heat sinks at 27°C has an efficiency of 25%. By how many degrees should the temperature of the source be changed to increase the efficiency by 100% of the original efficiency?

What is the 2nd Law of Thermodynamics? Questions, Advantage & Limitations

Updated on May 13, 2025 12:44 IST

The second law of thermodynamics is a physical law based on universal empirical observation concerning heat and energy interconversions. The second law of thermodynamics establishes the concept of entropy as a physical property of a thermodynamic system. It predicts whether processes are forbidden despite obeying the requirement of conservation of energy as expressed in the first law of thermodynamics and provides necessary criteria for spontaneous processes. For example, the first law allows the process of a cup falling off a table and breaking on the floor, as well as allowing the reverse process of the cup fragments coming back together and 'jumping' back onto the table, while the second law allows the former and denies the latter.

The second law is concerned with the direction of natural processes. It asserts that a natural process runs only in one sense, and is not reversible. That is, the state of a natural system itself can be reversed, but not without increasing the entropy of the system's surroundings, that is, both the state of the system plus the state of its surroundings cannot be together, fully reversed, without implying the destruction of entropy.

The Second Law of Thermodynamics gives a fundamental limitation to the efficiency of a heat engine and the co-efficient of performance of a refrigerator. In simple terms, it says that efficiency of a heat engine can never be unity. For a refrigerator, the Second Law says that the co-efficient of performance can never be infinite. The following two statements, one due to Kelvin and Planck denying the possibility of a perfect heat engine, and another due to Clausius denying the possibility of a perfect refrigerator or heat pump, are a concise summary of these observations.

Table of content

Kelvin-Planck statement
Clausius statement
Perpetual Motion Machine of the Second Kind (PMM2)

Kelvin-Planck statement

No process is possible whose sole result is 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 is the transfer of heat from a colder object to a hotter object.

It can be proved that the two statements above are completely equivalent.

Other Statements of Second Law of Thermodynamics

The second law of thermodynamics can break down into a couple of statements:

1. Entropy Increases: The total entropy of an isolated system will always increase over time. Entropy is a measure of the system's disorder or randomness. This means that, left to its own devices, a system will naturally evolve towards a more chaotic state.

2. Irreversibility: Many natural processes are irreversible. If you break an egg, it's not spontaneously going back together. The second law implies that certain changes in a system are easy to accomplish in one direction (like breaking an egg) but nearly impossible to undo.

3. Heat Flows from Hot to Cold: Heat energy tends to flow from regions of higher temperature to regions of lower temperature. This is the reason your coffee cools down in a room-it's the natural tendency for heat to disperse.

4. Efficiency Limits: The second law of thermodynamics places limits on the efficiency of thermal machines, like engines. No engine can be $100 %$ efficient; some energy is alwavs lost as waste heat.

In addition to these statements, a French physicist named Nicolas Léonard Sadi Carnot, also known as the "father of thermodynamics", introduced the second law of thermodynamics. However, as per his statement, he emphasised the use of caloric theory for the description of the law. Caloric (self-repellent fluid) relates to heat, and Carnot observed that some caloric was lost in the motion cycle.

Perpetual Motion Machine of the Second Kind (PMM2)

The device that produces work while interacting with a single heat reservoir is known as a perpetual motion machine of the second kind (PMM2). Also, a device that violates the second law of thermodynamics is a perpetual motion machine of the second kind.

Advantage of Second law of thermodynamics

1. Second law helps us to determine the direction in which energy can be transferred

2. It helps us to predict whether a given process or a chemical reaction can occur spontaneously.

3. It helps us to know the equilibrium conditions of a chemical reaction.

4. It helps us to calculate the maximum fraction of heat that can be converted to work in a given process.

The second law of thermodynamics has limitations primarily in its focus on closed systems and its inability to fully explain the direction of heat transfer in irreversible processes. It also doesn't address the underlying microscopic mechanisms responsible for entropy increase. Additionally, the second law assumes a constant external environment, which isn't always realistic, and it doesn't account for the possibility of systems returning to a low-entropy state through complex mechanisms.

Here's a more detailed look at the limitations:

1. Closed System Focus:

The second law is primarily formulated for closed systems, where there's no exchange of matter or energy with the surroundings.

2. Irreversible Processes and Entropy Increase:

The second law focuses on the irreversibility of heat conversion and the increase in entropy, particularly in cyclic processes.

While it correctly states that a heat engine's efficiency is always less than 1 , it doesn't explain the underlying microscopic mechanisms that drive entropy increase or the extent to which entropy can be reduced in some complex systems.

3. Direction of Heat Transfer:

The second law, while correctly stating that heat doesn't flow spontaneously from a cold body to a hotter one, doesn't provide a complete explanation for the direction of heat transfer in irreversible processes.

It doesn't explain why heat can't be spontaneously converted into work in all cases or why some processes, like water spontaneously cooling and freezing, don't occur.

4. Constant External Environment:

The second law assumes a constant external environment, which may not be the case in many real-world scenarios.

For instance, the behavior of a system in a changing environment (like a star) or a system with a non-constant source of energy might not be fully captured by the second law.

In summary, while the second law provides a powerful framework for understanding thermodynamic systems its limitations lie in its focus on closed systems, its inability to fully explain the direction of heat transfer in irreversible processes, and its lack of detail regardin the microscopic mechanisms and quantum effects that might influence system behavior.