Thermodynamics is the branch of physics that deals with heat, work, temperature, energy, and other aspects of energy transformation—the laws of thermodynamics deal with the underlying laws of nature. In thermodynamics, a collection of matter or space is chosen for our study (e.g., water kettle, aircraft engine) known as a system. The types of systems are classified further:

Closed System: In a closed system, no mass crosses the boundary; only heat and work can transfer. An example of a closed system is a gas in a piston-cylinder device.

Open System: In an open system, both mass and energy can cross the boundary. An example of an open system is a water heater or car radiator.

Isolated System: In an isolated system, there is no interaction with the surroundings, and no mass or energy transfer occurs. A thermos flask is an example of an isolated system.

State, Path, and Process

State: The condition of a system is defined by fixed values of all its properties. For example, a gas in a sealed container with a specific temperature and pressure is in a certain state.

Path: The sequence of states a system passes through during a change. For instance, heating a gas might involve a path from low temperature and pressure to high temperature and pressure.

Process: A change from one state to another, including the path and interactions with surroundings. For example, boiling water involves a process where the system changes from a liquid state at a specific temperature to a vapor state.

Intensive Vs Extensive Properties

Intensive Properties: These properties are independent of the system’s size or mass. Examples include temperature (T), pressure (P), and density. For example, the temperature of a cup of water remains the same whether it’s a small cup or a large pot.

Extensive Properties: These properties depend on the system’s size or mass. Examples include volume (V), mass (M), and total energy. For example, if you double the amount of water in a container, the total volume will also double.

Zeroth Law of Thermodynamics

According to the Zeroth Law of Thermodynamics, if two systems are in thermal equilibrium with a third system, they must also be in thermal equilibrium with one another. Thus, temperature measurement is based on the zeroth rule of thermodynamics.

For example, if you have a hot cup of coffee and you place a metal spoon in it, after a few minutes, the spoon and coffee will have reached the same temperature. According to the Zeroth Law of Thermodynamics, if the coffee is in thermal equilibrium with the spoon, and the spoon is in thermal equilibrium with your hand when you touch it, the coffee and your hand will be at the same temperature. This concept helps to explain why temperature measurement is possible.

First Law of Thermodynamics

The first law of thermodynamics, also known as the Law of Conservation of Energy, states that energy cannot be created or destroyed in an isolated system; it can only be transferred or transformed from one form to another. It also affirms that energy is a thermodynamic property.

When a system undergoes a process, if a quantity changes and this change depends solely on the initial and final states, that quantity is a property of the system. The change in a property’s value is independent of the path taken. Consequently, in a complete cycle, the net change in every property is zero.

Heat and work are not properties because they depend on the path and end states; their net change in a cycle is not zero. Therefore, they are called path functions. On the other hand, energy, temperature, pressure, internal energy, density, and enthalpy are state variables or functions because these variables do not depend on the path taken by the thermodynamic system to change from the initial to the final state.

According to the First Law of Thermodynamics, 

Energy (E) consists of kinetic energy, potential energy, and internal energy. The first law of thermodynamics states that to obtain work, you must supply heat, i.e., you can’t get something for nothing. This law connects a system’s kinetic and potential energy to the work it can do and the heat it can transfer. 

Additionally, it introduces an extra state variable, enthalpy, which is sometimes used to describe internal energy.

Internal energy is a system’s molecular energy and can take several forms, including sensible, latent, chemical, and nuclear.

Second Law of Thermodynamics

The second law of thermodynamics states that it is impossible to create a device that operates in a cycle and only performs work, such as raising a weight while exchanging heat with a single reservoir. In other words, natural processes occur spontaneously in one direction, and reversing these processes requires external energy. For example, heat always flows from a hotter body to a cooler one, unless an external force intervenes.

We can’t construct a heat engine with just one positive heat interaction.

The second law of thermodynamics is crucial because it deals with the direction and feasibility of natural processes, which the first law does not address. While the first law states that energy is conserved (e.g., the heat lost by coffee equals the heat gained by the surrounding air), it does not specify the direction in which energy transformations occur. For example, while the first law allows for the hypothetical scenario where hot coffee heats up in a cooler room (as long as energy is conserved), this is physically impossible in practice.

The second law introduces the concept of entropy, which dictates that natural processes have a preferred direction—typically from states of higher to lower energy quality. The coffee example, explains why the coffee cools down rather than heats up: the overall entropy of the system increases. Thus, the second law provides the missing principle that not only is energy conserved, but its quality also degrades over time, defining the direction of natural processes.

Therefore, according to the second law of Thermodynamics, the total entropy of an isolated system can never decrease over time; it can only stay the same or increase. This law explains the direction of natural processes and the concept of irreversibility. The second law applies only to the cycle and not to the process. 

A process must satisfy both the first and second laws of thermodynamics to proceed. The first law addresses the quantity of energy and its transformations, while the second law focuses on the direction of processes and the quality of energy, ensuring that natural processes move towards increased entropy.

The two popular statements of the Second Law of Thermodynamics are:

Kelvin-Planck Statement: It is impossible for any device operating in a cycle to receive heat from a single reservoir and produce a net amount of work. In other words, no heat engine can be 100% efficient; some heat must be expelled to a cold sink.

Clausius Statement: It is impossible to construct a device operating in a cycle that transfers heat from a lower-temperature body to a higher-temperature body without external work input. In other words, heat cannot spontaneously flow from cold to hot without external work.

These statements are interconnected: if an engine or refrigerator violates one, it also violates the other. A process must satisfy both the first and second laws of thermodynamics to occur. A device violating either law is called a perpetual-motion machine. A device that creates energy, violating the first law, is a perpetual-motion machine of the first kind (PMM1). A device that violates the second law is a perpetual-motion machine of the second kind (PMM2). Despite numerous attempts, no perpetual-motion machine has ever worked.

Reversible vs Irreversible Process

Reversible Process: A reversible process can be reversed without leaving any trace on the surroundings. This means that both the system and the surroundings return to their initial states with zero net heat and work exchange in the combined process. Reversible processes are idealizations and do not occur naturally, but they can be approximated. These processes proceed infinitesimally slowly, maintaining equilibrium at each stage, and can move in either direction with a small driving force. This allows them to achieve maximum work and return to the initial state without altering the surroundings. 

Examples of reversible processes:

• Constant Volume and Constant Pressure Heating and Cooling
• Isothermal and Adiabatic Processes
• Evaporation and Condensation
• Elastic Expansion/Compression

Irreversible Process: In contrast, an irreversible process occurs rapidly, moving from the initial to the final state without maintaining equilibrium between the system and surroundings. It proceeds in only one direction, requires a definite driving force, and results in less work than a reversible process. Irreversible processes cannot return to the initial state without changing the surroundings and take a finite time to complete, with equilibrium established only after the process is finished.

Irreversible processes include motion with friction, spontaneous chemical reactions, heat transfer, unrestrained expansion, substance mixing, and current flow through a resistance. These processes cannot be reversed without external work.

Factors that cause a process to be irreversible are known as irreversibilities. These include friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, and chemical reactions.

Internally and Externally Reversible Process

A process is internally reversible if no irreversibilities occur within the system boundaries. The system proceeds through a series of equilibrium states, and reversing the process returns the system through the same states. For example, slow and controlled compression or expansion of a gas, where the gas remains in equilibrium throughout, is internally reversible.

A process is externally reversible if no irreversibilities occur outside the system boundaries. For example, heat transfer between a system and a reservoir is externally reversible if the system’s outer surface is at the same temperature as the reservoir.

A totally reversible process, or simply reversible, involves no irreversibilities within the system or its surroundings. It has no heat transfer through a finite temperature difference, no non-quasi-equilibrium changes, and no friction. For example, the isothermal heat transfer to a system at a constant pressure, where the temperature difference between the system and the reservoir is infinitesimally small, is totally reversible.

Entropy

Entropy is a concept used in thermodynamics and statistical mechanics to describe the microscopic disorder or randomness of a system. As entropy increases, the level of disorder or randomness also increases. According to the second law of thermodynamics, natural processes tend to move towards a state of maximum entropy, which explains why many processes are irreversible. 

Entropy measures how energy is distributed within a system and helps predict spontaneity. A process is considered spontaneous if it leads to an increase in the total entropy of the system and its surroundings. In information theory, higher entropy indicates greater uncertainty and a need for more information to describe the system.

For an isolated system, entropy either increases or remains constant, reaching a maximum at equilibrium. 

If heat Q flows reversibly from the system to the surroundings at temperature To, the entropy of the system decreases by Q/To, while the entropy of the surroundings increases by the same amount. Entropy in a closed system can increase due to dissipative effects or internal irreversibilities, and by heat interactions where entropy transfer occurs. 

The entropy change between two equilibrium states is the same regardless of whether the process is reversible or irreversible because entropy is a state function. This means that the change in entropy depends only on the initial and final states, not on the process path.

Entropy is not a conserved property; there is no principle of conservation of entropy. In idealized reversible processes, entropy remains constant. However, it increases in real processes, reflecting the tendency towards disorder and irreversibility.

According to Clausius Inequality, for all cycles, 

Where ds is a change in entropy. Entropy is a point function and an extensive property. The unit of entropy is J/K.

Third Law of Thermodynamics

The third law of thermodynamics states that absolute zero, which is 0 Kelvin, is unattainable because it would require an infinite amount of work to reach it. The law explains that as a system approaches absolute zero, the entropy of a perfectly crystalline substance also approaches zero. In practice, it is impossible to achieve absolute zero because no matter how much work is done to cool a system, reaching 0 Kelvin is not achievable.

Violation of Thermodynamics Law

Attempting to achieve the impossible: 

1. Perpetual Motion Machines of the First Kind would violate the First Law of Thermodynamics by creating energy from nothing. 
2. Perpetual Motion Machines of the Second Kind would violate the Second Law of Thermodynamics by converting heat completely into work with no waste. 
3. Achieving Absolute Zero (0 K) would violate the Third Law of Thermodynamics, as it is unattainable and requires infinite work.