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The study of Physics Topics involves the exploration of matter, energy, and the forces that govern the universe.
What is the Importance of the Three Laws of Thermodynamics?
The Second Law of The Thermodynamics
Let us consider a man holding a glass of hot tea. From our experience we know that in such a case the hand gradually becomes hotter and the glass cooler. This is because some heat (say, 10 calorie per second) is absorbed by the hand from the glass. Now, consider the opposite process. If 10 calorie of heat is given by the cold hand to the hot glass per second, the energy would still be conserved, i.e., the first law of thermodynamics would still be obeyed. But, this opposite process never occurs in nature. In general, there is a natural direction in every real process. The first law of thermodynamics cannot determine this natural direction. So it is important to formulate a new law—the second law of thermodynamics.
Scientists expressed the second law in different forms. However, all of the different forms are equivalent. They provide alternative statements of the same physical law.
Clausius statement: No self-acting machine can transfer heat from a lower to a higher temperature.
Kelvin-Planck statement: No self-acting machine can convert some amount of heat entirely into work.
Entropy
In the example given in the beginning of section 1.12, the glass and the hand together, is now regarded as a closed system. Energy can be transmitted from one part to another or can be transformed from one form to another in a closed system. But this type of process is irreversible. Direction of an Irreversible process is determined by a change in a special property, called the entropy of the system.
The analysis of a reversible process, using the second law of thermodynamics, leads to the concept of entropy. Entropy, denoted by the letter S, is a property of all thermodynamic systems. It is defined by the relation
dS = \(\frac{d Q}{T}\) …… (1)
where, dQ = heat exchange of a system in an infinitesimal reversible process at temperature T and dS = corresponding, change in entropy of the system.
From (1), dQ = TdS. Using the relation in the first law of thermodynamics, we get,
dQ = dU + dW or, TdS= dU + pdV ……… (2)
Now, we note that dQ and dW are quantities exchanged between the system and the surroundings in a process. So they depend on whether the process is reversible or irreversible. But equation (2) does not contain any such exchange quantity. So it is true for reversible as well as irreversible processes. Then, if we use equation (2) in the thermodynamics, we need not worry about the nature of the process. This is the beauty of the ideal concept of reversibility.
The entropy principle: Thermodynamic analysis shows that in every real process in nature, the sum of the entropies of a system and its surroundings always increases. The opposite process, in which the sum of the entropies decreases, is not allowed in nature.
We may compare the situation with the law of conservation of energy (first law of thermodynamics). This law states that the total energy of the universe is a constant – it can never increase or decrease. In analogy, the second law of thermodynamics states that the total entropy of the universe increases in every process-it can never decrease.
Every real process in nature occurs in such a direction that the total entropy of the universe increases. Alternatively, no process, in which the total entropy of the universe decreases, can occur in nature. This is known as the principle of increase of entropy.
For a reversible process the total entropy of the universe remains constant while for an irreversible process the entropy of the universe increases. As all natural processes in general are irreversible, every natural process results in an increase in entropy of the universe.
This principle of increase of entropy is the most general statement of the second law of thermodynamics. The Clausius and the Kelvin-Planck statements can easily be derived from the principle.
In an adiabatic process, dQ = 0. The equation (1) gives that dS = 0 or, S = constant. So the entropy of a system remains constant in an reversible adiabatic process (just like temperature in an isothermal process). For this reason, a reversible adiabatic process is called an isentropic process.
In essence, each of the three laws of thermodynamics defines one important property of all thermodynamic systems:
- Zeroth law: Temperature (T)
- First law: Internal energy (U)
- Second law: Entropy (S)