The 2nd law of thermodynamics is one of the fundamental principles of physics, with profound implications for both natural systems and technological applications. This law states that:
"The amount of entropy in the universe tends to increase over time."
In simple terms, this means that natural processes tend toward disorder, and that the useful energy available to perform work decreases over time.
Energy, heat and work: what is true and what is not
One of the essential aspects derived from this law is that, although all mechanical work can be completely converted into heat, the reverse is not true: not all heat can be transformed into work. This limitation defines a theoretical maximum efficiency, known as the Carnot efficiency, which depends solely on the temperatures of the hot and cold sources.
Comparison with the first law of thermodynamics
According to the first law of thermodynamics, or the law of conservation of energy, energy is neither created nor destroyed; it is only transformed. This idea is summarized in the equation:
\[ \Delta U = Q - W \]
Where:
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\( \Delta U \) is the change in the internal energy of the system.
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\( Q \) is the heat absorbed.
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\( W \) is the work done by the system.
This law, however, does not indicate in which direction the processes occur; that is, it does not distinguish between what is possible or natural and what is not. For this, we need the second principle.
Entropy and the second law
The key concept in the second law is entropy, a measure of the disorder or randomness of a system. The greater the entropy, the more disorganized or dispersed the system. The second law states that:
In a closed or isolated system, total entropy cannot decrease; it can only remain constant (in reversible processes) or increase (in real, irreversible processes).
Therefore, for heat to flow from a cold body to a warm one (i.e., against the thermal gradient), external work is required. This explains, for example, the operation of refrigerators or air conditioners, which require energy to force heat to move "against" its natural direction.
In spontaneous processes—such as the mixing of gases, the cooling of a hot object, or the dissolution of a substance—the entropy of the system and its surroundings increases. However, if you want to decrease the entropy of one part of the system, you need to increase it even further elsewhere, so the total entropy continues to increase.
Systems in equilibrium
The second law applies primarily to systems that are close to or in a state of thermodynamic equilibrium. In these cases, the evolution of the system can be predicted based on entropy changes. If a process causes an increase in overall entropy, then it is thermodynamically permissible. If not, the process is impossible.
Furthermore, lower entropy production in a process is often associated with greater energy efficiency. This is especially important in chemical engineering, the energy industry, and other technological fields.
Examples of the second law of thermodynamics
The second law manifests itself in many everyday natural phenomena. Some clear examples of the second law include:
1. A compressed gas that expands
If a valve connecting a container of compressed gas to a lower-pressure area is opened, the gas spontaneously expands, increasing its entropy. The reverse process (the gas spontaneously compressing again) does not occur without external work.
2. Heat always flows from hot to cold
When a hot object is placed in contact with a cooler object, heat flows from the hotter object to the cooler object until thermal equilibrium is reached. The reverse has never been observed without energy input.
3. A cup of coffee that gets cold
When you leave a hot cup of coffee on a table, the heat is transferred to the surrounding air. The cup cools, and that heat disperses into the environment. The opposite process—the coffee heating itself—is impossible without intervention.
4. How a refrigerator works
The refrigerator extracts heat from its interior (making it colder) and expels it to the exterior (making it warmer). To achieve this "reverse flow," it requires electrical energy to power the compressor. This fulfills the second law: order is imposed at the cost of increasing disorder in the environment.
5. The mixture of liquids or gases
When two initially separate liquids or gases mix, they do so spontaneously until they reach a uniform distribution. Entropy increases, since the final state is more disordered than the initial one. They will not separate on their own.
6. The combustion of a fuel
When we burn gasoline, energy is released in the form of heat and gases that are dispersed. Although some of that energy can be transformed into work (for example, moving a car), there will always be thermal losses. Efficiency is limited by the Second Law.