Question

Use the second law of thermodynamics to explain why we can use oil only once as a fuel, or in other words, why we cannot recycle its high-quality energy.

Short answer

The second law of thermodynamics prevents recycling oil's energy, as using it increases entropy irreversibly, dispersing energy.

Step-by-step solution

Understand the Second Law of Thermodynamics

The second law of thermodynamics states that in any energy transfer or transformation, the total entropy (disorder) of a system and its surroundings will always increase over time. Entropy is a measure of energy dispersal, and as it increases, energy becomes less available to do useful work. This law implies that some energy will always be 'lost' to disorder or as waste heat in any process.

Energy Quality and Oil as a High-Quality Fuel

Oil is considered a high-quality energy source because it has a low entropy and high energy density, which means it can do a significant amount of work when burned. However, once oil is used for combustion, its energy is converted into heat, mechanical work, and other forms of energy, while increasing the entropy of the universe.

Energy Degradation Through Use

When oil is burned as fuel, its energy spreads into the environment as heat and other forms of energy that have higher entropy and lower availability for further work. This dispersal is an irreversible process due to the increase in entropy, making it impossible to completely convert the products back into usable oil without expending even more energy, resulting in a net loss.

Conclusion on Recycling High-Quality Energy

The second law of thermodynamics explains that because burning oil irrevocably increases entropy, we cannot recycle its energy to its original high-quality state without greater energy input. Therefore, the high-quality energy from oil is inherently lost to disorder, preventing recycling.

Key concepts

Energy Quality

Energy quality is a concept that helps us understand the usability and efficiency of an energy source. High-quality energy, such as that found in oil, is characterized by low entropy and high energy density. This means it contains a concentrated amount of energy that can perform a significant amount of work.
For example, when oil is combusted in an engine, it releases a large amount of kinetic energy that powers vehicles. However, this form of energy is valuable because it can be directly converted into work with minimal losses initially.
High-quality energy sources, like oil, are limited in their ability to be recycled efficiently. Once used, they typically transform into lower quality forms of energy that are less capable of doing useful work. This transformation is primarily due to the increase in entropy, which we will discuss further.

Entropy

Entropy is a key player in the world of thermodynamics and essentially measures energy's tendency to spread out and become less ordered. Imagine energy as a neatly organized deck of cards. As you use the energy, it's like shuffling the deck—its order breaks down.
According to the second law of thermodynamics, every time energy is transformed, the total entropy of a system and its environment increases. This means the energy becomes more dispersed and more random.

• Higher entropy means energy is less orderly and hence, less usable.
• Lower entropy means energy is more organized and therefore, more useful.

When you burn oil, its transformation raises the entropy, dispersing the energy into less useful forms, like heat. Thus, the once-useful energy becomes spread thinly across its environment, difficult to harness again.

Energy Transfer

Energy transfer involves moving energy from one system or form to another. In the combustion of oil, chemical energy stored in the oil is converted to thermal energy, which can be used for mechanical work.
This process of energy transfer is not perfectly efficient, partly due to increased entropy during the transformation. Not all the chemical energy is converted into mechanical work or useful output; some is inevitably released as waste heat.
This waste is a form of energy with high entropy, spread out too thinly to do further useful empirical work. Therefore, while energy is conserved in quantity, its quality degrades with each transfer, limiting its potential to be used for work again.

Energy Conservation

At its heart, energy conservation is a principle stating that energy cannot be created or destroyed, only transformed from one form to another. This concept, however, does not account for the quality of energy, which can decrease even though the total energy amount remains the same.
The second law of thermodynamics accentuates this by emphasizing that energy transformations increase overall entropy. Although energy cannot disappear, its ability to perform work does.

• Oil, initially with high-quality energy, loses this property when burned, as the energy becomes heat and mechanical work.
• The energy remains but in a degraded form that offers less opportunity for recycling.

Consequently, although we conserve energy's essence, its utility diminishes, leading to an unavoidable loss of useful high-quality energy over time.