Question

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

Short answer

A barrel of oil can only be used once as fuel because the second law of thermodynamics mandates that energy transformations increase entropy, making the original high-quality energy unrecoverable.

Step-by-step solution

Understanding the Second Law of Thermodynamics

The second law of thermodynamics states that in any energy transfer or transformation, the total entropy of a system and its surroundings always increases. This means that energy transformations are not 100% efficient and every time energy changes form, some is lost as waste, generally in the form of heat.

High-Quality Energy to Low-Quality Energy

High-quality energy, like the chemical potential energy in oil, can be efficiently converted to do work. However, when this energy is used to perform work, it is transformed into lower-quality energy (thermal energy dispersed into the environment) which is less organized and more disordered, increasing entropy.

Irreversibility of Energy Transformations

Because entropy increases, most energy transformations are irreversible. Once high-quality energy converts to low-quality energy and is dispersed as heat, it cannot be efficiently converted back into high-quality energy. This means the energy in the oil is depleted and cannot be reused in its original form.

Concluding Statement on Energy Recycling

As a result of increased entropy and energy dispersion, a barrel of oil can only be burned once for energy production. The high-quality chemical energy becomes low-quality heat energy that cannot be recycled, adhering to the second law of thermodynamics.

Key concepts

Entropy

Entropy is a foundational concept in thermodynamics that reflects the level of disorder within a system. Consider it as a measure of chaos or randomness. When a system experiences changes, especially in terms of energy transformation, its entropy tends to increase. This principle underlines why energy transformations are not perfectly efficient.
Imagine a clean and neatly stacked library; this represents a system with low entropy. As the library becomes messier with books scattered around, its entropy rises. Similarly, during energy transformations, such as burning fuel, some energy is lost as waste heat, thus increasing the system's overall disorder. It's important to understand that while energy is conserved, according to the first law of thermodynamics, entropy increases with every energy interaction, aligning with the second law of thermodynamics.

Energy Transformation

Energy transformation is when energy changes from one form to another. This is a common process in nature and our daily life. For example, when a car engine burns fuel, the chemical energy within is converted into kinetic energy to move the car.
However, energy transformation is not 100% efficient. According to the second law of thermodynamics, some energy will be dispersed as heat, which contributes to an increase in entropy. When you burn a barrel of oil, its high-quality chemical energy is transformed into heat and kinetic energy. As a result, you can't gather all this energy back to reconstitute the original fuel form. This dispersion and change in energy form highlight the inherent inefficiency in energy transformations, as some energy becomes unusable for work.

High-Quality Energy

High-quality energy is energy that can be easily used to do work because it is highly organized. Fossil fuels, like oil, possess high-quality energy due to their potent chemical bonds. When utilized, they can drive engines or generate electricity efficiently.
Unfortunately, once high-quality energy is used, it tends to become low-quality energy. Oil, when burned, transforms its orderly chemical structure into less useful thermal energy. This increased disorder means that thermal energy disperses into the environment, raising the system's entropy. Thus, high-quality energy's value decreases as it disperses, explaining why once oil is burned, its potent energy cannot be recycled back to its original state.

Irreversibility of Energy Transformations

The irreversibility of energy transformations is a fundamental concept rooted in the increase of entropy. When energy converts from one form to another, like the burning of oil, it involves a loss of usable energy as it transitions into less organized forms.
This irreversible process means that once high-quality energy is diffused as low-quality heat, it cannot revert to its previous state naturally. Consider trying to un-toast a piece of toast or reeling in ink from water; it's practically impossible. These examples illustrate how energy transformations inherently lead to a one-way street. The increase in entropy ensures that high-quality energy will diminish with every successive transformation, making it impossible to reuse a barrel of oil once it's burned.