Question

State the third law of thermodynamics and explain its significance.

Short answer

The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero is zero, establishing absolute zero as an unattainable reference point, and providing the basis for determining absolute entropies of substances.

Step-by-step solution

Understanding the Third Law of Thermodynamics

The third law of thermodynamics states that as the temperature of a system approaches absolute zero (0 K), the entropy, or disorder, of a perfect crystal approaches a constant minimum. This law implies that it is impossible for any process, no matter how idealized, to reduce the entropy of a system to its absolute-zero value in a finite number of steps.

Significance of the Third Law

The third law is significant because it establishes that the entropy of a perfect crystal at absolute zero is exactly zero. This provides a reference point for the measurement of entropy. The third law also implies that it is impossible to reach absolute zero through any physical process, making absolute zero unattainable.

Implications in Thermodynamics

The third law has important implications for thermodynamic processes and the behavior of materials at very low temperatures. It explains why certain physical properties of materials, such as heat capacity and thermal conductivity, approach zero as the temperature approaches absolute zero. Additionally, it allows for the absolute entropy of any substance to be determined relative to this universal reference point.

Key concepts

Entropy

Entropy is a fundamental concept in thermodynamics representing the measure of a system's disorder or randomness. It is a central idea associated with the Second Law of Thermodynamics which states that the entropy of an isolated system never decreases over time. As entropy increases, energy becomes more spread out and less available for doing work.

In the context of the Third Law of Thermodynamics, entropy takes on a special significance. According to the law, as a perfectly orderly crystal approaches absolute zero, its entropy approaches a constant minimum. This minimum entropy is often taken to be zero, though for real substances there are often residual entropies associated with quantum ground states. Nevertheless, the concept provides a baseline for understanding how energy and disorder are interlinked. At absolute zero, the hypothetical point where all molecular motion ceases, the system's entropy would be minimal because the molecules are perfectly ordered.

Understanding entropy allows us to grasp why certain energy transitions and transformations are possible or impossible. For example, it helps explain why heat naturally flows from hotter to colder bodies - doing so increases the total entropy of the system.

Absolute Zero

Absolute zero is the theoretical lowest possible temperature where a thermodynamic system's entropy would achieve its minimum value, as postulated by the Third Law of Thermodynamics. This temperature corresponds to 0 Kelvin (K), -273.15 degrees Celsius, or -459.67 degrees Fahrenheit. At absolute zero, classic descriptions of thermodynamics predict that all thermal motion of particles would stop. As such, it represents a limit to how cold matter can get.

However, reaching absolute zero is impossible according to the third law of thermodynamics. This is because doing so would require an infinite number of steps or infinite time, as each step in cooling a system reduces its temperature and thus its entropy, but never to zero. This theoretical limit shapes our understanding of various physical phenomena, from the behavior of gases to the operation of lasers. It also has implications for quantum mechanics, as many quantum effects become pronounced at temperatures close to absolute zero.

While absolute zero is unattainable, scientists have reached temperatures fractions of a Kelvin above it, enabling the study of exotic states of matter such as Bose-Einstein condensates, showcasing the strange behaviors of atoms under extreme conditions.

Thermodynamic Processes

Thermodynamic processes refer to the ways in which a thermodynamic system changes from one state to another. These processes are governed by the laws of thermodynamics, which dictate energy and entropy changes within the system. Thermodynamic processes can be categorized in various ways: isothermal (constant temperature), adiabatic (no heat exchange), isobaric (constant pressure), and isochoric (constant volume), among others.

In light of the Third Law of Thermodynamics, these processes are particularly interesting as a system approaches absolute zero. The law implies that while systems can be driven towards lower entropy, none can perfectly reach the entropy of a perfect crystal at absolute zero. Consequently, no process can completely remove the thermal energy from a system, aligning with the principle that absolute zero is unattainable.

Thermodynamic processes are crucial in engineering and physics, giving rise to everything from the operation of heat engines and refrigerators to the understanding of stellar structures. They demonstrate how energy is transported within a system and how it can be converted from one form to another with the consequence of increasing entropy, consistent with the second law of thermodynamics, and the limitations imposed by the third law at very low temperatures.