7.1 Absolute zero and the Third Law
2 min read•july 23, 2024
Absolute zero, the coldest possible temperature, is a fundamental concept in thermodynamics. At 0 K or -273.15°C, all molecular motion theoretically stops. This temperature serves as a reference point for many calculations and helps explain material behavior at extremely low temperatures.
The states that a perfect crystal's is zero. It's impossible to reach absolute zero in finite steps, but understanding this concept helps explain phenomena like and superfluidity in materials at very low temperatures.
Absolute Zero and the Third Law
Definition of absolute zero
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- Lowest possible temperature equal to 0 K or -273.15 ℃
- System has minimum kinetic energy and maximum order at absolute zero
- All molecular motion theoretically ceases at this temperature (particles stop vibrating)
- Serves as a reference point for temperature scales like the Kelvin scale
- Many thermodynamic equations and calculations use absolute temperature (Kelvin) instead of relative scales (Celsius or Fahrenheit)
- Efficiency of heat engines (steam turbines) and behavior of gases (ideal gas law) analyzed using absolute temperature
Third Law of Thermodynamics
- Entropy of a perfect crystal at absolute zero is zero
- Entropy measures disorder or randomness in a system (messy room has high entropy)
- As temperature approaches absolute zero, entropy of a system approaches a constant minimum value
- Impossible to reach absolute zero in a finite number of steps or amount of time
- Provides a reference point for determining the absolute entropy of a substance
- Helps explain behavior of materials at extremely low temperatures
- Superconductivity (zero electrical resistance)
- Superfluidity (zero viscosity in fluids like liquid helium)
Unattainability of absolute zero
- Impossible to cool a system to absolute zero in a finite number of steps
- Amount of work required to remove heat from the system increases exponentially as temperature approaches absolute zero
- Cooling methods become increasingly inefficient close to absolute zero
- Adiabatic demagnetization
- Cost and energy required to maintain extremely low temperatures become prohibitively high
- Quantum mechanical effects like prevent complete removal of energy from a system
System behavior near absolute zero
- Changes in physical properties as temperature approaches absolute zero
- Electrical resistance of pure metals decreases and approaches zero (superconductivity in lead, niobium)
- Some materials exhibit superfluidity (liquid helium) with zero viscosity
- Thermal expansion of materials decreases and approaches zero (contraction)
- Thermodynamic property changes near absolute zero
- Specific heat capacity of materials decreases and approaches zero (less energy to change temperature)
- Thermal conductivity may increase or decrease depending on the substance (copper vs rubber)
- Behavior of gases deviates from the ideal gas law ()
- Quantum mechanical effects become more prominent at extremely low temperatures
- Zero-point energy, the minimum energy a system can possess, becomes significant (quantum harmonic oscillator)
- Quantum tunneling and other quantum phenomena may affect particle behavior (alpha decay)
Key Terms to Review (15)
Cryogenics: Cryogenics is the study and application of materials at extremely low temperatures, typically below -150°C (-238°F). This field focuses on the behaviors of substances when cooled to these temperatures, leading to significant insights about thermodynamic properties, phase transitions, and material stability. Understanding cryogenics is crucial for applications in superconductivity, liquefied gases, and various scientific experiments that require controlled thermal environments.
Entropy at absolute zero: Entropy at absolute zero refers to the concept that a perfectly ordered system would have zero entropy when cooled to the temperature of absolute zero (0 K). At this point, a system's microscopic states become completely ordered, leading to the conclusion that the entropy reaches its minimum value, consistent with the Third Law of Thermodynamics. This principle implies that as temperature approaches absolute zero, the entropy of a perfect crystalline substance approaches zero, emphasizing the relationship between temperature and disorder in a system.
Ideal crystals: Ideal crystals are perfectly ordered structures where the arrangement of atoms is consistent and uniform throughout, with no defects or impurities. This concept is essential for understanding the behavior of materials at very low temperatures and forms a foundation for the Third Law of Thermodynamics, which states that as a system approaches absolute zero, the entropy of a perfect crystal approaches zero.
Information entropy: Information entropy is a measure of the uncertainty or randomness associated with a random variable, quantifying the amount of information that is produced when the state of that variable is known. It plays a significant role in understanding how disorder and energy distribution relate to thermodynamic systems, particularly as they approach absolute zero.
Irreversibility: Irreversibility refers to the natural tendency of processes to move towards a state of increased disorder, meaning they cannot spontaneously revert to their original state without external work or intervention. This concept is central to understanding the directionality of thermodynamic processes and plays a crucial role in concepts like entropy and the second law of thermodynamics, as well as in analyzing both equilibrium and non-equilibrium states.
Laser cooling: Laser cooling is a technique used to reduce the temperature of atoms or molecules by using laser light to slow down their motion, effectively lowering their kinetic energy. This method is crucial for achieving states of matter like Bose-Einstein condensation and getting closer to absolute zero, as it allows scientists to manipulate atomic motion with high precision and control.
Lord Kelvin: Lord Kelvin, also known as William Thomson, was a prominent physicist and engineer who played a vital role in the development of thermodynamics. He is best known for his work on the absolute temperature scale, which established a framework for understanding temperature and heat transfer in thermodynamic processes. His contributions laid the groundwork for concepts such as the second law of thermodynamics and the Carnot cycle, influencing how we perceive heat engines and the limits of efficiency, as well as the fundamental understanding of absolute zero and the implications of the Third Law of Thermodynamics.
Nernst Heat Theorem: The Nernst Heat Theorem states that as a system approaches absolute zero, the entropy of a perfect crystal approaches zero. This concept is tied to the Third Law of Thermodynamics, emphasizing that it is impossible to reach absolute zero in a finite number of steps. The theorem highlights the behavior of materials at low temperatures and has significant implications for understanding the thermodynamic properties of substances.
Quantum effects: Quantum effects refer to the observable phenomena that arise from the principles of quantum mechanics, particularly at very small scales, such as those of atoms and subatomic particles. These effects highlight the departure from classical physics, showcasing behaviors like superposition and entanglement, which are not explained by classical theories. At extremely low temperatures, such as near absolute zero, quantum effects become more pronounced, leading to behaviors like Bose-Einstein condensation.
Quantum entanglement: Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles become interconnected in such a way that the state of one particle instantly influences the state of another, regardless of the distance separating them. This concept is fundamental to quantum mechanics and has profound implications for our understanding of information transfer and the behavior of particles at extremely low temperatures, where traditional thermodynamic principles may begin to blur with quantum effects.
Superconductivity: Superconductivity is a phenomenon where certain materials exhibit zero electrical resistance and the expulsion of magnetic fields when cooled below a critical temperature. This remarkable behavior occurs close to absolute zero and is deeply connected to the Third Law of Thermodynamics, as it highlights the behavior of materials at extremely low temperatures, where entropy reaches its minimum. Superconductivity has profound implications for technology, allowing for lossless energy transmission and powerful magnetic applications.
Thermodynamic equilibrium: Thermodynamic equilibrium is the state in which a system's macroscopic properties are constant over time and there are no net flows of energy or matter within the system or between the system and its surroundings. In this state, the various forms of energy, such as thermal, mechanical, and chemical, are balanced, leading to uniform temperature and pressure throughout the system. Understanding this concept is crucial for analyzing state variables, processes, and the implications of laws like absolute zero and the Third Law of thermodynamics.
Third Law of Thermodynamics: The Third Law of Thermodynamics states that as the temperature of a perfect crystalline substance approaches absolute zero, the entropy of that system approaches a minimum value, typically taken to be zero. This concept is crucial for understanding the behavior of entropy in various processes, the significance of absolute zero, and phenomena like residual entropy and Bose-Einstein condensation.
Walther Nernst: Walther Nernst was a German physical chemist known for his significant contributions to thermodynamics, particularly in relation to the Third Law of Thermodynamics and the concept of absolute zero. His work established a quantitative understanding of the behavior of systems at very low temperatures, paving the way for advancements in cryogenics and low-temperature physics.
Zero-point energy: Zero-point energy is the lowest possible energy that a quantum mechanical system can have, even at absolute zero temperature. This concept arises from the Heisenberg uncertainty principle, which implies that particles cannot be at rest and must possess some intrinsic energy. This energy has significant implications in various fields such as thermodynamics and quantum mechanics, influencing phenomena like the behavior of particles at low temperatures and the electronic properties of materials.