CONTENTS

• Definition.
• Statement.
• Implications of Third Law.
• Examples.
• Conclusion.

Third Law of Thermodynamics-Definition

The Third Law of Thermodynamics states that as the temperature of a system approaches absolute zero (0 K), the entropy of a perfect crystalline substance approaches zero. This law provides an anchor point for the measurement of entropy and has profound implications for low-temperature physics.

Statement of the Third Law

1. Entropy at Absolute Zero:
   • At absolute zero (0 Kelvin), a system exists in its ground state (minimum energy state), and the number of microstates (Ω) is exactly 1. Therefore, according to Boltzmann’s entropy formula:
   • S=kBln⁡Ω=kBln⁡(1)=kB​ln(1)=0 where S is entropy, kB​ is the Boltzmann constant, and Ω is the number of accessible microstates.
2. Practical Statement:
   • It is impossible to reach absolute zero through any finite number of steps or processes. This is due to the diminishing energy differences between states as temperature decreases, making it harder to remove the remaining thermal energy.

Key Implications of the Third Law

1. Absolute Entropy:
   • The Third Law allows for an absolute scale of entropy by assigning zero entropy to a perfect crystal at 0 K.
   • Perfect Crystal: A substance is considered a perfect crystal when its atoms are arranged in a completely ordered and defect-free lattice.
2. Low-Temperature Behavior:
   • As temperature approaches absolute zero, physical properties of matter, such as heat capacity, approach zero because the system has no additional energy to give up.
3. Unattainability of Absolute Zero:
   • In practice, no matter how efficient a cooling process is, reaching absolute zero is impossible due to the energy required to remove the last bit of heat becoming infinitesimally small.

Practical Examples

1. Perfect Crystal at Absolute Zero:
   • In a perfect diamond lattice at 0 K, all atoms are fixed in their lattice positions with no randomness or disorder, and hence entropy is zero.
2. Superconductivity:
   • Materials like certain metals become superconducting at very low temperatures. This transition involves the reduction of entropy as the material enters a highly ordered quantum state.
3. Helium and Quantum Effects:
   • Helium-3 and Helium-4 do not solidify at standard pressures near absolute zero because of quantum mechanical effects, demonstrating unusual low-temperature behaviors.

Comparison with Other Laws

• First Law: Conservation of energy.
• Second Law: Entropy tends to increase in an isolated system.
• Third Law: Absolute zero is unattainable, and entropy approaches zero at 0 K.

Conclusion

The Third Law of Thermodynamics provides a fundamental understanding of how entropy behaves at very low temperatures. It asserts that as temperature approaches absolute zero, entropy approaches a minimum value, typically zero for a perfect crystal. While real systems may never reach absolute zero, the third law allows scientists to make predictions and understand the behavior of systems at extremely low temperatures. This law has profound implications in fields like cryogenics, low-temperature physics, and quantum mechanics.

ALSO READ:

• First Law of Thermodynamics.
• Second Law of Thermodynamics.
• Zeroth law of Thermodynamics.