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 Third Law of Thermodynamics states that a perfect crystal's entropy at absolute zero is zero. It's impossible to reach absolute zero in finite steps, but understanding this concept helps explain phenomena like superconductivity and superfluidity in materials at very low temperatures.

Absolute Zero and the Third Law

Definition of absolute zero

• 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
  • Laser cooling
• Cost and energy required to maintain extremely low temperatures become prohibitively high
• Quantum mechanical effects like zero-point energy 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 ($PV = nRT$)
• 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)