Third Law of Thermodynamics – 3rd Law
The third law of thermodynamics was developed by the German chemist Walther Nernst during the years 1906–12. For this research Walther Nernst won the 1920 Nobel Prize in chemistry. Therefore the third law of thermodynamics is often referred to as Nernst’s theorem or Nernst’s postulate. As can be seen, the third law of thermodynamics states that the entropy of a system in thermodynamic equilibrium approaches zero as the temperature approaches zero. Or conversely the absolute temperature of any pure crystalline substance in thermodynamic equilibrium approaches zero when the entropy approaches zero.
Nernst Heat Theorem (a consequence of the Third Law) is:
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 operations.
The Nernst heat theorem was later used by a German physicist Max Planck to define the third law of thermodynamics in terms of entropy and absolute zero.
Some materials (e.g. any amorphous solid) do not have a well-defined order at absolute zero. In these materials (e.g. glass) some finite entropy remains also at absolute zero, because the system’s microscopic structure (atom by atom) can be arranged in a different ways (W ≠ 1). This constant entropy is known as the residual entropy, which is the difference between a non-equilibrium state and crystal state of a substance close to absolute zero.
Note that the exact definition of entropy is:
Entropy = (Boltzmann’s constant k) x logarithm of number of possible states
S = kB logW
This equation, which relates the microscopic details, or microstates, of the system (via W) to its macroscopic state (via the entropy S), is the key idea of statistical mechanics.
Absolute zero is the coldest theoretical temperature, at which the thermal motion of atoms and molecules reaches its minimum. This is a state at which the enthalpy and entropy of a cooled ideal gas reaches its minimum value, taken as 0.
lim ST→0 = 0
S = entropy (J/K)
T = absolute temperature (K)
Classically, this would be a state of motionlessness, but quantum uncertainty dictates that the particles still possess a finite zero-point energy. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, and −459.67 °F on the Fahrenheit scale.