Everyone Can Understand The Third Law of Thermodynamics — Including You!
The third law of thermodynamics states, “the entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K)”.
But what does this statement mean? In this article, I will discuss the meaning behind this law and present it in a clear, comprehensible manner. Continue reading if you want to broaden your physics knowledge like me ,̶ ̶o̶r̶ ̶i̶f̶ ̶y̶o̶u̶ ̶w̶a̶n̶t̶ ̶t̶o̶ ̶l̶e̶a̶r̶n̶ ̶s̶o̶m̶e̶t̶h̶i̶n̶g̶ ̶c̶o̶m̶p̶l̶e̶x̶ ̶a̶n̶d̶ ̶s̶h̶o̶w̶ ̶o̶f̶f̶ ̶t̶o̶ ̶y̶o̶u̶r̶ ̶f̶r̶i̶e̶n̶d̶s̶!
Entropy and Microstates
You may have heard that Entropy is a measurement of disorder or randomness. This is how entropy is often described, but this definition has its drawbacks. The classical thermodynamic view asserts that “entropy is the measure of energy that is unavailable to do work”.
Nevertheless, I believe that a more comprehensible definition is that it is a measure of distribution of energy in the system.
Another relevant term is the microstate. A microstate is defined as the arrangement of each molecule in the system at a single instant. Thus, with a higher entropy, there are more possible distributions of energy in the system, thus more microstates can possibly exist.
So what is a perfect crystal?
A perfect crystal is a crystal that contains no point, line, or planar defects. These crystals have no imperfections, which implies that it is in an orderly state.
Absolute Zero
Absolute Zero (0K or −273.15 °C) is the theoretical lowest temperature a thermodynamic system can reach. This is the state in which the enthalpy and entropy of an ideal gas reaches their minimum value. At this temperature, particles in a substance are essentially motionless.
Piecing together the Third Law of Thermodynamics
The Third Law of Thermodynamics can also be expressed in the following equation:
S represents the entropy of the system, k_B represents the Boltzmann Constant, and Ω represents the total number of microstates.
A perfect crystal in absolute zero’s particles are not moving, meaning that there is no kinetic energy. Thus, the total number of microstates, or how energy is distributed, is one.
Ω equals to 1, so ln Ω equals 0. Thus, entropy of the system is equal to zero, which proves the Third Law of Thermodynamics valid.
And that’s it! Let me know if you want to learn more about thermodynamics in the comments :)
