The Third Law of Thermodynamics

Introduction

State the Third Law of Thermodynamics

The Third Law of Thermodynamics is stated as: at the temperature of 0 K, the entropy of perfect crystals is zero.

Absolute zero

Most people use the Celsius scale to talk about temperature, whereas a few countries use the Fahrenheit system. On the other hand, Kelvins are the fundamental unit of absolute temperature measurement used by scientists all around the world. 

This scale is based on the following physical principles: The temperature at which all molecular motion halt is absolute zero Kelvin. In the most basic sense, heat is molecular motion, so no motion implies no heat. A temperature of zero Kelvin equals no heat. 

This is not the same as a freezing point, such as 0 degrees Celsius, because ice molecules still have tiny internal motions, sometimes known as heat. However, as the options for different molecular arrangements of a substance suddenly and rapidly grow or decrease with temperature, phase shifts between solid, liquid, and gas cause significant variations in entropy.

Details of Third Law of Thermodynamics

 It has a significant impact on how modern industries operate and the ideas on which our equipment is built. Everything in the universe is subject to these laws. These laws are described in terms of thermodynamics and explain physical circumstances in terms of a standard. The third law of thermodynamics, in particular, discusses the universe as a whole and the degree of randomness that exists inside it. This law treats everything as a single system and establishes rules for the randomness of the system’s components in terms of temperature dependence. Fixed criteria and an understanding of what is practically possible are critical in science. As a result, the third law uses absolute zero as a starting point.

As the temperature approaches absolute zero, the third law of thermodynamics is concerned with the limiting behaviour of systems. Because most thermodynamics calculations rely solely on entropy differences, the entropy scale’s zero point is frequently overlooked. On the other hand, the third law is discussed for completeness’ sake because it depicts the state of zero entropy.

The concept of “absolute zero” is addressed in this law. The lowest temperature on the Kelvin Scale, absolute zero, is zero. On a Celsius scale, this is -273.15, and on a Fahrenheit scale, it is -459.7.

According to the third Thermodynamics law, when a system’s temperature approaches absolute zero, its entropy approaches a constant value. This shows that a system’s randomness has a minimum value, which is reached when the temperature reaches absolute zero. When two thermodynamic systems are combined to produce an isolated system, all energy exchange between the two systems is constrained in any form.

This can also be explained by the fact that the system has no heat at absolute zero, indicating the presence of all molecules at their lowest energy levels. 

This equation should make sense since, according to our basic knowledge of thermodynamics, when the temperature of a system lowers, the heat of the system, which is simply a collection of kinetic energy, reduces as well. As a result, the kinetic energy eventually comes to a full stop, implying no randomness.

The Third Law: A Mathematical Explanation

The Third Law of Thermodynamics is represented as follows in mechanics:

S – S0 = kB lnΩ

S = the system’s entropy,

S0 = its entropy in the initial stage,

kB= is the Boltzmann constant,

Ω = the total number of microstates that make up the system’s macroscopic configuration.

Entropy

Entropy is a physical number that shows a system’s molecular disorder or unpredictability. It is the amount of thermal energy in a system that cannot be used to perform useful work. This is due to the fact that work is obtained from a system’s ordered movement of molecules. The concept of entropy aids in understanding the spontaneity of any process and determining which processes are thermally viable or impossible. The phenomena of the irreversibility of reactions are explained in this way.

The entropy of a system is inversely proportional to its temperature. 

ΔS=Q/T, where

The entropy change in the system is denoted by ΔS.

The heat absorbed is denoted by the letter Q.

Temperature is denoted by T.

The Third Law of Thermodynamics contradictions with Other Laws of Thermodynamics

The second law of thermodynamics, on the other hand, says that the temperature can never reach absolute zero, although the third law talks about it as a state. According to the second law, heat cannot be transferred spontaneously from a colder to a hotter body. When a system attempts to approach absolute zero, it tends to pull thermal energy from the surrounding environment, which means it will never achieve absolute zero.

Because the first law specifies that energy cannot be created or destroyed, heat energy must be extracted from somewhere outside the system, thus putting an end to the system’s chances of reaching absolute zero.

The Various Applications of the Third Law of Thermodynamics

While absolute zero does not exist in nature and cannot be achieved in the laboratory, the concept of absolute zero is essential for temperature and entropy calculations. Many measurements imply a connection to some point of origin. When we state a distance, we must first consider measuring it against it. When we specify a time, we must follow up with the question, “Time since when?” Positive values on the temperature scale gain meaning when the zero value is defined. When a temperature is written as 100 K, it signifies that it is 100 degrees Celsius above absolute zero, which is twice as far as 50 K and half as 200 K.

At first glance, the Thermodynamics Third Law appears to be straightforward and obvious. It does, however, clearly discuss the nature of heat and thermal energy at the end.

Conclusion

The Third Law of Thermodynamics statement mentions that a system’s entropy at the temperature of absolute zero attains a constant value. This value is influenced by factors such as pressure and magnetic field. At absolute zero, the system’s least possible energy value is zero.