The Consequences of The Third Law of Thermodynamics

Third law of thermodynamics

We can find a few hints of the third law of thermodynamics in the interpretation of the second law. Like in refrigeration the co-efficient of performance (COP) is:

Third law of thermodynamics

It is the ratio of the heat removed from a cold object to the work supplied for achieving the same removal of heat.

If the COP is highwe have to do less work to achieve the required heat removal. From equation (1) it is obvious that COP is zero if the temperature of the cold body goes to zero. It means we have to do an infinite amount of work to cool down an object up to absolute zero temperature.

Absolute Entropy and Third Law of Thermodynamics

In the second law of thermodynamics, we observed two interpretations of entropy. One discussed by Rudolph Clausius is a change in entropy. It is a thermodynamic property.

The second discussed by Ludwig Boltzman is absolute entropy. It is a statistical property.

The absolute entropy tells us a system which has a non-degenerated ground state has zero entropy at absolute zero temperature. However, Clausius entropy can have a different value at absolute zero.

The third law of thermodynamics confirms that both the interpretations of entropy represent the same quantity. In other words, a change in entropy (Clausius) is the same as the change in the disorder of the system (Boltzman). Whenever we add or remove heat from a system, the number of possible microstates in which the system can rearrange itself changes.

Applications of Third Law of Thermodynamics

Interesting things happen at very low temperatures. The concept of absolute zero temperature and the probabilities of attaining it fascinated scientists to a great extent. This fascination led to the discovery of a whole new spectrum of physics and the scientists also came up with some wonderful inventions as well.

Third law of thermodynamics

The scientists discovered that if we cool certain objects near absolute zero temperature, they show zero resistance to the flow of current. H.K Onnes discovered this wonderful phenomenon in 1911 when he succeeded in cooling substances up to 4K.

It is the temperature at which the Helium gas liquefies and remains in the same phase. The biggest application of superconductivity so far is the MRI (Magnetic Resonance Imaging) technique used in medical science.

The countries like Japan, and South Korea built high-speed magnetic levitating trains by using the concept of superconductivity. The Maglev (Magnetic levitation) is a system in which they use two sets of strong magnets.

One pushes the train up off the track and the other set moves it ahead. This amazing technique almost eliminates the effect of friction and as a result, the train achieves huge speeds.

Third law of Thermodynamics
Magnetic Levitation

Another important property of substances at very low temperatures is superfluidity. Helium acts as a superfluid at 1K. Helium has zero viscosity at 1K. It acts like a thin film and climbs over the walls of the container. It creeps over the surface of the container in which we keep it.

For more information about the superfluidity of helium, read Superfluid helium-4.

Cool things happened at very low temperatures. So the scientists got obsessed with achieving zero K temperature. Therefore, they started working on different techniques and experimentations to go near absolute zero.

The advent of quantum mechanics made us realize that every molecule has some extent of magnetism in it. The electrons spin randomly in their orbitals.

Adiabatic Demagnetization

The idea of adiabatic demagnetization is to apply a strong magnetic field which will thus align the spins of electrons. Then as we demagnetize the electrons, the system loses some amount of heat which results in decreasing the temperature.

Electron Spins with and without external magnetic field

The concept was to repeat this experiment to and fro until we reach the absolute zero temperature. The first step was isothermal magnetization, i.e. the application of a strong magnetic field. And the second one was adiabatic demagnetization.

Since scientists performed this experiment a number of times before they came to the conclusion that it was not possible in a finite number of steps. Because both the curves meet each other before they reach 0 K.

Third law of thermodynamics
Adiabatic Demagnetization

This is what the third law of thermodynamics tells us:

“It is impossible to reach zero kelvin in a finite number of steps”.

As we see the above plot between Temperature and Entropy it says that if we reach absolute zero, the entropy of a perfect crystal would be zero.

The third law of thermodynamics, therefore, establishes a reference point for the calculation of the absolute entropy of a system. Since the absolute entropy for a system with its non-degenerate ground state is zero at zero kelvin.

Scope of Third Law

It is quite evident from the above discussion that the third law troubles those who try to reach absolute zero. For it has no practical limitations on the daily life processes. The first three laws however have a significant amount of dictation on how thermodynamic changes would occur on daily basis.

Scientists considered that a perfect ideal gas could be regarded as the starting point of many theoretical formulations and discussions. But the possibility of such an idea vanishes at T=0. Since we’ve already discussed that the product of pressure and temperature (PV) is a function of temperature.

Therefore, at absolute zero this product turns out to be zero but the thermodynamic properties of a perfect gas suggest that the PV product becomes infinite at T=0.


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