Third Law of Thermodynamics- Absolute zero, Cryogenics, Adiabatic Demagnetization

In this article we will be talking about third law of Thermodynamics: Absolute zero, Cryogenics, Adiabatic demagnetization. Change of properties. Applications. Microscopic foundations of thermodynamics.

The second principle suggests that an absolute temperature scale with absolute temperature zero occurs. The Third Law of Thermodynamics states that no procedure consisting of a finite number of steps can reach absolute zero. Absolute zero can be approached indefinitely, but can never be reached. At absolute zero the system has the least possible energy (kinetic plus potential).

Absolute zero

It is the lowest temperature theoretically possible. Absolute zero corresponds to -273.15 ° C, or zero on the thermodynamic scale or Kelvin (0 K).

In relation with gas experiments, the idea of an absolute zero temperature emerged first; when a gas is cooled without affecting its volume, its pressure decreases with temperature. Although this experiment can’t be performed beyond the gas dew point, the graph of pressure versus temperature’s experimental values could be extrapolated to zero pressure. The temperature at which the pressure will be is the absolute zero of temperature. This derived notion was shown to be consistent with theoretical definitions of zero.

Atoms and molecules of an object at absolute zero would have as little movement as possible. They would not be completely at rest, but they could not lose more movement energy, so they could not transfer heat to another object.

According to the third law of thermodynamics, the entropy (or disorder) of a pure crystal would be null at absolute zero; this is of considerable importance in the analysis of chemical reactions and in quantum physics. Materials have strange properties when they are cooled to very low temperatures. Some completely lose their electrical resistance.

You cannot physically reach absolute zero, but you can get as close as you want. To achieve very cold, or cryogenic, temperatures, special procedures are needed. Liquid helium, which has a normal boiling point of 4.2 K (-268.9 ° C), can be obtained from cryostats, extremely well-insulated containers. If this helium is evaporated under reduced pressure, temperatures of up to 0.7 K can be reached. For lower temperatures, it is necessary to resort to the subsequent magnetization and demagnetization of paramagnetic substances (not very magnetizable), such as chromium alum.

This method employs a magnetic field that aligns the electronic spins of the material, which is cooled in a bath of liquid helium. When the magnetic field is removed, the spins revert to a random orientation, which reduces the thermal energy of the material and therefore its temperature. With the demagnetization of paramagnetic salts, temperatures of only 0.002 K have been reached, and the demagnetization of atomic nuclei has allowed temperatures of only 0.00001 K.

Measuring temperatures close to absolute zero presents special problems. Gas thermometers can only be used above the dew point of helium. At lower temperatures, electrical and magnetic measurements must be used to determine the actual temperature.

The concept of absolute zero is also important from a theoretical point of view. According to the thermodynamics, the entropy – or disorder – of a pure crystal would be null at absolute zero; this is of considerable importance in the analysis of chemical reactions and in quantum physics. Materials have strange properties when they are cooled to very low temperatures. Some completely lose their electrical resistance. Thermodynamics third law,this effect was first observed in mercury a few degrees above absolute zero, but it is being obtained at increasingly higher temperatures with new materials.

Cryogenics

Study and use of materials at very low temperatures. An upper limit for cryogenic temperatures has not been agreed, but has suggested that the term cryogenics be applied for all temperatures below -150 ° C (123 K). Some scientists consider the normal boiling point of oxygen (-183 ° C) as the upper limit.

Cryogenic temperatures are achieved by rapid evaporation of volatile liquids or by expansion at pressures between 150 and 200 atmospheric pressure of confined gases. Expansion can be simple, that is, through a valve that communicates with a lower pressure region, or take place in the cylinder of an alternative engine, where the gas drives the engine piston. The second method is more efficient, but it is also more difficult to apply.

Humphry Davy and Faraday generated gases by heating a suitable mixture at one end of an inverted V-shaped watertight tube. The other end was kept in a mixture of ice and salt to cool it down. The combination of low temperatures and high pressures caused the generated gas to liquefy. Upon opening the tube, the liquid evaporated rapidly and cooled to its normal boiling point. By evaporating solid carbon dioxide mixed with ether at low pressures, Faraday achieved a temperature of approximately 163 K (-110 ° C).

If a moderate temperature gas expands through a valve, its temperature increases. But if its initial temperature is below the so-called inversion temperature, the temperature reduction is caused by the expansion: this is what the Joule-Thomson effect is called. The inversion temperatures of hydrogen and helium, two basic cryogenic gases, are extremely low and must first cool below their inversion temperatures to achieve temperature reduction by expansion: hydrogen by liquid air and helium by liquid hydrogen. Generally, this method does not achieve gas liquefaction in one step, but by chaining the cascading effects, Cailletet and Pictet independently managed to produce a few drops of liquid oxygen.

Physicist Onnes set up the first liquid air production plant, using the cascade principle. Over the years, different researchers developed various process improvements. Chemist Dewar was the first to liquefy hydrogen, and Onnes did the same with helium, the most difficult gas to liquefy. One of the challenges has continued to be improving efficiency by having the refrigerant gas operate in an alternative engine or turbine. The works of Kapitsa and Collins were notable. A helium blender based on the Collins design has made it possible for many non-specialized laboratories to conduct experiments at the normal boiling point of helium, 4.2 K (-268.9 ° C).

Evaporation of liquid helium at reduced pressure produces temperatures of up to 0.7 K (-272.45 ° C). Even lower temperatures can be achieved by adiabatic demagnetization. In this process, a magnetic field is established around a paramagnetic substance held in liquid helium to cool it. The field aligns the electronic spins; When disconnected, the spins regain their random orientation, reducing the thermal energy of the entire sample. This allows the temperature to drop to levels of only 0.002 K. Likewise, the alignment of the nuclear spins followed by the disconnection of the magnetic field has produced temperatures close to 0.00001 K.

Dewar flasks or thermos flasks have proven useful for storing liquids at cryogenic temperatures. These containers are made up of two flasks, one inside the other, separated by a space in which the vacuum has been made. The outside of the inner bottle and the inside of the outer bottle are coated with a reflective coating to prevent heat from passing through the vacuum by radiation. Substances cooler than liquid air cannot be handled in open Dewar flasks, because the air would condense on the sample or form a solid stopper that would prevent the release of the released vapors; These would accumulate and eventually break the container.

The measurement of temperatures in the cryogenic zone presents difficulties. One procedure is to measure the pressure of a known amount of hydrogen or helium, but this method fails at the lowest temperatures. The use of the vapor pressure of helium 4, that is, helium of atomic mass 4, or helium 3 (atomic mass 3), improves the previous procedure. Determining the electrical resistance or magnetic properties of metals or semiconductors further extends the scale of measurable temperatures.

Change of Properties

At cryogenic temperatures, many materials behave in an unknown way under normal conditions. Mercury solidifies and the rubber becomes as brittle as glass. The specific heat of gases and solids decreases in a way that confirms the predictions of quantum theory. The electrical resistance of many (and not all) metals and metalloids drops sharply to zero at temperatures of a few kelvins. If an electric current is introduced into a cooled metal ring until it becomes a semiconductor, the current continues to circulate through the ring and can be detected hours later. The ability of a semiconductor material to sustain a current has allowed experimental computer memory modules to be designed to operate at these low temperatures. However,

The behavior of helium 4 at low temperatures is surprising in two respects. First, it remains liquid even after extreme cooling. To solidify helium 4, it is necessary to subject the liquid to a pressure greater than 25 atmospheres. Furthermore, liquid helium 4 reaches a state of super fluidity at temperatures below 2.17 K (-270.98 ° C). In this state, its viscosity seems to be almost nil. It forms a film on the surface of the container, through which it flows without resistance. Helium 3 is also super fluid, but only at temperatures below 0.00093 K.

Applications

Among the many important industrial applications of cryogenics is the large-scale production of oxygen and nitrogen from the air. Oxygen has many uses: for example, in rocket engines, in blast furnaces, in cutting torches and welding or to make breathing possible in spacecraft and submarines. Nitrogen is used in the production of ammonia for fertilizers or in the preparation of frozen foods that cool quickly enough to prevent cell tissues from being destroyed. It is also used as a refrigerant and for the transport of frozen food.

The commercial carriage of liquefied natural gas has been made possible by Cryogenics. Without cryogenics, nuclear physics research would lack liquid hydrogen and helium for particle detectors and for the strong electromagnets used in large particle accelerators. These magnets are used in experiments into nuclear fusion as well. Some infrared devices, lasers, and lasers also require cryogenic temperatures.

Cryogenic surgery or cryosurgery is used in the treatment of Parkinson’s disease: tissue is selectively destroyed by freezing it with a small cryogenic probe. A similar technique has also been used to destroy brain tumors and stop the progression of cervical cancer.

Microscopic Foundations of Thermodynamics

The discovery that all matter is made up of molecules provided a microscopic basis for thermodynamics. A thermodynamic system consisting of a pure Material Could be described as a Pair of Equivalent molecules, each of which has an individual movement that can be described with mechanical variables such as speed or linear momentum. If so, it needs to be possible, at least in principle, to calculate the collective properties of the system by solving the equations of motion of the molecules. In this sense, thermodynamics could be considered as a simple application of the laws of mechanics to the microscopic system.

Objects of normal dimensions, on a human scale, contain immense amounts of molecules (according to Avogadro on the order of 10 24 ). Assuming that the molecules were spherical, it would take three variables to describe the position of each and another three to describe their speed. It would be a challenge to characterise a macroscopic device in this manner that not even the largest modern machine could do. Also, a complete solution of those equations will tell us where each molecule is and what it is doing at each moment. Such an enormous amount of information would be too detailed to be useful and too fleeting to be important.

Hence, statistical systems were created to get the mean values ​​of the mechanical factors of the molecules of a machine and to deduce from the overall qualities of this machine. These general features prove to be the macroscopic thermodynamic factors. The statistical treatment of molecular mechanisms is known as statistical mechanics, also it supplies thermodynamics using a mechanical foundation.

From a statistical perspective, temperature represents a measure of the average kinetic energy of the molecules in a system. The increase in temperature reflects an increase in the intensity of molecular movement. When two systems are in contact, as a consequence of collisions, energy is transferred between their molecules. This transition continues until uniformity, which corresponds to thermal equilibrium, is achieved in the statistical sense. The kinetic energy of the molecules corresponds to heat, and, along with the Possible energy about the Connections between the molecules, constitutes the internal energy of a system.

The law of conservation of energy becomes the first principle of thermodynamics, and the concept of entropy corresponds to the magnitude of the disorder on a molecular scale. Assuming that all combinations of molecular motions are just as likely, thermodynamics shows that the more disordered the state of an isolated system, the more combinations there are that can lead to that state, so it will occur at a higher frequency.

The probability of the most disordered state occurring is overwhelmingly greater than that of any other state. This possibility gives a statistical foundation for defining the balance condition and entropy. Ultimately, temperature may be diminished by eliminating energy from a method, in other words, by lessening the intensity of molecular movement. Total zero corresponds to the condition of a system where all of its elements are in rest. But this theory belongs to classical physics. An investigation of the statistical foundation of the next principle goes beyond the constraints of the discussion.

Recommended Books:

The Laws of Thermodynamics: A Very Short Introduction