LOW TEMPERATURE PHYSICS

LOW TEMPERATURE PHYSICS

 INTRODUCTION: The branch of physics which deals with the production and interpretation of natural phenomena at very low temperatures i,e  between absolute zero and ice point is called low temperature physics. It is also called cryogenics.

At low temperatures atoms are arranged in an orderly state. The behavior of matter at low temperatures is different from its behavior at high temperatures. This leads to numerous applications. Low temperature physics is essential to study the properties of matter such as atomic heat, susceptability, conductivity, viscosity etc.

Different methods  of producing low temperatures

1. Freezing mixture,eg; Nacl+ice,Kcl+ice
2. Adiabatic expansion
3. Adiabatic demagnetization
4. Joule Thomson effect
5.  

In this chapter we are going to discuss only Joule Thomson effect and Adiabatic Demagnetization

 Joule Thomson effect

Whenever any gas is allowed to pass from a constant high pressure region to a constant low pressure region through a porous plug there is change in temperature. There may be either fall in temperature or rise in temperature depending on the nature of gas and its initial temperature. This phenomenon is called Joule Thomson effect.

• The difference in temperature is directly proportional to the pressure difference between the ends.
• At ordinary temperatures almost all gases show cooling effect except Hydrogen and helium.
• As the initial temperature increases cooling effect decreases. For every gas at one particular temperature called inversion temperature, there is neither heating nor cooling. Above this temperature there is heating effect.
• For hydrogen and helium the inversion temperatures are -80 degrees and -240 degrees respectively. That is the reason they show heating effect at ordinary temperatures.

POROUS PLUG EXPERIMENT

The experimental arrangement to study joule Thomson effect is shown in the figure.

It consists of a cylindrical tube as shown in the figure. The porous plug divides the tube into two parts .the lower part of the tube is connected to a compression pump through the spiral tube. The gas under investigation can be pumped through the compressor. There are two platinum resistance thermometers to measure the temperature at the two ends. The manometer at the lower end is used to measure pressure. At the top end the pressure is equal to atmospheric pressure. Whole arrangement is kept in constant temperature water bath. The upper portion of the tube is surrounded by a cylindrical vessel and is isolated from water bath.

Working:

The gas under investigation is pumped through the compressor. This gas passes through the spiral tube and enters into the bottom portion the tube .Heat of compression is compensated by the water bath. At the lower end the pressure of the gas is measured using manometer. The gas throttles through the porous plug. Platinum resistance thermometers at the input and output ends show the temperature.

The following observations are made:

1. The difference in temperature is directly proportional to the pressure difference between the ends.
2. At ordinary temperatures almost all gases show cooling effect except Hydrogen and helium.
3. As the initial temperature increases cooling effect decreases. For every gas at one particular temperature called inversion temperature, there is neither heating nor cooling. Above this temperature there is heating effect.
4. For hydrogen and helium the inversion temperatures are -80 degrees and -240 degrees respectively. That is the reason they show heating effect at ordinary temperatures.

Adiabatic Demagnetization

This method Is also called magneto calorific method. Whenever a paramagnetic substance which is already magnetized is demagnetized under adiabatic conditions there is fall in temperature. If the substance is already at low temperature the fall is considerable. The final temperature depends on the paramagnetic substance chosen and the magnetic field applied to magnetize it.

The paramagnetic substances used for this purpose are

Gadolinium sulphate,potassium  Alum, Iron ammonium alum, Cerium Ethyl sulphate ,Double Sulphate of Potassium and Aluminum etc.

The experimental arrangement to obtain low temperatures by the method of adiabatic demagnetization is shown in the following figure

A paramagnetic substance in the form of ring is suspended by using silk or rayon thread into the innermost chamber this chamber is connected to vacuum pump through which required gas at required pressure can be pumped. This chamber is surrounded by another chamber which contains liquid helium at 1k.this chamber is surrounded by another chamber containing liquid hydrogen. The whole arrangement is placed between the pole pieces of electromagnet which produces a strong magnetic field of about 10000 gauss. A coil is wound around the innermost chamber which measures temperature by measuring the susceptibility.

Working:

1. To begin with, He gas at 1k is pumped into the innermost chamber using vacuum pump.
2. The magnetic field is turned on .the paramagnetic substance gets magnetized .Heat of magnetization is transferred to the surrounding chamber through the he gas.ie magnetization is carried under isothermal condition.
3. He gas is removed .Magnetic field is turned off .the magnetic substance uses its internal energy for demagnetization and its temperature falls.
4. The final temperature can be known by measuring the susceptibility of coil surrounding the chamber A.
5. Using this method we can produce lowest temperature of the order of 10-4

Liquefaction of gases:

The physical conversion of gas to liquid is called liquefaction of gases. To convert a gas into liquid two factors should be considered

1. Decreasing the intermolecular distance and
2. Increasing the intermolecular forces.

     A gas cannot be converted into liquid just by applying pressure alone. First its temperature should be reduced below its inversion temperature .then pressure should be applied. Lesser the temperature below critical temperature lesser will be the pressure needed.eg to liquefy nitrogen at -146 degrees pressure needed is 30 atmosphere, at -182 degrees the pressure required is 4 atmosphere and at -196 degrees it can be liquefied at pressure of 1 atmosphere pressure alone.

Liquefaction of gases has major importance, on liquefying a gas it can be stored easily. It occupies less volume, it is easy to transport. Oxygen and Hydrogen on liquefaction can be used as rocket fuel. Liquid oxygen is used in hospitals for artificial respiration.lpg is used for cooking.

Ammonia is the first gas to be liquefied and helium is the last gas to be liquefied.

LIQUEFACTION OF HELIUM BY KAPITSA METHOD :

This method uses both adiabatic expansion and joule thomson effect. This method is different from other methods processes in the sense no lubricant is used here. Any lubricant solidifies at these temperatures. Piston is loosely fitted.

The experimental arrangement to liquefy helium by Kapitza method is shown in the figure.

It consists of compressor p,cold water jacket,refrigerant,heat exchanger and expansion engine.

1. Pure and dry helium is send through the compressor p.this gas passes through the tube which is surrounded by cold water jacket.
2. Cold water jacket removes the heat of compression.aaaaaaaafter that gas enters the spiral tube which is surrounded by liquid nitrogen here the gas is cooled.
3. Here after the gas enters the heat exchanger where the gas is divided into two parts. At A about 10% of the gas traverses downwards where it experiences joule Thomson effect at the throttle valve.
4. The liquefied helium is collected in the Dewar flask. Remaining gas along with 90% of gas at A enters into the expansion engine where it undergoes adiabatic expansion.
5. The cooled gas rises up the tube and enters the compressor to restart the new cycle. In this way after passing through several cycles the gas gets liquefied.

 DIFFERENCE BETWEEN JOULE THOMSON EFFECT AND ADIABATIC EXPANSION

JOULE THOMSON EXPANSION	ADIABATIC EXPANSION
Gas passes from high pressure region to low pressure region through porous plug.	A compressed gas is allowed to expand suddenly.
Gas may experience either cooling effect or heating effect or no effect depending on the initial temperature.	Gas always experience cooling effect.
Ideal gas does not show joule thomson effect.	Both ideal gas and real gas experience cooling effect.
Enthalpy is constant.	Entropy is constant
This effect is irreversible	This is reversible.

EFFECT OF CHLORO FLUORO CARBONS ON OZONE LAYER:

Ozone is present at all altitudes above the earth surface. But its concentration is more at an altitude of 25km in the stratosphere. The thick layer of ozone is called ozonosphere or ozone umbrella. Chemically ozone is O3.this ozonosphere blocks the U.V rays and cosmic rays to enter into the earth atmosphere. These rays cause damage to the life on earth. They lead to spurt of skin cancers, reduction of food production and genetic mutations. The presence of chlorofluorocarbons in the atmosphere causes an impayable damage to the ozonosphere. The chlorine reacts with O3and converts into ordinary oxygen. Each chlorine atoms can react with as much as 1000 chlorine atoms. This leads to the formation of holes in ozone layer allowing the harmful U.V rays into the earth atmosphere causing great damage to plant and animal life. Generally the coolants used in refrigerators and air conditioners release the harmful chlorofluoro carbons into the atmosphere. If we don’t control the usage our future generations will be in danger.

Advantages of substances at low temperatures

The behavior of substances at low temperatures will be different from their behavior at ordinary temperatures this made them useful in applications in the branches of science industry and research.

1. Conductors at low temperature become superconductors. This enables them to use in transmission of electrical energy without loss of power.
2. At 90k almost all the chemical reactions cease. Therefore animal and vegetable matter will not perish at these temperatures.
3. Liquid oxygen and liquid hydrogen are used in rockets as fuel.
4. Liquid oxygen is used in hospitals for artificial respiration.
5. Magnetic levitation is based on superconductivity which is used in superfast trains in cold countries.

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