Showing posts with label pressure. Show all posts
Showing posts with label pressure. Show all posts

WHAT IS LEAK RATE IN VACUUM SCIENCE?



Ideally it is impossible to build a vacuum system with out any leak. Without active pumping the pressure in a real system will rise with time. 

Pressure rise is produced by outgassing and by gas molecules penetrating through leaks from the outside into the vacuum system.


 The leak rate must be small enough not to prevent to reach the required pressure level.

A real leak will yield a linear pressure rate-of rise curve. The slope of the curve is a function of the leak rate and the volume of the system: 

 
Leak is related to how many molecules per unit time are being admitted into volume.
According to the gas laws, if volume, temperature and pressure are specified, it is possible to determine how many molecules are there inside the volume.

Torr-liter defines the molecules contained in a one liter volume at a pressure of 1 Torr at 0 °C.

 22.4 liters of gas at 760 Torr and 0 °C equals 6.02x1023 molecules (Avogadro’s number)

 1 Torr-liter would then contain about 3.5 x 1019 molecules.

std cc represents the number of molecules contained in a 1 cc volume at a pressure of 760 Torr and 0°C

1 std. cc contains about 2.7 x 1019 molecules 

 The leak rate is defined as the pV-throughput of a gas through a leak.
 It is a function of the type of gas, pressure difference and temperature.

 In a system of volume V the leak



Here Δp is the pressure rise during the time interval Δt .




What is Boyles Law?

Boyle's Law is a statement of the relationship between the pressure and volume of gasses. Specifically it states that under isothermic conditions, i.e. when temperature remains constant, the product of the pressure and volume remains constant, or
P1 x V1 = P2 x V2
where P1 is the pressure before some change, V1 is the volume before the change, P2 and V2 are the new values after the change.

What is desorption?

When a material is placed in Vacuum, the gas which was previously adsorbed begins to desorb i.e. to leave the material.

The desorption is influenced by

1.       Pressure
2.       Temperature
3.       Shape of material
4.       Kind of its surface

The pressure has a basic influence on the desorption phenomenon since according to its tendency of increasing over or decreasing below the equilibrium , the phenomenon of sorption or that of desorption appears.

The temperature has a clear influence on desorption phenomena. Desorption is endothermic, thus it is accelerated by increase of temperature.

The shape of the material influences desorption either if the gas is adsorbed or absorbed.


If the gas is adsorbed, then only the amount of surface is the influencing factor, but if the gas has to diffuse from the interior of the material to the surface, then the third dimension “thickness” is also influencing the rate of desorption. 

Third Law of Thermodynamics ( The Law of zero entropy )


"Nernst" in 1906 proposed a general priniciple supported by series of experimental tests on problem of atomic heat at low temperatures. it was proposed as " The new heat theorem " and is called as third law of thermodyanmics.

Nernst statement

                       " The heat capacities of all solids tend to zero as the absolute zero of temperature is approached and   that the internal energies and entropies of all substances become equal there, approaching their common value asymptotically".

This law neither follows from first law or second law nor is totally a new law.

Other statement of Nernst:

                        " No entropy change takes place when pure crystalline solid reacts at absolute zero".

 Plank statement:

                        " The entropy of a solid or a liquid is zero at absolute zero of temperature".

Lewis and Randall statement 

                       "Every system has finite positive entropy, but at the absolute zero of temperature the entropy may become zero and does so become in the case of a pure crystalline substance".      

But this statement is confined to pure crystalline solids because theoretical argument and experimental evidence have shown that the entropy of solutions and super cooled liquids is not zero even at absolute zero.

For instance, ice always has residual entropy at absolute zero. It also doesn't apply to amorphous class of substances like glass etc.

Importance of third law of thermodynamics

  • Third law is useful in explaining the nature of bodies in neighborhood of absolute zero.
  • It permits the calculations of absolute values of entropy and physical interpretation of thermodynamic properties such as Helmholtz & Gibbs free energies etc.
  • It can be conceived that as the temperature of system tends to absolute zero, its entropy tends to a constant value which is of pressure and state of aggregation etc. 
 "Nernst" formulated that "the entropy change in isothermal reversible process of condensed system approaches zero as temperature at which the process occurs approaches zero".

The principle of Barthelot states that "every chemical transformation which takes place with out the intervention of external energy tends towards the production of that substance or systems of substance which will give the greatest development of heat i.e that process is realized which is most exothermic.
       

All about Second Law of Thermodynamics?


Why second law of thermodynamics introduced? 

We know that some processes occur spontaneously but if we try to reverse the direction of process, the process do not occur spontaneously and further some external energy is required to move the given system away from state of equilibrium. 

The question is that "why such reversed processes do not occur spontaneously?" could not be answered by first law because the total energy of system would remain constant in the reversed process as it did in the orginal path and ther is no voilation of first law. Therfore there must be some other natural principle in addition to first law which determines the direction in which a process can take place in an isolated system. This principle is "second law of thermodynamics" .



Second law infers us that "the entropy of universe tends to maximum". 


Second law of thermodynamics in terms of entropy:

  "The entropy of an isolated system is fully conserved in every reversible process i.e. for every reversible process the sum of all changes in entropy taking place in an isolated system is zero. If the process is not a reversible one, then the sum of all changes in entropy taking place in an isolated system is greater than zero. In general we can say that in every process taking place in an isolated system the entropy of system either increases or remains constant."  

Condition for equilibrium of an isolated system

“If an isolated system is in such a state that its entropy is maximum, any change from that state would evidently lead to decrease in entropy and hence will not happen. Thus the necessary condition for equilibrium of an isolated system is that its "Entropy shall be maximum."


 Other forms of second law of thermodynamics

Kelvin-Planck statement : It is impossible to construct a device which, operating in a cycle has the sole effect of extracting heat from a single reservoir and performing equivalent amount of work.

Clausius statement : It is impossible for heat to flow from a cooler body to another hotter body  without the aid of external energy.

 "Study on heat engines is based on the above law"