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Q. State the postulates of kinetic molecular theory of gases.
Kinetic theory of gases
The behavior of gases is well described by the kinetic theory It relates macroscopic properties (T. P. and V etc) of gases to microscopic properties (K E etc) . It provides a mathematical model to study the behavior of gases Postulates
A finite, volume of gas consist of very large number of molecules The size of the molecules is much smaller than the separation between molecules The gas molecules are in random motion and may change their direction of motion after every collision
Collisions between gas molecules themselves and with walls of container are assumed to be perfectly elastic.Molecules do not exert force on each other except during a collision
Q. Discuss the term "Thermodynamics". Also describe the terms: System, Surrounding. Boundary and State variables?
Answer
Thermodynamics
Thermodynamics deals with the science of motion (dynamics) and/or the transformation of heat (thermo) and energy into various other energy-containing forms The flow of energy is of great importance to engineers involved in the design of the power generation and process industries. Thermodynamics provides an understanding of the nature and degree of energy transformations, so that these can be understood and suitably utilized
System
A system is a region containing energy and/or matter that is separated from its surroundings by arbitrarily imposed walls or boundaries
In a thermodynamic analysis, the system is the subject of the investigation Closed system (control mass):
energy, but not matter can be exchanged with the environment Examples a tightly capped cup of coffee
open system (control volume):
Both energy and matter can be exchanged with the environment Example an open cup of coffee
Isolated system
Neither energy nor mass can be exchanged with the environment in fact, no interactions with the environment are possible at all
Example coffee in a closed, well-insulated thermos bottle Surrounding
Everything external to the system is the surroundings
Boundary
A boundary is a closed surface surrounding a system through which energy and mass may enter or leave the system
State variables
State Variables are Path Independent meaning that the change in the value of the state variable will be the same no matter what path you take between the two states This is not true of either the work W or the heat
If a system is formed through a cycle that returns it to its original state, then a variable will only be a state variable if variable returns to its original value.
State Variables are only measurable when the system is in Equilibrium Examples of State Variables Temperature Pressure Volume, Entropy, Enthalpy. Internal Energy Mass Density
Q. Write a note on Internal Energy and show that it is independent of the path
Answer
Internal energy
The sum of all the forms of molecular energies (such as kinetic or potential energy) of a substance is called internal energy
Explanation
It is the study of thermodynamics an ideal gas is usually considered as a working substance The molecules of an ideal gas are mere mass points which exert no force on one another So the internal energy of an ideal gas system is generally the translational KE of the molecules Since Take>, this the internal energy of an ideal gas is directly proportional to its temperature
How can we increase the internal energy
1. By heating
When we heat a substance, energy associated with its atoms or molecules is increased ie heat is converted to internal energy
2. By doing mechanical work
When two objects are rubbed together, their internal energy increases because of mechanical work The increase in temperature of the object indicates an increase in the internal energy
Note Similarly, when an object slides over any surface and comes to rest because of frictional forces, the mechanical work done on or by the system is partially converted into internal energy Internal energy in a state function
In thermodynamics internal energy is function of state Consequently, it does not depend on the path but depends on initial and final states of the system
Explanation
Consider the system which undergoes a pressure and volume change from P, and V, to Pe and V respectively regardless of the process by which the system changes from initial to final state By experiment it has been seen that the change in internal energy is always the same and is independent of the paths C. and C; Internal energy is similar to the gravitational P E, so like the gravitational P E we take the change in internal energy and not its absolute value, which is important
Q. Write a note on reversible and irreversible processes.
Answer
Reversible Process
A reversible process is one which can be retracted inexactly reverse order, without producing any change in the surroundings
Explanation
In the reverse process, the working substance passes through the same stages as in the direct process but thermal and mechanical effects at each stage are exactly reversed If heat is absorbed in the direct process, it will be given out in the reverse process If work is done by the substance in the direct process, work will be done on the substance in the reverse process Hence, the working substance is restored to its original conditions
Cycle
A succession of events which bring the system back to its initial condition is called a cycle
Examples of Reversible Process
1 The process of liquefaction and the evaporation of a substance performed slowly are reversible processes
2 Slow compression of a gas in a cylinder is reversible process as the compression can be changed to expansion by decreasing the pressure on the piston
Irreversible Process
A reversible process is one which cannot be retraced inexactly reverse order, without producing any change in the surroundings
Explanation
All changes which occur suddenly or which involve friction or dissipation of energy through conduction, convection and radiation are irreversible
Examples
i ) Explosion is an example of highly irreversible process
ii ) Work done against friction
Q. Why we cannot use the large amount of heat energy in oceans and atmosphere?
Answer
Thus it is true for oceans and our atmosphere which contains a large amount of heat energy but cannot be converted into useful mechanical work.
Reason
The reason for our inability to utilize the heat contents of oceans & atmosphere is that there is no reservoir at a temperature lower than anyone of the two
Note In practice the petrol engine of a motor car extracts heat from the burning fuel and converts a Fraction of this energy to mechanical energy or work & expels the rest to atmosphere
Q. Discuss the working principle of refrigerator in terms of second law of thermodynamics?
Answer
Refrigerator
In nature, heat flows from high-temperature regions to low temperature ones. The reverse process however, cannot occur by itself.
The transfer of heat from a low temperature region to a high-temperature one requires special devices called refrigerators
Refrigerators are cyclic devices, and the working fluids used in the cycles are called refrigerant
Working principle
A refrigerator, consisting of a fluid pumped through a closed system, involves a four-step process
Step 1 - The fluid passes through a nozzle and expands into a low-pressure area Similar to the way carbon dioxide comes out of a fire extinguisher and cools down the fluid turns into a gas and cools down This is essentially an adiabatic expansion
Step 2- The cool gas is in thermal contact with the inner compartment of the fridge it heats up as heat is transferred to it from the fridge This takes place at constant pressure so it's an isobaric expansion
Step 3 The gas is transferred to a compressor which does most of the work in this process The gas is compressed adiabatically, heating it and turning it back to a liquid
Step 4. The hot liquid passes through coils on the outside of the fridge and heat is transferred to the room This is an isobaric compression process
Q. Describe environmental crisis as entropy crisis.
Answer
Environmental Crisis As Entropy Crisis
According to 2 law of thermodynamics, every real process causes to increase the disorder or entropy of the universe Any increase in the disorder of a system procures and even greater increase in the disorder of the environment which is called environmental crisis. The disorder producing activities due to all industries may result a great increases of disorder which affect the overall life support system Our mechanical energy producing processes are not efficient
For example
petrol engine has ts efficiency about 30% and diesel engine's about 40% Hence most of energy is transferred into the environment in form of heat which causes to increase the entropy of it The second law of thermodynamics impose limit on the efficiency of mechanical energy produced by engines which says that thermal pollution is an inevitable result of second law of thermodynamics Due to the thermal pollution in environment temperature change may occur But a small change in environment may have serious effects on metabolic rate implants and animals This may disturb ecological balance.
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