THERMODYNAMICS QUESTION ANSWERS CLASS NOTES

THERMODYNAMICS QUESTION

ANSWERS CLASS NOTES

  1. What is the Difference between a control volume and control surface?

ANS:

Fig. Control Volume & Control Surface

Control volume is a 3 dimensional space in a thermodynamic system (say  a compressor or pump). There will be mass transfer, energy transfer and momentum transfer across the control volume. Select control volume arbitrarily as per convenience. At a time, it will be a fixed volume in space. It is some portion (or complete plant) of a system for analyzing the system in parts or as a whole. This control volume may be static or moving with a constant velocity. Gas or liquid or solid flows through it. It is very much similar to free body diagram in classical mechanics.

Control surface is 2 dimensional. It is the surface area of the control volume. Mass, energy and momentum transfer can take place through a control surface.

2. What is difference between a reversible and an irreversible process?

ANS:

In a reversible process, there are no losses of any kind. Path followed in moving from state 1(p1,v1,t1) to state 2(p2, v2 and t2) and back from state2(p2, v2 and t2)  to state 1 (p1,v1,t1) is exactly the same. It is an ideal case. In actual practice, it is not possible. Examples of reversible processes are motion without friction. These are

(i) extension and compression of a spring

(ii) elastic deformation

(iii) friction less compression and expansion of a gas

(iv) an electric circuit with a zero resistance.

In a irreversible process, there are losses of one or more kinds. Path followed in moving from state 1(p1,v1,t1) to state 2(p2, v2 and t2) and back from state2 is totally different. In actual practice and in nature too, all processes are irreversible processes. Examples of irreversible processes are

(i) motion with friction

(ii) extension and compression of a spring

(iii) plastic deformation

(iv) free expansion of a gas

(v) an electric circuit with some resistance

(vi) combustion of fuel with air

(vii) cooking food

(viii) eating food

(ix) development of a plant

3. Which of the  followings is a thermodynamic  properties?

(i ) p dv, (ii) ʃ v dp and (iii) ʃ(p dv + v dp)

ANS:

For a certain quantity to become a property, it should be an exact differential requiring no additional information.

Find pdv integral with a known relation between p and v. There p dv is not a property of a thermodynamic system.

Find Integral ʃv dp with a known relation between v and p. There v dp is not a property of a thermodynamic system.

Integral ʃ(p dv + v dp) is equal to differential of p v or d(p v).

ʃ(p dv + v dp) is a thermodynamic property.

Q. What is the difference between non-flow and flow work?

ANS:

Non-flow work means work done on a constant mass. There is w no inflow or outflow of the gas is taking place.

 Work done by gas on the piston =δW = force x distance dx

= p x A x dx= p dv

Total non flow work =∫p dv

Flow work is the net work from inlet to outlet of a turbine.

 Flow work = work done ( suction+ compression – exhaust)

= p1V1 + (p2V2-p1V1)/(γ-1) –p2V2= p1V1 — p2V2 + (p2V2-p1V1)/(γ-1)=[(γ/(γ –1)][ (p2V2-p1V1)] = ∫V dp

Q. Discuss the followings. 

(i) Work done if dv=0

(ii)No work done with dv ≠0

ANS:

  • Case 1

  • Work done if dv=0

  • It is possible in the case of pedal work.  Here work done without any change in volume.

  • Case 2

  • NO work done if dv ≠0

  • No work done in free expansion of a gas.

Q. Differentiation between temperature, heat and internal energy of a thermodynamic system.

Sr. No.
Temperature
Heat
Internal energy
1.
Represents the degree of hotness or coldness of a body.  Heat flows towards a cold body.
It is a poor grade of energy. Produce it with a slightest action anywhere and everywhere. There is a natural flow of heat from a body at higher temperature to a body of lower temperature. Reverse is not possible.
 Internal means it is invisible to the naked eye. It is the energy of the molecular arrangement and motion of the molecules.
IE= Molecular potential energy + molecular kinetic energy
Internal energy is  product of mass m, specific heat cv at constant volume and the temperature. Total internal energy, U =mcvt .
Specific internal energy, u = cvt
2.
Its symbol is t.
 Symbol of heat energy is Q.
 Symbol of internal energy is is U.
3.
 Units of temperature are 0C
  Heat units are kJ.
 Units of internal energy are kJ.
4.
Temperature is a thermodynamic property as it is a point function.
Heat is a not a thermodynamic property as it is a path function.
Internal energy is a thermodynamic property as it is a point function.
5.
Temperature is visible.
It is non- visible.
It is non- visible.

Q. Discuss Thermodynamic temperature & Thermodynamic Scale

Development of Thermodynamic Temperature  

Temperature is because of random translation, rotational motions and vibrations of the submicroscopic particles. These motions constitute the internal energy of a substance. Thermodynamic temperature is the measure of the average kinetic energy per degree of freedom of its constituent particles. It is one of the important thermodynamic parameters. Kinetic energy is zero at absolute zero. Absolute zero is the Null point.”

Practical applications of Thermodynamic temperature

 Thermodynamics does not use temperature in C, o F. Use temperature difference. Thermodynamic temperature is most common in thermodynamics .Further,  Boyle’ Law, Charles Law, Perfect gas equation use thermodynamic temperature. It determines is the efficiency of

(i) Carnot heat engine,

(ii)COP of a Reversed Carnot cycle for a refrigerator

(iii) COP of a heat pump.

Thermodynamic Scale

All other scales of temperature depend on the properties of the working substance. While thermodynamic temperature scale is independent of the properties of the working medium. Further, it is based on the fact the efficiency of a reversible steam engine depends only on two temperatures. This efficiency does not depend on the properties of the working substance. Thermodynamic scale differs from empirical scales in that it is absolute. It is based on basic laws of thermodynamics. Thermodynamic scale is based on the efficiency of the reversible heat engines. Second law of thermodynamic gives thermodynamic temperature scale.

S I Units

The International System of Units has defined a scale for the thermodynamic temperature. Its lower limit is absolute zero (0 K) or (-273C). The upper point is the triple point of water at 273.16 K or o.16C.

Features Of The Thermodynamic Temperature Scale

(i) One Kelvin is 1/273.16 of difference between absolute zero and the triple point of water

(ii) It fixes one Kelvin equal to 10C.

(iii) Thermodynamic scale covers temperatures from absolute zero (0 K) to triple point of water (273.16 K or 0.01 °C).

(iv) For temperature outside this range, different thermometers  cover the entire range. Such thermometers are as given below:

  1.  Helium vapor pressure thermometer

  2.  Helium gas thermometer

  3.  Standard platinum resistance thermometer

  4.   Monochromatic radiation thermometers

  5.  Liquid (mercury) thermometers

Q. Discuss Salient Features of Thermodynamics.

 Thermodynamics is the process of energy conversion. It is applicable to all power producing and power consuming machines.

  1. First law does not give the direction of a process. It does not specify the quantum of conversion.

  2. A heat engine is one which converts heat energy into work or mechanical motion. In this, input is heat and output is work. Example is steam engine, steam turbine and  I.C. engines.

  1. The ratio of output to input in a heat engine is its efficiency.

  2.  A revered heat engine input is work.  Heating or cooling is the output. Example is refrigerator or heat pump.

  3. The ratio of output/input in a reversed heat engine is the Coefficient of Performance.

  4. Heat and work flow across the heat engine boundary. These are not the properties of the system.

  5. Kelvin-Planck  says that no engine can convert a certain quantity of heat into an equivalent amount of work.

  6. A perpetual motion machine of second kind is not possible.  In this, there is no net work by exchanging heat with a single reservoir.

  7. Clausius Statement: Heat cannot transfer from lower to higher temperature without an external agent.

   10.The entropy of an isolated system never decreases.  Ir-reversibility increases the entropy.

  1. The second Law of thermodynamics defines the entropy.

  2. The second Law of Thermodynamics tells about the feasibility of a process.

   13, This Law helps to determine the efficiency of a system (heat engine).

  1. A reversible heat engine is the most efficient engine. In practice, all heat engines are non-reversible.

  2. All reversible engines working between same two fixed reservoirs have the same efficiency.

  3. The efficiency of a reversible engine working between two reservoirs with fixed temperatures  is independent of the working substance.

   17. In a reversible cycle, there is no net change in energy. In  a  irreversible cycle, there is a net loss of energy.

  1. A Carnot Cycle is a reversible (Theoretical ). This cycle consists of two isothermal and two isentropic processes. The sequence is isothermal-isentropic-isothermal-isentropic.

  2. The efficiency of the Carnot Cycle is (T1–T2)/T1

        Where T1 and T2 are absolute higher and lower reservoir temperatures

   20. A reservoir is one whose temperature remains constant on receipt or rejection of heat