Thermodynamics Interview Questions & Answers

  1. Question 1. State The Third Law Of Thermodynamics. Give Its Limitations And Importance.?

    Answer :

    The third law of thermodynamics states that:

    “The entropy of a perfect crystal of each element and a compound is zero at absolute zero.”

    Limitations: If any disorder like impurity or imperfection is found in a substance then the entropy of such crystal is non-zero at 0 K. For example: The entropy of pure carbon dioxide and nitric oxide is zero at 0K. This shows that there exists disorder in the arrangement of such molecules.

    This law is applicable only to pure compounds. Thus we can say that, this law is not applicable to glass which is a supercooled liquid. It is also not applicable to amorphous substance and supercooled solutions.


    1. With the help of this law Thermodynamic properties can be calculated and chemical affinity can be measured. 
    2. This law helps in explaining the behaviour of solids at very low temperature.
  2. Question 2. Explain The Laws Of Thermodynamics.?

    Answer :

    1. Zeroth law: If any two systems are in thermal equilibrium with the third system, then they are also in thermal equilibrium with each other. 
    2. First law: First law of thermodynamic states that energy can neither be created nor be destroyed but it can only be converted from one form to another. 
    3. Second law: This law states that “all processes in nature tend to occur with an increase in entropy and the direction of change always lead to the increase in entropy.” 
    4. Third law: This law states that “The entropy of a perfect crystal of each element and a compound is zero at absolute zero.”
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  4. Question 3. Explain The Following Terms: Isolated System, Open System And Closed System And Give Example Where Ever Possible.?

    Answer :

    1. Isolated system: A system that can neither exchange matter nor heat with the surrounding is known as an isolated system.
      For example: Water placed in a vessel that is closed as well as insulated. 
    2. Open system: A system that can exchange both matter and energy with the surrounding is said to be an open system.
      For example: A reaction taking place in an open vessel exchanges both energy and matter with the surrounding. 
    3. Closed system: A system that exchanges only energy and not matter with the surrounding is said to be a closed system.
      For example: A reaction taking place in a closed metallic vessel.
  5. Question 4. Write A Short Note On Gibbs Free Energy And Derive The Equation For The Same.?

    Answer :

    This thermodynamic quantity states that the decrease in value during a process is equal to the useful work done by the system. It is denoted by G and the mathematical equation is:

    G = H – TS


    H = heat content 
    T = absolute temperature 
    S = entropy of the system

    For isothermal process we have

    G1 = H1 – TS1 for the initial state 
    G2 = H2 – TS2 for final stage


    G2 – G1 = (H2 – H1) – T(S2 – S1)


    ΔG = G2 – G1 is the change in Gibbs free energy 
    ΔH = H2 – H1 is the change in enthalpy of the system 
    ΔS = S2 – S1 Is the change in entropy of the system

    Thus the above equation becomes:

    ΔG = ΔH – TΔS is known as Gibbs-Helmoholtz equation.

  6. Question 5. Why Entropy Decreases With Increase In Temperature?

    Answer :

    ds=dQ/T. Entropy is inversely proportional to the temperature so, as temp. Increases, entropy decreases.

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  8. Question 6. Why Different Types Of Sound Are Produced In Different Bikes, Though They Run On Si Engines?

    Answer :

    Engine specifications are different in different manufactures like as Bore Diameter (CC), Ignition timing. Also the exhaust passage takes more responsible for sound.

  9. Question 7. How Much Watt Means 1hp?

    Answer :

    746.2 Watt

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  11. Question 8. Explain Bicycle Rear Wheel Sprocket Working?

    Answer :

    Rear wheel sprocket works under the principle of ratchet and pawl.

  12. Question 9. Definition Of Octane Number And Cetane Number?

    Answer :

    Octane No:-Octane number is defined as the percentage, by volume, of iso octane in the mixture of iso octane and h-heptane. It is the measure of rating of SI engine.

    Cetane No:– Cetane number is defined as the percentage, by volume, of n-cetane in the mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.

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  14. Question 10. Which Mechanism Is Used In Automobile Gearing System?

    Answer :

    Differential mechanism

  15. Question 11. When Crude Oil Is Heated, Which Hydrocarbon Comes First?

    Answer :

    Natural gas (Gasoline)… at 20 Celsius

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  17. Question 12. How To Calculate Bearing Number Ti Diameter Of The Inner And Outer?

    Answer :

    Divide the shaft diameter size by 5, it will give last two digit of the bearing no. and according to type of load we have to choose the type of bearing and that will give prior number of the bearing.

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  19. Question 13. The Fatigue Life Of A Part Can Be Improved By?

    Answer :

    Improving the surface finish by Polishing & providing residual stress by Shot peening.

  20. Question 14. What Happens If Gasoline Is Used In A Diesel Engine, Siesel Engine Will Work?

    Answer :

    No, It will not work, as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may blast.

  21. Question 15. Why Poisson Ratio Is Higher In, Rubber/steel/wood?

    Answer :

    When a material is compressed in one direction, it usuallytends to expand in the other two directions perpendicular tothe direction of compression. This phenomenon is called the Poisson effect. Poisson’s ratio is a measure of the Poisson effect.

    For rubber = 0.5
    For steel = 0.288
    For wood < 0.2

    Thus Poisson’s ratio is higher in RUBBER.

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  23. Question 16. When A Real Gas Behaves Like Ideal Gas?

    Answer :

    A real gas behaves like an ideal gas in low pressure and high temperature conditions.

  24. Question 17. What Is The Significance Of Entropy?

    Answer :

    As per the second law of thermodynamics, any heat input to the system (Heat engine) cannot be converted completely into useful work. Some energy is lost and that is called ‘unavailable work’. The amount of unavailable work increases as the entropy increases.

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  26. Question 18. What Is The Difference Between Heat Transfer And Thermodynamics?

    Answer :

    Heat transfer deals with the energy analysis which in transition and and depends on the modes of heat transfer like conduction, convection and radiation or combination of any modes. Heat transfer deals in non equilibrium domain and conditions while thermodynamics deals with study of system at equilibrium and does not depend on how heat transfer is calculated.

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  28. Question 19. At Which Temperature Thermal Radiation Can Become Zero?

    Answer :

    Not possible. Because thermal radiation becomes only zero at absolute zero temperature which can never be attained by the third law of thermodynamics.

  29. Question 20. Difference Between Tube And Pipe?

    Answer :

    • There always a debate between differentiation of pipe and tube. Pipe is always designated by its Inside Diameter (ID) because main purpose of pipe is to carry fluid, while the 
    • Tube is always designated by its Outside Diameter (OD). Pipe’s thickness is sometimes called as schedule and tube’s thickness is gauge.
    • As per the tolerance basis, tubes requires higher tolerances as to that of pipe and tubes are more costlier in manufacturing compared to pipe production. 
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  31. Question 21. Difference Between Flywheel And Governor?

    Answer :

    The function of flywheel is to store the energy/power produced during the power stroke of an engine and this stored energy is used during remaining stroke to make piston up and down during working of engine while governor tries control the speed of an engine due to variation in load. It does this work by reducing or increasing the amount of fuel passing to the engine.

  32. Question 22. What Is The Importance Of The Thermodynamics In The Field Of Mechanical Engineering?

    Answer :

    All the mechanical engineering systems are studied with the help of thermodynamics. Hence it is very important for the mechanical engineers.

  33. Question 23. How Many Laws Of Thermodynamics Are There?

    Answer :

    There are three laws of the thermodynamics.

    1. First Law : Energy can be neither created nor destroyed. It can only change forms.In any process in an isolated system, the total energy remains the same.
    2. Second Law : When two isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium in itself, but not in equilibrium with each other at first, are at some time allowed to interact, breaking the isolation that separates the two systems, and they exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium. The sum of the entropies of the initial, isolated systems is less than or equal to the entropy of the final exchanging systems. In the process of reaching a new thermodynamic equilibrium, entropy has increased, or at least has not decreased.
    3. Third Law : As temperature approaches absolute zero, the entropy of a system approaches a minimum.
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  35. Question 24. State Laws Of Conservation Of Energy?

    Answer :

    According to the laws of conservation of energy, “energy can neither be created nor be destroyed. It can only be transformed from one form to another.”

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  37. Question 25. Is The Boiler A Closed System?

    Answer :

    Yes definitely the boiler is a closed system.

  38. Question 26. What Is Carnot Engine?

    Answer :

    It was being designed by Carnot and let me tell you that Carnot engine is an imaginary engine which follows the Carnot cycle and provides 100% efficiency.

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  40. Question 27. Which Formula Forms A Link Between The Thermodynamics And Electro Chemistry?

    Answer :

    Gibbs Helmholtz formula is the formula which forms the link between the thermodynamics and electromagnetism.

    ∆Hs/R = [∂ lnp /∂ (1/T)]x

    where: x – mole fraction of CO2 in the liquid phase 
    p – CO2 partial pressure (kPa) 
    T – temperature (K) 
    R – universal gas constant 
    α – mole ratio in the liquid phase (mole CO2 per mole of amine)

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  42. Question 28. Which Is The Hardest Compound Known?

    Answer :


  43. Question 29. What Is Hess Law?

    Answer :

    According to the Hess law the energy transfer is simply independent of the path being followed. If the reactant and the product of the whole process are the same then same amount of energy will be dissipated or absorbed.

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  45. Question 30. Which Has More Efficiency: Diesel Engine Or Petrol Engines?

    Answer :

    Off course Diesel engine has the better efficiency out of two.

  46. Question 31. Explain The Second Law Of Thermodynamics?

    Answer :

    The entropy of the universe increases over time and moves towards a maximum value.

  47. Question 32. How Do You Measure Temperature In A Wet Bulb Thermometer?

    Answer :

    Wet bulb temperature is measured in a wet bulb thermometer by covering the bulb with a wick and wetting it with water. It corresponds to the dew point temperature and relative humidity.

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  49. Question 33. What Is Bending Moment?

    Answer :

    When a moment is applied to bend an element, a bending moment exists in the element.

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  51. Question 34. What Are The Points In The Stress Strain Curve For Steel?

    Answer :

    Proportional limit, elastic limit or yield point, ultimate stress and stress at failure.

  52. Question 35. Define Reynolds Number.?

    Answer :

    Reynolds number is the ratio of inertial force and viscous force. It is a dimensionless number. It determines the type of fluid flow.

  53. Question 36. What Is A Newtonian Fluid?

    Answer :

    A Newtonian fluid possesses a linear stress strain relationship curve and it passes through the origin. The fluid properties of a Newtonian fluid do not change when any force acts upon it.

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  55. Question 37. How Many Joules Is 1 Btu?

    Answer :

    1 BTU is equal to 1055.056 joules.

  56. Question 38. What Is Ps?

    Answer :

    PS is Pferdestarke, the German unit for Horsepower.

  57. Question 39. Explain Otto Cycle?

    Answer :

    Otto cycle can be explained by a pressure volume relationship diagram. It shows the functioning cycle of a four stroke engine. The cycle starts with an intake stroke, closing the intake and moving to the compression stroke, starting of combustion, power stroke, heat exchange stroke where heat is rejected and the exhaust stroke. It was designed by Nicolas Otto, a German engineer.

  58. Question 40. Explain The Nomenclature Of A 6203-zz Bearing?

    Answer :

    6 is the type code, which shows it is a single-row ball bearing, 2 is the series, means light, 03 is the bore, which is 17 mm and ZZ is the suffix meaning double shielded bearing.

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  60. Question 41. What Is Gear Ratio?

    Answer :

    It is the ratio of the number of revolutions of the pinion gear to one revolution of the idler gear.

  61. Question 42. What Is Annealing?

    Answer :

    It is a process of heating a material above the re-crystallization temperature and cooling after a specific time interval. This increases the hardness and strength if the material.

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  63. Question 43. Define Torque.?

    Answer :

    Torque is defined as a force applied to an object that results in rotational motion.

  64. Question 44. What Is Ductile-brittle Transition Temperature?

    Answer :

    It is the temperature below which the tendency of a material to fracture increases rather than forming. Below this temperature the material loses its ductility. It is also called Nil Ductility Temperature