RGU Question Paper Physics Semester II 2021: Heat and Thermodynamics

 JUNE-2021

B. Sc. (II Semester) Examination

PHYSICS

Paper: PHY-121 (New Course)

(Heat and Thermodynamics)

Full Marks : 60

Pass Marks : 35%

Time : Three Hours

Note: 1. Students have to answer any three questions from Section A and any three questions from Section B.

2. The figures in the margin indicate full marks for the questions

Section A

(Heath and Radiation)

1. (a) Write any four postulates of kinetic theory of gases. 2

(b) Derive the expression for pressure exerted by a gas in a vessel and show that pressure is proportional to density. 4

(c) Calculate the RMS speed of molecule of gas which has density of $1.24\times {10}^{-5}gm/c{m}^3$ at a temperature of 273 K and pressure of ${10}^{-2}$ atm. 2

(d) Calculate the values of $C_p$ and $C_v$, if $\gamma =1.33$ and $R=8.31 J/mol K$. 2

2. (a) What is Brownian Motion and what do you understand by mean free path? 2

(b) State and prove Maxwell’s law of equipartition of energy. 4

(c) Using the law of equipartition of energy, find the expression for $\gamma$ (ratio of two specific heats) in terms of degrees of freedom ƒ. Estimate the value of $\gamma$ for monoatomic, diatomic and polyatomic gases. 4

3. (a) What do you mean by liquefaction of gasses? Explain liquefaction of gases by Joule-Thomas Effect. 2+4=6

(b) Write a short note on Clausius-Clapeyron’s equation. 4

4. (a) What is first order phase transition? Explain it. Write the Gibb’s phase rule. 2+2+2=6

(b) Explain thermocouple thermometer. 4

5. (a) State Newton’s law of cooling. 2

(b) Describe Wien’s displacement law. If the sun emits maximum energy at wave length 4753 $A^o$, calculate the temperature of its surface. Wien’s constant $b=0.288 c\mathrm{m. }$3+2=5

(c) State and explain Rayleight Jeans’s law and its failure. 3

Section B

6. (a) What is specific heat? Why gas possesses two specific heats? 1+1=2

(b) What is isothermal and adiabatic process? Find the expression for work done in isothermal and adiabatic process. 1+1+3+3=8

7. (a) What do you mean by quasi-static process? Is it possible to realize in practice? 1+1=2

(b) What is indicator diagram? Explain its importance. 1+2=3

(c) Explain how first law of thermodynamics leads to the concept of internal energy. Why does the temperature of a gas drop when it is subjected to adiabatic expansion? Explain. 3+2=5

8. (a) Define efficiency of heat engine. Obtain an expression for the efficiency of a reversible Carnot’s engine with perfect gas as the working substance. Mention the effective ways to increase its efficiency. 2+5 = 7

(b) Efficiency of a Carnot’s cycle changes from 1/6 to 1/3 when source temperature is raised by 100 K. Calculate the temperature of the sink? 3

8. (a) What are Kelvin-Planck and Clausius statement of second law of thermodynamics? 2+2=4

(b) State and prove Carnot’s two theorems. 6

9. (a) Explain the concept of entropy. State and prove the principle of increase of entropy. 2+2+3=7

(b) Explain Kelvin’s thermodynamical scale of temperature. 3

10. Mention the four thermodynamic potentials and write their relation with state variables. Deduce the Maxwell’s thermodynamic relations from four thermodynamic potentials. 2+2+6=10