Thermal Physics — DU B.Sc. (Hons) Physics Previous Year Question Papers

(b) Derive Wein's displacement law and Rayleigh- Jeans' law, Stefan's Law from the Planck's law of blackbody radiation. (9) (b) At what temperature a black body will radiate thermal energy at the rate of 8 watt/cm2? Given a = 5.7 x 10-8 watt in-2 K-4. (3) 6. Write short notes on: (ii) Define and establish relation between Entropy and Thermodynamic probability. (6) (iii) Obtain an expression for thermodynamic probability and the most probable distribution function for a system obeying Fermi Dirac statistics. (9) (a) State the Zeroth Law of Thermodynamics. (b) Using Clausius- Clapeyron equation, discuss the effect of pressure on melting point of solids. (c) Distinguish between the reversible and irreversible processes. (d) Define a perfectly black body. Give one example. (e) The mean free path of nitrogen molecules at 0°C and 1 atm is 0.8 x 10-5 cm, having 2.7 x |019 molecules/cm3. Calculate the molecular diameter. (g) What is thermodynamic probability? 2. (a) Derive an expression for the efficiency ofcarnot's cycle in terms of temperature of the source and sink. (15) (b) Draw the T-S diagram for Carnot Cycle. What is the significance of T-S diagram? (3) 3. (a) Define the four thermodynamic potentials (U, F, G and H) and give their physical significance. Using the above, derive Maxwell's four thermodynamic relations. (12) (b) Prove from the laws of thermodynamics cp -c„ = (p + (#,I(#,P Hence prove that for an ideal gas Cp-Cv=R. 4. (a) Derive Maxwell- Boltzmann distribution law of velocity. (12) (b)How does the mean free path depend on temperature and pressure of a gas? (3) (c) The mean free path of molecules of a gas at pressure P and temperature T is 3 x 10-5cm. Calculate the mean free path when the temperature is raised to 2T. (3) 5. (a) Explain the spectral distribution of blackbody radiation with proper graphical representation.