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Valliammai Engineering College Kattankulathur Department of Physics Question Bank Engineering physics II – PH6251 UNIT-I – Conducting Materials PART – A 1. What are the classification of conducting materials? 2. What are the sources of electrons in metals. 3. Give any three postulates of classical free electron theory. 4. Define drift velocity. How is it different from thermal velocity of an electron? 5. Define the terms relaxation time, collision time and Mean free path of an electron. 6. The mobility of electron in copper is 3x10 -3 m 2 /Vs. Assuming e = 1.6x10 -19 C. and m e = 9.1x10 -31 kg, calculate the Mean free time. 7. Differentiate between electrical conductivity and thermal conductivity. 8. Mention the drawbacks of classical free electron theory of metals. 9. What are the merits of classical free electron theory of metals. 10. Find the drift velocity of electrons copper wire whose cross sectional area is 1 mm 2 . When the wire carries a current of 10 A. Assume that each copper atom contributes one electron of the electron gas. Given n = 8.5x10 28 /m 3 11. A conducting rod contains 8.5x10 28 electrons per m 3 . Calculate the electrical conductivity at room temperature if the collision time for electron is 2 X 10 -14 s. 12. State any three assumptions of quantum free electron theory. 13. What are the merits and demerits of quantum free electron theory? 14. Define Fermi level and Fermi energy with its importance 15. Write down the expression for Fermi-Dirac distribution function and plot it as a function of energy. 16. Calculate the Fermi energy of copper at 0 ° K if the concentration of electrons is 8.5x10 28 m -3 17. Define Density of Energy states. 18. Define Lorentz number 19. The thermal conductivity of a metal is 123.92 W/m/k . Find the electrical conductivity and Lorentz number when the metal possess relaxation time 10 -14 seconds and 300 K.(Density of electrons = 6 x 10 28 /m 3 ). 20. The Fermi temperature of a metal is 24600 K. Calculate the Fermi velocity. www.Vidyarthiplus.com www.Vidyarthiplus.com
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PART – B 1. i) Define Electrical conductivity. Derive an expression for electrical conductivity of a metal by using classical free electron theory. ii) Find the mobility of electrons in copper if there are 9x10 28 valence electrons/m 3 and the conductivity of copper is 6x10 7 mho/m. 2. i) Define thermal conductivity Derive an expression for thermal conductivity of a metal. ii) Calculate the electrical and thermal conductivities for a metal with a relaxation time 10 -14 second at 300 K. Also calculate Lorentz number using the above result. (density of electrons =6x10 28 m -3 ). 3. Deduce mathematical expression for electrical conductivity and thermal conductivity of a conducting material and hence obtain Wiedemann-Franz law. 4. State and prove Wiedmann-Franz law. Why does the Lorentz number determined experimentally does not agree with the value calculated from the classical theory.
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