Tribhuvan University
Institute of Science and Technology
TU Exam Year: 2081
Candidates are required to give their answers in their own words as far as practicable.
SECTION A
Attempt all questions.
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1.Define the terms critical potential, excitation potential, and ionization potential. Describe Frank-Hertz experiment for determining the critical potentials. Show how the results of the experiment confirm Bohr’s postulates.
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2.What are the postulates of wave mechanics? Derive Schrödinger wave equation. Discuss the significance of the wave function.
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3.Explain potential energy barrier for the charge carriers in BJT. Describe how the transistor works as a voltage amplifier.
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4.Develop differential equation for the oscillation of a mass–spring system and write its solution.
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5.Discuss electrical conductivity of a semiconductor.
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6.Explain Bloch theorem for a particle moving in a periodic potential.
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7.Suppose the body of an ice skater has a moment of inertia 4 kgm² and her arms have a mass of 5 kg each with the center of mass at 0.4 m from her body. She starts rotating at 0.5 rev/s on the point of her skate with arms outstretched. She then pulls her arms inward so that the center of mass is at the axis of her body. What will be her speed of rotation?
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8.An electron is placed midway between two fixed charges q1 = 2.5×10⁻¹⁰ C and q2 = 5×10⁻¹⁰ C. If the charges are 1 m apart, what is the velocity of the electron when it reaches a point 10 cm from q1?
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9.The uncertainty in the position of a particle is equal to the size of the de Broglie wavelength of the particle. Calculate the uncertainty in the velocity of the particle in terms of the velocity of the de Broglie wave associated with the particle.
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10.Consider a copper wire of cross-sectional area 1 mm² carrying a current 1 A. What is the drift velocity of electrons? Cu is monovalent, i.e., there is one free electron per atom. The density and molecular weight of Cu are 9 g/cc and 64 g/mole respectively.
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11.Suppose that the effective mass of holes in a material is four times that of electrons. At what temperature would the Fermi level be shifted by 10% from the middle of the forbidden energy gap? Consider Eg = 1 eV.
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12.Find the truth table for the circuit shown in the given figure. What logic function does the circuit perform?
