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Friday, 28 March 2014

Modern physics-Model question paper for Msc


1(a) Explain the terms time dilation and length contraction. (6 Marks)
(b) In the photoelectric effect, how can a photon moving in one direction eject an electron moving in a different direction? What happens to conservation of momentum? (6 Marks)
(c) How does the total intensity of thermal radiation vary when the temperature of an object is doubled? (4 Marks)
(d) In a nuclear reactor, each atom of Uranium releases about 20 MeV when it fissions. What is the change in mass when 1 kg of Uranium is fissioned? (4 Marks)
2(a) What difficulties does the uncertainty principle cause in trying to pick up an electron with a pair of forceps? (4 Marks)
(b) Explain Davisson-Germer experiment. (6 Marks)
(c) Assume that electron beam in a television tube is accelerated through a potential difference of 25 kV and then passes through a deflecting capacitor of interior width 1 cm. Are diffraction effects important in this case? Justify your answer with a calculation. (6 Marks)
(d) The speed of an electron is measured to within an uncertainty of 20000 m/s. What is the size of the smallest region of space in which the electron can be confined? (4 Marks)
3(a) Assuming a pendulum to behave like a quantum oscillator, what are the energy differences between the quantum states of a pendulum of length 1m? Are such differences observable? (6 Marks)
(b) Compare the probabilities for an oscillating particle in its ground state to be found in a small interval at the center of the well and at the classical turning points. (6 Marks)
(c) Does the Thomson model fail at large scattering angles or at small scattering angles? Why? (4 Marks)
(d) The first excited state of sodium decays to the ground state by emitting a photon of wavelength 590 nm. If sodium vapour is used for the Franck-Hertz experiment, at what voltage will the first current drop be recorded? (4 Marks)
4(a) How does a quantized angular momentum vector differ from a classical angular momentum vector? (4 Marks)
(b) The photon has a spin quantum number of 1, but its spin magnetic moment is zero. Explain. (6 Marks)
(c) Explain why the Bohr theory gives a poor accounting of optical transitions but does well in predicting the energies of X-ray transitions. (6 Marks)
(d) A certain excited state of an atom has the configuration 4d15d1. What are the possible L and S values? (4 Marks)
5(a) How do the molecular force constant k compare with those of ordinary springs? What do you conclude from this comparison? (6 Marks)
(b) Explain how equilibrium separation in a molecule can be determined by measuring the absorption or emission lines for rotational states. (6 Marks)
(c) Why does an atom generally absorb radiation only from the ground state, while a molecule can absorb from many excited rotational or vibrational states? (4 Marks)
(d) At what temperature would 25% of a collection of HCl molecules be in the first excited state vibrational state? Ignore the rotational structure. (4 Marks)
6 (a) Would you expect the photoelectric effect to depend on the temperature of the surface of the metal? Explain. (4 Marks)
(b) Explain the salient features of Fermi-Dirac statistics. (8Marks)
(c) Semiconductors are sometimes called 'nonohmic' materials. Why? (4 Marks)
(d) From the Fermi energy for Mg, find the number of free electrons per atom. The molar mas of Mg is 24.3 g and its density 1.74 g/cubic cm. (4 Marks)
7(a) Why is the binding energy per nucleon relatively constant? Why does it deviate from a constant value for low mas numbers? (6 Marks)
(b) Distinguish between a slow neutron and a delayed neutron. (4 Marks)
(c) Explain why a fusion reactor requires a high particle density, a high temperature and a long confinement time. (6 Marks)
(d) A radiation detector is in the form of circular disc of diameter 3 cm. It is held 25 cm from a source of radiation, where it records 1250 counts per second. Assuming that the detector records every radiation incident upon it, determine the activity of the sample in Curies. (4 Marks)
8(a) List some similarities and differences between the properties of photons and neutrinos. (6 Marks)
(b) Why does the radius of a white dwarf or neutron star depend inversely on the number of nucleons? Shouldn't a star with more mater have a larger radius? (6 Marks)
(c) Why is it difficult to obtain precise values for the Hubble parameter and the deceleration parameter? (4 Marks)
(d) A satellite is in a orbit at an altitude of 150 km. We wish to communicate with it using a radio signal of frequency 1000 MHz. What is the gravitational change in frequency between a ground station and the satellite? Assume g does not change appreciably. (4 Marks)

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