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07 AL Physics/Essay/P.1 HONG KONG ADVANCED LEVEL EXAMINATION AL PHYSICS 2007 Essay Type Question 1. (a) (i) Describe an experiment using an air track to verify that momentum is conserved for a completely inelastic collision in which a body collides with another body initially at rest. (ii) Show that the law of conservation of momentum for two bodies colliding in one dimension can be deduced from Newton’s laws of motion. (7 marks) (b) A bullet is fired horizontally towards a wooden sphere suspended from point O by a light rigid rod. The rod is free to rotate about O. On striking the sphere, the bullet is embedded in it and the sphere then swings upwards. Discuss and explain whether the law of conservation of momentum and the law of conservation of mechanical energy can be applied to the system (bullet + sphere) in the following processes: Process 1: From the time when the bullet just strikes the sphere until they start to move together, assuming this time interval is short so that the position of the sphere remains almost unchanged. Process 2: From the time when the system (bullet + sphere) starts to swing upwards until it reaches its highest position. Neglect air resistance in your discussion. (5 marks) (c) The figure shows an elastic oblique collision between two spheres in two dimensions. Sphere B is initially at rest. Spheres A and B move away at 90 after collision. Draw a diagram to illustrate the relation among the momentum vectors before and after collision. Hence show that the two spheres must be of equal mass. (4 marks) 07 AL Physics/Essay/P.2 2. (a) State ONE feature to distinguish longitudinal and transverse waves. Give ONE example for each kind of wave. (2 marks) (b) Describe how a sound wave is produced and how it passes through air to cause our sensation of sound. Draw a graph to represent the spatial variation of the displacement of the air particles along the path of a travelling sound wave of a certain wavelength and indicate the locations corresponding to compressions (C) and rarefactions (R). (6 marks) (c) A loudspeaker L, a microphone M, a signal generator and a dual trace oscilloscope are connected as shown below. (i) With the aid of a diagram, describe how you would perform an experiment in the school laboratory to show that the phase of the vibration of the air particles changes for different points along the path of a travelling sound wave. (ii) State and explain how to find the wavelength and speed of the sound in air. Given that the speed of sound in air is of the order of 102 ms-1, suggest an appropriate separation between the loudspeaker and the microphone, and state the frequency of the signal generator to be used. Explain briefly. (iii) Explain why the loudspeaker and the microphone should be placed close to the bench. (8 marks) 3. (a) (i) The figure shows a circuit consisting of a cell of e.m.f. and internal resistance r, and two uniform resistance wires BC and CD. The two wires are equal in length and are made of the same material, but the cross- 07 AL Physics/Essay/P.3 sectional area of BC is double that of CD. The total resistance of the two wires is R. All other connecting wires are of negligible resistance. Sketch a graph showing the readings of the voltmeter versus the path length along the circuit when its flying lead is connected to various points along the path ABCDE. Indicate on your graph the voltmeter readings at various points in terms of , r and R. Explain the conversion of electrical potential energy of a charge +q passing through each section of the complete circuit. (ii) Using a simple model of electron conduction, describe how heat is generated in a resistance wire by a current. (9 marks) (b) (i) The figure shows a metallic conductor of length L carrying a current I. The number of conduction electrons per unit volume is n and the charge of an electron is e. The conductor is placed perpendicular to a uniform magnetic field of flux density B. It is known that the magnetic force acting on a conduction electron due to its average drift velocity v is Bev. Hence, derive the magnetic force acting on the whole length of the conductor in terms of current I. (ii) With no current flowing in the conductor, it is being moved with uniform velocity u inside the uniform magnetic field B pointing into the paper as shown, a potential difference is developed across its ends XY. Referring to the force acting on a conduction electron, briefly explain how is produced and find an expression for its magnitude. (7 marks) 07 AL Physics/Essay/P.4 4. Kinetic theory relates the macroscopic behaviour of an ideal gas with the microscopic properties of its molecules. (a) The gas pressure is due to the collisions of the gas molecules with the walls of the container. (i) Despite the fact that the incident angle and the incoming speed of the gas molecules vary greatly among the molecules, the pressure on the container wall is steady if the volume and the temperature of the gas are kept constant. Explain. (ii) The collisions are assumed to be elastic. What would happen macroscopically if the collisions were inelastic? Explain how you can deduce from observation that the collisions are in fact elastic. (5 marks) (b) (i) In terms of the microscopic interpretation of pressure, explain qualitatively why the pressure of a gas can be kept constant when its volume increases with the temperature. (ii) Describe an experiment to find the relationship between the volume and the temperature of a gas when its pressure is kept constant. Sketch and label the set-up of the experiment. How can the absolute zero be estimated from the experiment? (9 marks) (c) A gas at low temperature and under high pressure would deviate from ideal gas behaviour. Explain briefly with reference to the assumption(s) of the kinetic theory. (2 marks) 5. (a) What is meant by the binding energy of a nucleus? Sketch the graph of binding energy per nucleon against atomic number, and use the graph to explain (i) which of the following reactions can produce more energy per unit mass of reactant: (I) fission of uranium in a nuclear reactor (II) fusion of hydrogen into helium-4 in the sun’s nucleus (ii) why iron ( 56 26 Fe ) is one of the most stable nuclei. (5 marks) (b) The energy levels of the electron in a hydrogen atom is given by En = 13.6 eV n2 07 AL Physics/Essay/P.5 (i) Explain the meaning of ground state, excitation energy and ionization energy of a hydrogen atom with this expression. (ii) Explain the production of emission line spectra in terms of light quanta and energy levels. (5 marks) (c) (i) Explain what Fraunhöfer lines in the solar spectrum are and how they are produced. Describe how scientists can obtain information about the composition of the sun from these lines. (ii) Describe briefly how the velocity of a distant star relative to the earth along the line of sight can be estimated from the light spectrum of the star. (6 marks) END OF PAPER