97 AL Physics/Essay/P.1

HONG KONG ADVANCED LEVEL EXAMINATION

AL PHYSICS

1997 Essay Type Question

1.(a)A man pushes a heavy rock resting on the ground, but it does not move. A student says that this is because the pushing force is balanced by the reaction of this force. Comment, with the aid of a diagram, on whether the student’s argument is correct. (2 marks)

(b)Using a spring balance, a small object is found to weigh heavier at the north pole than at the equator.

(i)State and explain TWO reasons for this observation.

(ii)What would the result be if the object is weighed again at the two places using a beam balance? Explain briefly.

(5 marks)

(c)Consider the cases in which (i) a man is inside a lift falling freely and (ii) he is inside a space-craft moving in a circular orbit round the earth. Identify THREE similarities between these physical environments. (3 marks)

(d)(i)The equation of state and kinetic theory equation of an ideal gas can be written as pV = nRT and pV = . State the meaning of the symbols excluding pressure p and volume V.

(ii)Two identical vessels containing hydrogen and oxygen respectively are at the same temperature and pressure. What can you say about the number of molecules, the average molecular kinetic energy and the mean square speed of the molecules in the two vessels? Explain briefly. (Assume that the gases behave ideally.)

(6 marks)

2.(a)Plane monochromatic light waves of wavelength  are incident normally onto a plane transmission grating of slit separation d to produce an interference pattern.

(i)Using the principle of superposition describe briefly how the principal maxima are formed and deduce the formula for the angular positions of the principal maxima.

(ii)It is preferable to measure the wavelength of light by using a plane transmission grating rather than using a double slit. Explain briefly.

(5 marks)

(b)Describe an experiment for observing the absorption spectrum of iodine using a diffraction grating. Describe the spectrum observed and account for it in terms of the quantum nature of light and atomic structure. (8 marks)

(c)Briefly explain the principles involved in identifying the elements present in the atmosphere of the sun through studying the sun’s spectrum. (3 marks)

3.(a)Explain the meaning of the potential difference between two points in an electric field and hence state the meaning of the potential at a point in the field.

(3 marks)

(b)(i)An isolated spherical conductor is positively charged. Draw carefully on the same diagram

(I)the electrostatic lines of force,

(II)a series of equipotential surfaces with equal increment in electric potential around the spherical conductor.

(ii)With reference to the diagram drawn in (i),

(I)explain how the lines of force help in describing an electric field.

(II)illustrate the relationship between electric field strength and potential difference.

(6 marks)

(c)With the aid of a diagram, describe and explain an experiment to investigate the potential around a charged sphere. Briefly describe the experimental results. (7 marks)

4.(a)You are given two bar magnets, a long copper wire and a light beam galvanometer. Describe how you would use the apparatus to investigate qualitatively the factors affecting the e.m.f. induced in a coil by electromagnetic induction. (5 marks)

(b)(i)Consider a rectangular coil of N turns rotating uniformly in a uniform magnetic field about an axis perpendicular to the field. Derive an expression for the e.m.f. produced.

(ii)With the aid of a labelled diagram, describe the construction of a generator to provide a d.c. to a light bulb using the method in (i) [smoothing is not required]. Show that the current generated is always flowing in one direction through the bulb.

(iii)Explain carefully why a greater driving torque is needed to maintain the coil of the generator rotating at the original speed when an identical light bulb is connected in parallel with the first one. Also explain how this change agrees with the principle of conservation of energy.

(11 marks)

5.(a)(i)With the aid of a labelled diagram, explain the working principles of a diffusion cloud chamber. State, with brief explanations, TWO properties of the radiations that could be investigated by the cloud chamber.

(ii)The tracks of an -source are observed in a diffusion cloud chamber in which a trace amount of helium is introduced. Sketch the tracks observed when there is an oblique collision between an -particle and a helium atom. Show, with mathematical derivation, how the mass of an -particle can be deduced from these tracks. (The speed of the helium atom before collision is assumed to be negligible.)

(10 marks)

(b)Explain, through analogous comparison with throwing dice, what is meant by radioactive decay being a ‘random process’. Hence deduce from first principles the exponential law of decay of a radioactive source. (No need to describe the dice experiment.) (Given: ) (6 marks)

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