Physics Paper 3, Nov/Dec 2011

Questions:	1	2	3	Main

General Comments
Weakness/Remedies
Strength

Question 2
The diagram above illustrates a syringe with a petri-dish fixed to the top of its plunger. A known mass, M, is placed on the petri-dish. The piston of the syringe moves downward and the volume, V, of air beneath the piston is read and recorded. The procedure is repeated for four other values of M. Fig 2(a) represents the masses, Mi, placed on the petri-dish. Fig 2 (b) represents the corresponding volumes, V where = 1,2,3,4 and 5. (i) Read and record the values of M (ii) Read and record the corresponding values of V (iii) Evaluate V-1 (iv) Tabulate your readings. (v) Plot a graph with M on the vertical axis and V-1 on the horizontal axis (vi) Determine the slope, s, of the graph. (vii) What physical law does this experiment verify? (viii) State two precautions that are necessary to ensure accurate results when performing this experiment (b) (i) Explain the effect of decreasing the volume of a fixed mass of gas at constant temperature using the kinetic molecular theory of gas. (ii) A cylinder is filled with a gas at 27oC. If the volume of the gas is increased at constant pressure to 1.25 times its original value, calculate the final temperature of the gas.
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Observation
This question was attempted by majority of the candidates. The masses were read correctly but most candidates failed to determine correctly the values of V. as most of the responding candidates were reading the upper meniscus of the syringe instead of the lower meniscus. Many candidates lost marks for not evaluating V-1 to 3 d.p. Rather most of the candidates evaluated V-1 to 1.d.p or 2 d.p deliberately for the purpose of easy plotting of graph. The graph, slope and precautions were well attempted by majority of the candidates. The physical law that the experiment verified was missed by most responding candidates. In part b, the explanation on the effect of decreasing the volume of a fixed mass of a gas using the kinetic molecular theory was poorly attempted. However, the numerical part was well handled by most responding candidates. Candidates are expected to among other things: read and record five values of M to at least 1 d.p.in kg read and record five values of V to at least 1 d.p.in cm3 evaluate V-1 to at least 3 d.p present data in composite table plot graph of M against V-1 using reasonable scales draw line of best fit determine the slope of the graph state the law the experiment verified stale any two of the following precautions ensure only dry air is used ensure constant room temperature avoid parallax error in reading the mass/volume note/corrected zero error on metre rule/scale balance ensure metre rule is placed vertically ensure readings are repeated. The expected responses for part (b) are: b(i) If the volume of a fixed mass of gas is decreased, the molecules take less time to travel within the container. The collision rate with the walls is increased . Therefore the gas pressure increase. (ii) = = = T2 = 375K or 102oC