Physics Paper 3, Nov/Dec 2010
 Questions: 1 2 3 Main
Weakness/Remedies
Strength

Question 2

(a)    Diagram

The diagram above illustrates a set-up for verifying the relationship between the volume and pressure of a given mass of gas.

As the reservoir is moved up and down the difference, h in the levels of mercury in the burette (A) and
the reservoir (B) changes with the volume V, of trapped air. The metre rule placed vertically between the
burette and reservoir is used to measure h. The volume V is obtained by reading directly from the burette. The procedure is repeated for four other values of h and V.

Fig. 2 shows five difference volumes of the trapped air Vi against the corresponding heights of the mercury hi, where i = 1,2,4, and 5.

(i)           Read and record the values of A and B on the metre rule.
(ii)          Evaluate hi = B - A.
(iii)         Read and record the corresponding values of Vi
(iv)         Evaluate V-1
(vi)         Plot a graph with h on the vertical axis and Vi on the horizontal axis.
(vii)        Determine the slope, s, of the graph.
(viii)       State two precautions that are necessary to ensure accurate results when performing this experiment.

(b )(i) State Boyle's law

(ii) Determine from your graph the volume of air that corresponds to a pressure difference of 12.5 cmHg.

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Observation

This question was unpopular but the few candidates that attempted it performed fairly well. However, some candidates recorded the values on the burette for A and B instead of reading the values on the metre rule as directed by the question. Some candidates evaluated b = A - B instead of h = B - A as instructed. Among those that were able to determine b and V correctly did not record them to the required decimal places.

The (b) part was well tacked, however, some omitted the phrase "from a fixed mass of gas" and/or
constant temperature in their definition of Boyle's law.

In part (a) the tasks required of the candidates include:

- reading and recording values of A, B and V in trend;
- evaluation of five values of h and VI to the required decimal places;
- recording the data in composite table;
- distinguishing graph axes, selecting reasonable scales, plating of points correctly and drawing
- line of best fit;
- determine the slope of graph using large-right angled triangle;

Stating the precautions to ensure accurate results as follows:

- A void parallax error on metre rule/burette
- Note and correct for zero error on metre rule

The expected response to part (b) is as follows:

Boyle's law states that for a fixed mass of gas, the pressure is inversely proportional to the volume, provided that the temperature is kept constant