Experimental Design (OCR A Level Physics)

A group of students are conducting an experiment in the laboratory to determine the value of absolute zero by heating a fixed mass of gas. The volume of the gas is kept constant. Fig. 17.1 shows the arrangement used by the students.

Fig. 17.1

The gas is heated using a water bath. The temperature θ of the water is increased from 5 °C to 70 °C. The temperature of the water bath is assumed to be the same as the temperature of the gas. The pressure p of the gas is measured using a pressure gauge.

The results from the students are shown in a table.

Describe and explain how the students may have made accurate measurements of the temperature θ.

Fig. 17.2 shows the pressure gauge. Measurements of p can be made using the kPa scale or the psi (pounds per square inch) scale. The students used the psi scale to measure pressure and then converted the reading to pressure in kPa.

Fig. 17.2

i) Suggest why it was sensible to use the psi scale to measure p.

[1]

ii) The students made a reading of p of 37.0 ± 0.5 psi when θ was 44 ± 1°C.

Convert this value of p from psi to kPa. Complete the table for the missing value of p.

Include the absolute uncertainty in p.

1 pound of force = 4.448 N

1 inch = 0.0254 m

[2]

Fig. 17.3 shows the graph of p against θ.

Fig. 17.3

i) Plot the missing data point and the error bars on Fig. 17.3.

[1]

*ii) Explain what is meant by absolute zero. Describe how Fig. 17.3 can be used to determine the value of absolute zero.

Determine the value of absolute zero. You may assume that the gas behaves as an ideal gas.

[6]

Describe, without doing any calculations, how you could use Fig. 17.3 to determine the actual uncertainty in the value of absolute zero in (c)(ii).

The experiment is repeated as the water bath quickly cools from 70 °C to 5 °C. Absolute zero was found to be −390 °C.

Compare this value with your value from (c)(ii) and explain why the values may differ. Describe an experimental approach that could be taken to avoid systematic error in the determination of absolute zero.

A student makes a pendulum using a length of string with a ball of adhesive putty which acts as a bob. The mass of this bob is M.

A similar second pendulum is constructed with the same length of string but with a bob of a smaller mass. The mass of this bob is m.

The arrangement of the pendulums is shown below.

The bob of mass M is pulled back to a vertical height of H from its rest position. It is released and collides with the bob of mass m. The two bobs then stick together and reach a maximum vertical height h from the rest position.

The height h is given by the equation 

Describe how to perform an experiment to test the validity of this equation and how the data can be analysed.