A-LevelPhysicsForces, density and pressure9702/33 · Feb/Mar 2019 · Q220 Marks

In this experiment, you will investigate the relationship between the pressure in a balloon and its diameter.

(a)• You are provided with two partially inflated balloons. Place the larger balloon on the bench with its neck at the side, as shown in Fig. 2.1. • Put pieces of modelling clay around the balloon to stop it rolling, as shown in Fig. 2.1. • The diameter of the balloon is B, as shown in Fig. 2.1. Measure and record B. B = .....................................................[1]
(b)• Wet a small area on the top of the balloon with a damp paper towel. • Balance the transparent block on the top of the balloon, as shown viewed from above in Fig. 2.2. • The contact patch will be visible, as shown in Fig. 2.3. • The diameter of the contact patch is d, as shown in Fig. 2.3. Measure and record d. d = .....................................................[2]
(c)Estimate the percentage uncertainty in your value of d. percentage uncertainty = .....................................................[1]
(d)Calculate the pressure P in the balloon using the relationship P = 4mg/(πd²) where g = 9.81 Nkg⁻¹ and m is the mass of the transparent block written on the card. P = ..................................................... Nm⁻²[2]
(e)Justify the number of significant figures you have given for your value of P.[1]
(f)Repeat (a), (b) and (d) using the smaller partially inflated balloon. B = ..................................................... d = ..................................................... P = ..................................................... Nm⁻²[3]
(g)(i)It is suggested that the relationship between B and P is P = k/B where k is a constant. Using your data, calculate two values of k. first value of k = ..................................................... second value of k = .....................................................[1]
(g)(ii)Explain whether your results support the suggested relationship.[1]
(h)(i)Describe four sources of uncertainty or limitations of the procedure for this experiment.[4]
(h)(ii)Describe four improvements that could be made to this experiment. You may suggest the use of other apparatus or different procedures.[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.

[1] 2(a) value for B, with unit
[1] 2(b) measured value(s) for d, with unit, to nearest mm
[1] 2(b) evidence of repeat readings
[1] 2(c) absolute uncertainty in d value ≥ 0.2 cm and correct method of calculation to obtain percentage uncertainty. if several readings have been taken, then the absolute uncertainty can be half the range if the working is clearly shown (but not zero if values are equal)
[1] 2(d) calculation of P correct
[1] 2(d) POT of P value matches unit
[1] 2(e) justification based on s.f. in d, m and g
[1] 2(f) value for second B
[1] 2(f) value for second d
[1] 2(f) Quality: d smaller for smaller B
[1] 2(g)(i) two values of k calculated correctly
[1] 2(g)(ii) sensible comment relating to the calculated values of k, testing against a criterion specified by the candidate
[4 max] 2(h)(i) two k values are not enough to draw a valid conclusion
[4 max] 2(h)(i) difficult to measure B with reason, e.g. pressing on balloon / ruler scale doesn't start at zero / parallax
[4 max] 2(h)(i) difficult to balance Perspex block on balloon / balloon kept moving
[4 max] 2(h)(i) difficult to measure d with reason, e.g. not circular / changes when block touched / parallax / refraction changes apparent size
[4 max] 2(h)(i) difficult to see (edges of) contact patch (clearly)
[4 max] 2(h)(ii) take more readings and plot a graph / calculate more k values and compare
[4 max] 2(h)(ii) workable method of reducing parallax for B e.g. callipers
[4 max] 2(h)(ii) fix balloon in place with tape / glue / Blu-tack / trap between blocks at sides
[4 max] 2(h)(ii) grid on block / travelling microscope / thinner block
[4 max] 2(h)(ii) use contrasting colours for balloon and liquid