O-LevelPhysicsThermal Physics5054/21 · May/June 2019 · Q47 Marks

Fig. 4.1 shows a cylinder with a piston that contains a gas. [Figure 4.1]

(a)A liquid occupies a much smaller volume than a gas which has the same number of molecules. Explain why there is this difference.[2]
(b)(i)The volume of the gas changes from V₁ to V₂ as the piston moves into the cylinder. This increases the pressure from p₁ to p₂. The temperature remains the same. State the formula that relates V₁, V₂, p₁ and p₂.[1]
(b)(ii)The initial pressure of the gas in the cylinder is 1.2 × 10⁵ Pa and its initial volume is 100 cm³. The cross-sectional area of the piston is 5.0 cm². The piston is pushed in a distance of 8.0 cm. Calculate the final pressure of the gas in the cylinder.[3]
(b)(iii)Explain, using ideas about molecules, why the pressure of the gas changes.[1]

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Mark scheme

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

[1] no / small(er) force between gas molecules (so they separate) or large(r) force between liquid molecules (keeps them together)
[1] small(er) distance between liquid molecules or large(r) distance between gas molecules or density of liquid larger
[1] P₁V₁ = P₂V₂
[1] volume change = 8 × 5 or 40 cm³ or final volume = 60 (cm³) seen
[1] 1.2 × 10⁵ × 100 = p × 60
[1] 2(.0) × 10⁵ Pa
[1] more hits per second or hit more often / more frequently or more hits on walls

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