O-LevelPhysicsThermal Physics5054/22 · Oct/Nov 2015 · Q65 Marks

A gas is trapped at atmospheric pressure in a cylinder by a piston. The piston is held in a fixed position by a movable rod. Fig. 6.1 shows the cylinder. [Figure 6.1] The cylinder is heated. As the temperature of the gas increases, its pressure increases.

(a)Explain, in terms of molecules, why the pressure of the trapped gas increases.[2]
(b)The rod is pulled down and the piston is then free to move as shown in Fig. 6.2. [Figure 6.2] As the piston moves, the temperature of the gas remains constant. State and explain, in terms of molecules, what happens to the pressure of the gas.[3]

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 (Oct/Nov 2015). The mark or mark type is shown in brackets.

[1] they/molecules move/collide faster or gain kinetic energy
[1] they/molecules collide with walls more often or harder
[1] pressure decreases
[1] larger volume (of gas) or they/molecules move further between collisions
[1] fewer collisions per unit time/reduced collision frequency (of molecules with wall) or collide less often or pressure decreases to atmospheric pressure

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.