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A2 Daily A Level Physics question

2025-10-02 OCR A Thermal physics II: gas laws; kinetic theory model; pV = nRT; assumptions & limits 5.1.4(d)(i)

A rigid 2.0 L steel cylinder contains dry air sealed at 300 K and 100 kPa. It is warmed uniformly to 360 K with no leaks, and the cylinder’s volume is unchanged. According to the kinetic theory model of an ideal gas, what happens to the gas pressure?

  1. A Increases to about 125 kPa because the molecular kinetic energy rises by 25%.
  2. B Increases to about 120 kPa because, at fixed volume and amount, pressure is proportional to absolute temperature. (correct)
  3. C Rises to about 200 kPa because faster molecules collide more often and with greater force, doubling the pressure.
  4. D Stays at 100 kPa because the container volume is fixed, so the pressure cannot change.

Answer

The correct answer is B.

Correct: B — Increases to about 120 kPa because, at fixed volume and amount, pressure is proportional to absolute temperature. A uses an incorrect percentage change (360 K is 20% higher than 300 K, not 25%) and confuses the required Kelvin scaling. B is correct: with n and V fixed, p scales with T, so p = 100 kPa × (360/300) ≈ 120 kPa, assuming ideal-gas behaviour. C is a misconception that faster motion necessarily doubles pressure; doubling requires doubling the absolute temperature, which has not happened here. D is wrong because pressure can change at fixed volume when temperature (and hence collision rate/impulse) changes.

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