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How does the motion of gas particles create pressure, and how does temperature affect it?

Particle motion in gases: how the random motion of particles causes gas pressure, the link between temperature and average kinetic energy, and the effect of changing volume on pressure.

A focused answer to AQA GCSE Physics 4.3.3, covering how the random motion of gas particles creates pressure, the link between temperature and the average kinetic energy of particles, and how changing the volume of a fixed mass of gas at constant temperature changes the pressure.

Generated by Claude Opus 4.88 min answer

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  1. What this dot point is asking
  2. How gas pressure arises
  3. Temperature and kinetic energy
  4. Pressure and volume
  5. Try this

What this dot point is asking

AQA wants you to explain how the random motion of gas particles produces pressure, relate the temperature of a gas to the average kinetic energy of its particles, and explain and use the relationship between the pressure and volume of a fixed mass of gas at constant temperature. This is part of topic 4.3.3 of the AQA GCSE Physics (8463) specification.

How gas pressure arises

A single particle striking a wall and bouncing back changes its direction of motion, so its momentum changes, and by Newton's laws this means it exerts a force on the wall during the collision. Any one collision is tiny, but in even a small volume of gas there are enormous numbers of particles colliding with the walls every second, and the average effect of all these collisions, spread over the area of the walls, is the steady pressure we measure. Because the directions are random, the pressure is the same on every wall of the container regardless of orientation.

Temperature and kinetic energy

There are two distinct ways to raise the pressure of a gas, and AQA expects you to keep them separate. Raising the temperature (at constant volume) increases the speed of the particles, so each collision is harder as well as more frequent. Reducing the volume (at constant temperature) does not change the particle speeds at all; it simply packs the same particles into a smaller space, so they strike each unit area of wall more often. Both increase the pressure, but for different reasons, and confusing the two is a common way to lose marks.

When a gas is compressed quickly, work is done on the gas by the compressing force, which transfers energy to the internal energy of the gas and raises its temperature (this is why a bicycle pump warms up). The relationship below assumes the compression is slow enough for the temperature to stay constant.

Pressure and volume

For a fixed mass of gas at constant temperature, the pressure and volume are inversely proportional.

Try this

Q1. Explain how the particles of a gas create pressure on the walls of their container. [2 marks]

  • Cue. The particles collide with the walls; each collision exerts a force, and the total force over the area is the pressure.

Q2. A gas at 200 kPa200\,kPa occupies 0.5 m30.5\,m^3. It is compressed to 0.2 m30.2\,m^3 at constant temperature. Calculate the new pressure. [3 marks]

  • Cue. p2=p1V1V2=200×0.50.2=500 kPap_2 = \dfrac{p_1V_1}{V_2} = \dfrac{200 \times 0.5}{0.2} = 500\,kPa.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksA sealed syringe contains a fixed mass of gas at a pressure of 120 kPa120\,\text{kPa} and a volume of 30 cm330\,\text{cm}^3. The plunger is pushed in slowly, at constant temperature, until the volume is 18 cm318\,\text{cm}^3. Calculate the new pressure of the gas, and explain in terms of particles why the pressure changes.
Show worked answer →

For a fixed mass of gas at constant temperature, p1V1=p2V2p_1 V_1 = p_2 V_2 (1 mark). Rearranging, p2=p1V1V2=120×3018=360018=200 kPap_2 = \frac{p_1 V_1}{V_2} = \frac{120 \times 30}{18} = \frac{3600}{18} = 200\,\text{kPa} (2 marks). The pressure increases because the same number of gas particles is now squeezed into a smaller volume, so the particles hit the walls more frequently per unit area, giving a larger total force over the area and therefore a higher pressure (1 mark). Markers reward the inverse relationship, the correct value, and the particle explanation referring to more frequent collisions in the smaller volume.

AQA 20213 marksExplain, in terms of the motion of the particles, why increasing the temperature of a fixed volume of gas increases its pressure.
Show worked answer →

Increasing the temperature increases the average kinetic energy of the gas particles, so they move faster (1 mark). The faster particles collide with the walls of the container more frequently, and each collision exerts a larger force because the particles are moving faster (1 mark). With more frequent and harder collisions over the same wall area, the total force per unit area increases, so the pressure of the gas increases (1 mark). Markers reward the link from temperature to average kinetic energy and speed, then to harder and more frequent collisions, then to higher pressure. The condition of fixed volume must be respected.

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