Particle model and pressure (Physics only)

    AQA
    GCSE

    The particle model provides a microscopic explanation for the macroscopic behavior of gases, attributing pressure to the net force exerted by particles colliding with container walls. Candidates must analyze how changing conditions—specifically temperature and volume—affect the frequency and force of these collisions, thereby altering pressure. The topic necessitates a quantitative understanding of Boyle's Law (pV = constant) and a qualitative understanding of the pressure-temperature relationship. For Physics-only specifications, this extends to the thermodynamic principle that doing work on a gas increases its internal energy, resulting in a temperature rise.

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    Objectives
    3
    Exam Tips
    4
    Pitfalls
    4
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that pressure is caused by the force exerted by particles colliding with the walls of the container
    • Credit responses that explain increased pressure in terms of an increased frequency of collisions per unit area
    • Award 1 mark for correct substitution into the equation pV = constant to find an unknown pressure or volume
    • For Higher Tier, award credit for explaining that doing work on a gas increases the internal energy and average kinetic energy of the particles
    • Credit the specific link that increased average kinetic energy results in an increase in the temperature of the gas

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that pressure is caused by the force exerted by particles colliding with the walls of the container
    • Credit responses that explain increased pressure in terms of an increased frequency of collisions per unit area
    • Award 1 mark for correct substitution into the equation pV = constant to find an unknown pressure or volume
    • For Higher Tier, award credit for explaining that doing work on a gas increases the internal energy and average kinetic energy of the particles
    • Credit the specific link that increased average kinetic energy results in an increase in the temperature of the gas

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When explaining pressure changes, you must use the phrase 'frequency of collisions' or 'collisions per second' to secure the explanation marks; 'more collisions' is often insufficient.
    • 💡For Higher Tier questions involving bicycle pumps, explicitly state the energy chain: Work done on gas → Increases Internal Energy → Increases Kinetic Energy → Increases Temperature.
    • 💡In pV = constant calculations, you do not always need to convert to SI units (Pascals/m³) as long as the units on both sides are consistent, but be careful if the question asks for a specific output unit.

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that particles collide 'more' without specifying 'more frequently' or 'per second', which fails to define the rate
    • Believing that the particles themselves expand or contract when pressure changes, rather than the space between them changing
    • Failing to convert units consistently (e.g., mixing kPa and Pa) when performing pV = constant calculations
    • Attributing pressure to collisions between particles themselves rather than collisions with the container walls

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Describe
    Suggest
    Determine

    Practical Links

    Related required practicals

    • •{"code":"Non-Required Practical","title":"Investigation of Boyle's Law","relevance":"Common exam context involving syringes and pressure gauges to demonstrate pV=constant"}

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