Gas Pressure

    OCR
    GCSE

    Gas pressure is defined as the net force exerted per unit area on container walls caused by the frequent, random collisions of gas particles. Candidates must explain this phenomenon using the particle model, linking temperature directly to the average kinetic energy of particles and the resulting frequency and force of collisions. The relationship between pressure and volume at constant temperature (Boyle's Law) must be understood both qualitatively and quantitatively ($pV = \text{constant}$). Furthermore, the concept of doing work on a gas, such as compressing it rapidly, must be linked to an increase in internal energy and temperature.

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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 gas particles move in random directions at high speeds
    • Award 1 mark for linking pressure to the force exerted by particles colliding with the walls of the container
    • Credit responses that explain increased temperature leads to higher kinetic energy and more frequent/forceful collisions
    • Award 1 mark for correct application of $P = F/A$ or $P_1 V_1 = P_2 V_2$ with correct unit conversion
    • For Higher Tier: Credit explanation that doing work on a gas (compression) increases the internal energy/temperature of the gas

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly calculated the pressure, but check your units—did you convert $cm^2$ to $m^2$?"
    • "Good recall of the particle model. To improve, specify that collisions are with the *walls* of the container, not just other particles."
    • "You identified the relationship is inverse. Now, use the data to prove it by showing $P \times V$ gives a constant value."
    • "Excellent explanation of the temperature rise. For full marks, explicitly link the 'work done' in compression to the 'increase in kinetic energy' of the particles."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that gas particles move in random directions at high speeds
    • Award 1 mark for linking pressure to the force exerted by particles colliding with the walls of the container
    • Credit responses that explain increased temperature leads to higher kinetic energy and more frequent/forceful collisions
    • Award 1 mark for correct application of $P = F/A$ or $P_1 V_1 = P_2 V_2$ with correct unit conversion
    • For Higher Tier: Credit explanation that doing work on a gas (compression) increases the internal energy/temperature of the gas

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When explaining pressure changes, always use the phrase 'more frequent collisions per unit area' rather than just 'more collisions'
    • 💡In calculation questions involving $P_1 V_1 = P_2 V_2$, explicitly check that the units for Pressure and Volume are consistent on both sides before calculating
    • 💡For 6-mark extended response questions on the bicycle pump experiment, link the mechanical work done directly to the increase in the average kinetic energy of particles

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that pressure is caused by particles colliding with 'each other' rather than the container walls
    • Describing the relationship between Pressure and Volume as merely 'negative correlation' instead of 'inversely proportional'
    • Failing to convert area units correctly (e.g., using $cm^2$ directly in $P=F/A$ without converting to $m^2$ or adjusting the Pascal output)
    • Confusing 'heating' a gas with 'doing work' on a gas—candidates often miss the mechanical transfer of energy in compression contexts

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Kinetic theory and particle motion
    Pressure-Volume relationship (Boyle's Law)
    Pressure-Temperature relationship (Gay-Lussac's Law)
    Work done on a gas and internal energy

    Likely Command Words

    How questions on this topic are typically asked

    State
    Explain
    Calculate
    Describe
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"PAG P1","title":"Investigation of Gas Laws","relevance":"Demonstrating the inverse relationship between pressure and volume at constant temperature"}

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