Atmospheric Pressure

    OCR
    GCSE

    Atmospheric pressure arises from the cumulative force exerted by air molecules colliding with surfaces, defined as force per unit area. The magnitude depends on the density of the atmosphere and the height of the air column above a specific point. As altitude increases, the atmosphere becomes less dense, resulting in fewer molecular collisions and a corresponding decrease in pressure. Candidates must articulate the relationship between height, air density, and the weight of the air column to explain phenomena such as ear popping or the operation of barometers.

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

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that atmospheric pressure is created by air molecules colliding with a surface
    • Award 1 mark for explaining that as altitude increases, the density of the atmosphere decreases (fewer molecules per unit volume)
    • Credit responses that link lower density to a reduced frequency of collisions per unit area
    • Award 1 mark for recognizing that the weight of the air column above a specific point decreases as altitude increases

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly stated pressure drops with height, but you must explain *why* using the particle model"
    • "Avoid saying 'air is thinner'—instead, state that the 'density of air decreases'"
    • "Good use of the collision concept; now ensure you specify that there is less weight of air *above* the point"
    • "You identified the trend correctly, but check the graph shape—does it decrease at a constant rate?"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that atmospheric pressure is created by air molecules colliding with a surface
    • Award 1 mark for explaining that as altitude increases, the density of the atmosphere decreases (fewer molecules per unit volume)
    • Credit responses that link lower density to a reduced frequency of collisions per unit area
    • Award 1 mark for recognizing that the weight of the air column above a specific point decreases as altitude increases

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When explaining pressure changes, always use the phrase 'fewer collisions per second per unit area' rather than just 'less pressure'
    • 💡Distinguish clearly between the cause (gravity acting on air mass) and the mechanism (particle collisions)
    • 💡In graph questions, pay attention to the curve; atmospheric pressure does not reach zero linearly, so do not extrapolate a straight line

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that particles move slower at high altitudes to explain lower pressure (confusing temperature effects with density/pressure)
    • Using vague terminology like 'thinner air' without defining it as 'lower density' or 'increased particle spacing'
    • Assuming the relationship between altitude and pressure is linear (like liquid pressure) rather than non-linear/exponential

    Key Terminology

    Essential terms to know

    Particle collision model and kinetic theory
    Inverse relationship between altitude and pressure
    Atmospheric density gradients
    Force per unit area in gaseous fluids

    Likely Command Words

    How questions on this topic are typically asked

    State
    Describe
    Explain
    Suggest
    Analyze

    Practical Links

    Related required practicals

    • •{"code":"Demonstration","title":"Collapsing Can Experiment","relevance":"Demonstrates the magnitude of atmospheric pressure when internal pressure is reduced"}
    • •{"code":"Demonstration","title":"Magdeburg Hemispheres","relevance":"Historical context often used in OCR questions to test understanding of pressure difference"}

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