Brownian Motion

    OCR
    GCSE

    Brownian motion is the random, erratic movement of microscopic particles suspended in a fluid, typically observed using smoke particles in air or pollen grains in water. This phenomenon serves as critical empirical evidence for the kinetic particle model, confirming that matter is composed of discrete atoms or molecules. The observed motion results from the unequal, random collisions between the larger, visible suspended particles and the significantly smaller, invisible, and fast-moving fluid molecules. It demonstrates that fluid molecules possess kinetic energy and are in constant random motion.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for describing the movement of suspended particles (e.g., smoke) as random, zigzag, or unpredictable
    • Credit the explanation that large particles collide with smaller, invisible fluid molecules (e.g., air)
    • Candidates must state that the fluid molecules are moving rapidly in random directions
    • Award 1 mark for explaining that the random motion is caused by unequal bombardment/force on different sides of the suspended particle

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly described the motion as random; now explain *what* is colliding with the smoke particle"
    • "Ensure you mention that the air molecules are invisible and much smaller than the smoke particles"
    • "Good use of 'collisions', but remember to specify that the air molecules are moving fast and randomly"
    • "To access higher marks, explain how the motion would change if the temperature of the gas increased"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for describing the movement of suspended particles (e.g., smoke) as random, zigzag, or unpredictable
    • Credit the explanation that large particles collide with smaller, invisible fluid molecules (e.g., air)
    • Candidates must state that the fluid molecules are moving rapidly in random directions
    • Award 1 mark for explaining that the random motion is caused by unequal bombardment/force on different sides of the suspended particle

    Examiner Tips

    Expert advice for maximising your marks

    • 💡In 'Explain' questions, clearly distinguish the 'visible' particle (smoke) from the 'invisible' driver (air molecules)
    • 💡Avoid vague terms like 'jiggling'; use standard terminology such as 'random motion' or 'unpredictable path'
    • 💡If asked why the smoke changes direction, reference 'unbalanced forces' or 'unequal collisions' on different sides of the particle

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Attributing the random motion to collisions between the smoke particles themselves rather than with the air molecules
    • Describing the air molecules as 'vibrating' rather than moving with rapid translational velocity
    • Failing to explicitly state that the air molecules are much smaller and faster than the suspended particles

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Evidence for the kinetic particle model of matter
    Random and erratic motion of suspended particles
    Interaction between macroscopic and microscopic particles
    Relationship between temperature and particle velocity

    Likely Command Words

    How questions on this topic are typically asked

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    Describe
    Explain
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    Practical Links

    Related required practicals

    • •{"code":"P1 (Suggested)","title":"Observation of Brownian motion in a smoke cell","relevance":"Direct observation of particle behavior supporting kinetic theory"}

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