Pressure in Liquids

    OCR
    GCSE

    Pressure in liquids is defined as the force per unit area exerted by the weight of the liquid column above a specific point, acting equally in all directions perpendicular to any surface. The magnitude of this pressure is determined by the product of the column height, the liquid's density, and gravitational field strength, expressed mathematically as P = hρg. This relationship explains the mechanism of upthrust, where the pressure differential between the top and bottom of a submerged object generates a resultant upward force. Mastery of this topic requires understanding hydrostatic pressure, the incompressibility of liquids, and the transmission of pressure in hydraulic systems.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for correct substitution of depth (m), density (kg/m³), and gravitational field strength into P = hρg
    • Credit responses that identify the cause of upthrust as the greater pressure exerted on the bottom surface of an object compared to the top surface
    • Award 1 mark for stating that pressure acts equally in all directions at a specific depth within a fluid
    • Award 1 mark for concluding that an object sinks because its weight is greater than the maximum upthrust the displaced fluid can generate

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly calculated the pressure, but check your units—did you convert cm to m?"
    • "Good definition of upthrust. To improve, explain *why* the force is greater at the bottom of the object"
    • "You identified that the object sinks. Now, compare the specific values of weight and upthrust to justify this"
    • "Excellent use of the pressure formula. Ensure you are using the depth below the surface, not the height from the container floor"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for correct substitution of depth (m), density (kg/m³), and gravitational field strength into P = hρg
    • Credit responses that identify the cause of upthrust as the greater pressure exerted on the bottom surface of an object compared to the top surface
    • Award 1 mark for stating that pressure acts equally in all directions at a specific depth within a fluid
    • Award 1 mark for concluding that an object sinks because its weight is greater than the maximum upthrust the displaced fluid can generate

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Always convert depth to metres and density to kg/m³ before starting your calculation; OCR frequently penalises unit errors here
    • 💡When explaining floating, explicitly compare the magnitude of the weight force acting downwards and the upthrust force acting upwards
    • 💡Remember that 'h' in the pressure equation represents the vertical column of liquid above the point, not the object's height

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Substituting the height of the object or distance from the bottom instead of the depth below the liquid surface for 'h'
    • Failing to convert density from g/cm³ to kg/m³ or depth from cm to m, resulting in answers off by orders of magnitude
    • Describing upthrust vaguely as 'water pushing up' without explaining that it arises from the pressure difference at different depths

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Hydrostatic pressure relationship (P = hρg)
    Upthrust and the mechanism of buoyancy
    Transmission of pressure in fluids (Pascal's Principle)
    Factors affecting fluid pressure (depth and density)

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Describe
    Show
    Determine

    Practical Links

    Related required practicals

    • •{"code":"Demonstration","title":"Spouting Can Experiment","relevance":"Visualises relationship between pressure and depth"}
    • •{"code":"PAG P1","title":"Determination of Density","relevance":"Foundational concept for calculating liquid pressure"}

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