OCR · GCSE · Physics

    Course version: J249 · Version from Sept 2018

    Calculate the differences in pressure at different depths in a liquid (to…

    Full specification wording

    Pressure: calculate the differences in pressure at different depths in a liquid (to include: knowledge that g is the strength of the gravitational field and has a value of 10 N/kg near the Earth’s surface) (content found only in the physics separate science qualification) (Higher tier only)

    This Higher tier separate physics criterion asks you to calculate how pressure changes between two depths in a liquid.

    Checked against the OCR J249 specification (opens in a new tab) on .

    • Point 12 of 12
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    This Higher tier separate physics criterion asks you to calculate how pressure changes between two depths in a liquid. The pressure difference depends on the liquid's density, the gravitational field strength g and the vertical depth difference: Δp = ρ × g × Δh. Near the Earth's surface g is the strength of the gravitational field and equals 10 N/kg, so a liquid of density 1000 kg/m³ gives an extra pressure of 1000 × 10 × 1 = 10 000 Pa for each extra metre of depth. Work out the pressure at each depth, or the difference directly from the depth gap, and keep units consistent: density in kg/m³, depth in m, pressure in Pa (N/m²).

    What earns the marks

    • Uses Δp = ρ × g × Δh with g = 10 N/kg near the Earth's surface.
    • Substitutes density in kg/m³ and depth difference in metres to obtain pressure in pascals.
    • Calculates the pressure difference between two depths rather than the pressure at a single depth only.
    • Recognises that pressure increases with depth in a liquid and states the direction of the change.

    Common mistakes and how to put them right

    • Adds the two depths instead of finding the difference between them.

      Put it right: Subtract the shallower depth from the deeper one.

    • Leaves depth in centimetres while density is in kg/m³.

      Put it right: Convert depth to metres before multiplying.

    • Reports the answer in newtons rather than pascals.

      Put it right: Pressure is measured in pascals (N/m²).

    Exam tips

    • Write the equation Δp = ρ × g × Δh before substituting values so the method is visible.
    • Convert every depth to metres and check density is in kg/m³ before calculating.
    • State the unit of your final answer as Pa and check the size is sensible for the depth involved.

    Try the lesson

    The start of the Pressure: calculate the differences in pressure at different depths in a liquid (to include: knowledge that g is the strength of the gravitational field and has a value of 10 N/kg near the Earth’s surface) (content found only in the physics separate science qualification) (Higher tier only) lesson, made from checked teaching notes for this specification point.

    Have a go first

    A container of liquid of density 1200 kg/m³ has a tap 0.5 m below the surface and a second tap 2.5 m below the surface. Using g = 10 N/kg, what is the difference in pressure between the two taps?

    Listen

    This Higher tier separate physics criterion asks you to calculate how pressure changes between two depths in a liquid. The pressure difference depends on the liquid's density, the gravitational field strength g and the vertical depth difference: Δp = ρ × g × Δh. Near the Earth's surface g is the strength of the gravitational field and equals 10 N/kg, so a liquid of density 1000 kg/m³ gives an extra pressure of 1000 × 10 × 1 = 10 000 Pa for each extra metre of depth. Work out the pressure at each depth, or the difference directly from the depth gap, and keep units consistent: density in kg/m³, depth in m, pressure in Pa (N/m²). A strong answer earns marks for these points. Uses Δp = ρ × g × Δh with g = 10 N/kg near the Earth's surface. Substitutes density in kg/m³ and depth difference in metres to obtain pressure in pascals. Calculates the pressure difference between two depths rather than the pressure at a single depth only. Recognises that pressure increases with depth in a liquid and states the direction of the change. Exam tip: Write the equation Δp = ρ × g × Δh before substituting values so the method is visible.

    See it

    A top-to-bottom flowchart with eight boxes. The first box says identify the two depths in the liquid. An arrow leads to the second box: subtract the shallower depth from the deeper depth to find Δh. An arrow leads to the third box: convert Δh to metres. An arrow leads to the fourth box: check density is in kg/m³. An arrow leads to the fifth box: use g = 10 N/kg near the Earth's surface. An arrow leads to the sixth box: apply Δp = ρ × g × Δh. An arrow leads to the seventh box: pressure difference in pascals (N/m²). An arrow leads to the eighth box: pressure increases with depth.
    The steps for calculating the pressure difference between two depths in a liquid using Δp = ρ × g × Δh with g = 10 N/kg.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Use g = 10 N/kg near the Earth's surface
    2. 2. Pressure increases with depth
    3. 3. Apply Δp = ρ × g × Δh
    4. 4. Identify the two depths in the liquid
    5. 5. Subtract the shallower depth from the deeper depth to find Δh
    6. 6. Pressure difference in pascals (N/m²)
    7. 7. Convert Δh to metres
    8. 8. Check density is in kg/m³

    Key words

    Say what each word means before you turn the card.

    Gravitational field strength

    Try answering aloud or in your head before turning the card.

    Pressure in liquids

    Try answering aloud or in your head before turning the card.

    Density and depth

    Try answering aloud or in your head before turning the card.

    Unit consistency

    Try answering aloud or in your head before turning the card.

    See every definition
    Gravitational field strength
    Near the Earth's surface it has a value of 10 N/kg.
    Pressure in liquids
    Increases with depth in a liquid, and the difference between two depths is found using Δp = ρ × g × Δh.
    Density and depth
    The pressure difference depends on the liquid's density and the vertical depth difference.
    Unit consistency
    Density in kg/m³, depth in m and pressure in Pa (N/m²) must be kept consistent.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-to-bottom flowchart with eight boxes. The first box says identify the two depths in the liquid. An arrow leads to the second box: subtract the shallower depth from the deeper depth to find Δh. An arrow leads to the third box: convert Δh to metres. An arrow leads to the fourth box: check density is in kg/m³. An arrow leads to the fifth box: use g = 10 N/kg near the Earth's surface. An arrow leads to the sixth box: apply Δp = ρ × g × Δh. An arrow leads to the seventh box: pressure difference in pascals (N/m²). An arrow leads to the eighth box: pressure increases with depth.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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