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1.8Motion, Forces and Energy

Pressure — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 82 of 91 2 quizzes

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.8 Pressure — IGCSE PHYSICS notes: Define pressure, use p = F/A and describe how pressure in a liquid depends on depth and density.Pressure = the force per unit area.Equation — p = F/A.What the symbols mean: p = pressure (Pa = N m−2) · F = force acting at right angles to the sur…

  • Rearranged forms
  • Equation (Extended)
  • Total pressure at depth
  • Pascal (Pa)
  • Normal force
  • Atmospheric pressure

Part 1 of 7Pressure

Pressure
Pressure the force per unit area.
Equation
  • Equation p = F/A.
    What the symbols mean: p = pressure (Pa = N m−2) · F = force acting at right angles to the surface (N) · A = area (m2)
  • p is pressure in pascals (Pa), F is force in newtons (N) and A is area in square metres (m²).
Unit
  • 1 Pa = 1 N/m²; pressure may also be given in N/cm² when the area is in cm².
The force acts perpendicular (normal) to the surface.
If an object rests on several points of contact, add the areas to get the total area.

Part 2 of 7High pressure — small area

High pressure — small area
A small area gives a high pressure for the same force.
Knife blades, nails, drawing pins and needles concentrate the force to cut or pierce.
Stiletto or high heels have a tiny area and can dent soft floors.

Part 3 of 7Low pressure — large area

1
A large area gives a low pressure for the same force.
2
Snowshoes, skis, tractor tyres, caterpillar tracks and wide straps spread the weight.
3
A camel's wide feet stop it sinking in sand.
4
Wide foundations stop buildings sinking.

Part 4 of 7Calculating pressure

What the symbols mean:
Find the force (often the weight, W = mg) in newtons.
What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)
Find the area in contact in m² or cm².
Divide force by area.
Give the unit Pa or N/cm².
Rearranged forms
  • F = pA and A = F/p.
Total area for four legs is four times the area of one leg.

Part 5 of 7Pressure in a liquid

Pressure beneath the surface of a liquid increases with depth.
  • The pressure is caused by the weight of the liquid above.
It also increases with the density of the liquid.
  • Sea water is denser than fresh water, so the pressure is greater at the same depth.
Pressure at a given depth acts equally in all directions.
Dams are thicker at the bottom, and water spurts farthest from the lowest hole in a container.
Equation (Extended)
  • Δp = ρgΔh, with Δp in Pa, ρ in kg/m³, g in N/kg and Δh in m.
    What the symbols mean: p = pressure due to the liquid (Pa) · ρ = density of the liquid (kg m−3) · g = 9.81 N kg−1 · h = depth (m)
Total pressure at depth
Total pressure at depth atmospheric pressure (about 100 kPa) + ρgh.

Part 6 of 7Showing that pressure increases with depth

1
Make three holes at different heights up the side of a tall plastic bottle.
2
Cover the holes and fill the bottle with water.
3
Uncover the holes at the same time.
4
Observe the water jets; the lowest hole's jet travels farthest.
5
Conclude that pressure increases with depth below the surface.

Part 7 of 7More from the specification

1Hydrostatic thrust and perpendicular forces
  • Hydrostatic thrust and perpendicular forces fluids at rest exert uniform perpendicular compressive forces against all bounding solid surfaces.
2Pressure
  • Pressure equals force divided by area, p = F/A, measured in pascals (N/m2);
    • the same force on a smaller area gives greater pressure.
3Substituting into equations
  • Convert units first, then substitute the relevant quantities such as force in newtons and area in square metres.
4Pressure in liquids
  • Pressure in a fluid increases with depth and density: p = h x density x g, where g is the gravitational field strength.
5Measuring pressure
  • A manometer measures pressure from the height difference of a liquid column, and a change in column height shows a change in pressure.
6Applications
  • Sharp knives, wide tyres and skis show how area is changed to increase or reduce the pressure exerted on a surface.

Key terms

TermMeaning
PressureForce acting on a unit area; p = F/A.
Pascal (Pa)Unit of pressure; 1 Pa = 1 N/m².
Normal forceForce at right angles to a surface.
AreaSurface a force acts on, in m² or cm².
WeightForce of gravity on an object, W = mg.
Atmospheric pressurePressure from the weight of air above, about 100 kPa.

Avoid these mistakes

1
Writing pressure in N — N is the unit of force; pressure is in Pa.
2
Believing a larger area gives a larger pressure — it gives a smaller one for the same force.
3
Using mass in kg instead of weight in N for the force.
4
Forgetting to add atmospheric pressure when total pressure is asked.

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