Pressure in a Liquid (Cambridge O Level Physics)

• A fluid is either a liquid or a gas

When an object is immersed in a fluid, the fluid will exert pressure, squeezing the object

• This pressure is exerted evenly across the whole surface of the fluid and in all directions
• The pressure exerted on objects in fluids creates forces against surfaces
• These forces act at 90 degrees (at right angles) to the surface

 

Pressure on an Object in a Fluid

The pressure of a fluid on an object creates a force normal (at right angles) to the surface

 

• The pressure of a fluid on an object will increase with:
  • Depth within the fluid
  • Increased density of the fluid

• The pressure is more accurately the difference in pressure at different depths h in a liquid, since the pressure changes with the depth

• The pressure due to a column of liquid can be calculated using the equation

Δp = ρgΔh

• Where:
  • Δp = change in pressure in pascals (Pa)
    • Where 1 Pa = 1 N/m²
  • ρ = density of the liquid in kilograms per metre cubed (kg/m³)
  • g = gravitational field strength on Earth in newtons per kilogram (N/kg)
  • Δh = change in height of the column in metres (m)

• The force from the pressure is exerted evenly across the whole surface of an object in a liquid, and in all directions

Even Pressure on an Object in a Fluid

The force from the pressure of objects in a liquid is exerted evenly across its whole surface