Pressure in a Liquid (Cambridge (CIE) O Level Physics) : Revision Note

Pressure in Liquids

• 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

Liquid Barometers

• Barometers are instruments used to measure atmospheric pressure

• A liquid barometer consists of a glass tube filled with liquid, inverted into a container of the same liquid, with a vacuum at the top of the tube

• Atmospheric pressure pushes the liquid up the tube

• The height of the liquid column indicates the atmospheric pressure

• The higher the atmospheric pressure, the higher the liquid rises in the tube

  • The lower the atmospheric pressure, the lower the column height

• A common example is a mercury barometer

Calculating Pressure in Liquids

• 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