Pressure in a Liquid (OCR Gateway GCSE Physics)

• When an object is immersed in a liquid, the liquid will exert a pressure, squeezing the object
• This pressure is exerted evenly across the whole surface of the liquid and in all directions
  • The greater the depth of the liquid, the greater the pressure
  • The greater the density of the liquid, the greater the pressure

• In a liquid, the pressure at a point increases with the height of the column of liquid about that point
  • If there is more liquid above that point, then the pressure is more

• This is because the pressure in a liquid is caused by the weight of the liquid pushing against objects immersed in the liquid
  • As the liquid becomes deeper, the amount of liquid (and hence the weight) increases which causes the pressure to increase

• This is why, for example, the pressure increases with the depth of the ocean
  • The pressure on the seabed is far higher than that on the surface of the ocean

• The weight of the liquid also depends on its density
  • A more dense liquid has a greater weight and therefore will exert a higher pressure

Pressure in a column of water increases with depth, shown by the strong and weak jet of water

• In a column of water, the highest pressure would be at the bottom
  • If a hole is made at the bottom of the column, the water will pour out with a large force
  • If a hole was made at the top of the column, the water will pour out with a small force
  • This is because of the difference in pressure in the column caused by the weight of the water

Pressure Differences & Upthrust

• Upthrust is due to the difference in pressure between the top and the bottom of the submerged object
• A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface
  • This is because the pressure p is proportional to the depth h of the object

• The difference in pressure creates a resultant force upwards (upthrust)
• Upthrust is why objects appear to weigh less when immersed in a liquid

Upthrust is due to the different pressure at the top and bottom of this cylinder

• The amount of upthrust on an object depends on the weight of the fluid that the object displaces
  • Upthrust is equal to the weight of fluid displaced

Liquids exert an upwards force (upthrust) on objects immersed in them equal to the weight of the fluid displaced

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

p = h × ρ × g

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

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

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

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