ZIMSEC O Level Combined Science Notes: Pressure in liquids and the manometer

  • Gravity pulls a liquid down into its container
  • The liquid in turn exerts a pressure on the container
  • The pressure of a liquid at a certain depth is equal in all directions
  • The pressure of a liquid in a container increases with depth
  • This means in a container pressure is greatest at the bottom

 

Pressure increases with depth in liquids

Pressure increases with depth in liquids

  • This can be demonstrated by punching holes into a tin/container
  • These holes should be made at various depth points
  • The container is then filled with water (liquid)
  • Jets of water are observed on the holes
  • The strongest jet emanates from the hole at the bottom
  • The water from this hole travels the furthest from the container
  • The fact that pressure is greatest at the bottom is true even in dams and tanks

Calculating Pressure in Liquids

  • Pressure in liquids increases with density
  • The pressure of a liquid is found using the formula:
  • \text{Pressure} = \text{h x Px g}
  • g is for gravity
  • h is for height
  • ρ is for density
  • gravity is usually estimated as 10/ms2
  • Pressure in liquids depends on height
  • It is not affected by area

The manometer

 

A manometer. Image credit astarmathsandphysics.com

A manometer. Image credit astarmathsandphysics.com

  • gas/liquid pressure can be measured using a manometer
  • It is a U-tube partly filled with a liquid such as mercury or water
  • When both arms are open to atmospheric pressure the levels of the liquid are the same in both arms
  • However, if one arm is connected to a gas supply the levels change
  • The liquid in the left arm is forced downwards by the gas pressure
  • The pressure of the gas is then balanced by atmospheric pressure plus the column of liquid with height h
  • The pressure of the gas is equal to the atmospheric pressure at the time of measurement plus
  • The pressure of the water in column h

To access more topics go to the Combined Science Notes page


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