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

• 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

• 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

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