Pressure in fluids

Liquids and gases are fluids. A fluid is able to change shape and flow from place to place. Fluids exert pressure on surfaces, and this pressure acts at 90° to those surfaces – we say that it acts normal to the surface.

Atmospheric pressure

The atmosphere exerts a pressure on you, and everything around you. You may have seen a demonstration of the effects of this atmospheric pressure.

The Magdeburg hemispheres are two metal cups that fit together. If most of the air is removed from inside them using a vacuum pump, it is almost impossible to pull them apart again. The pressure of the atmosphere acting on their outside surface pushes them tightly together. Once the air is let back in, the pressure inside equals the pressure outside again, and the cups can easily be separated.

Rubber seals inside an air-filled vacuum chamber. The vacuum between the two rubber seals means they cannot be pulled apart without force because the weight of the atmosphere presses them together.
Once the air in the vacuum chamber was evacuated, the rubber seals fell apart as the atmospheric pressure keeping them together was lost.
Rubber seals being used as a demonstration of the Magdeburg hempispheres

The effects of pressure can be seen in the collapsing can experiment. Here some water is boiled in an empty drinks can and steam fills the can. If the can is turned upside down in a trough of cold water, the steam condenses and the air pressure inside goes down. The pressure of the air outside the can suddenly crushes the can.

Atmospheric pressure changes with altitude. The higher you go:

  • the lower the weight of the air above you
  • the lower the atmospheric pressure

For example, atmospheric pressure at sea level is about 100,000 Pa, but it is only about 21,000 Pa at the cruising height of an airliner.

Pressure in liquids

Just like the atmosphere, liquids exert pressure on objects. The pressure in liquids changes with depth. The deeper you go:

  • the greater the weight of liquid above
  • the greater the liquid pressure
A suspended bucket has water pouring from the numerous holes in it.Pressure in a liquid increases with depth so the jet coming from the bottom of the bucket travels further sideways

Liquid pressure is exerted on the surface of an object in a liquid. This pressure causes upthrust. An object placed in a liquid will begin to sink. As it sinks, the liquid pressure on it increases and so the upthrust increases. For a floating object, the upthrust is equal and opposite to the object’s weight. An object will continue to sink if its weight is greater than the maximum upthrust.

The two forces on a boat are the upthrust of the water and the weight of the boatThe weight of the boat is balanced by the upthrust from the water

Bang Goes the Theory presenter Dallas takes part in a free-diving challenge in a 30m deep water chamber

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