Fluid Pressure

Students explore how pressure affects flow of fluids.

What is Fluid Pressure?

Fluid pressure is the amount of force a fluid exerts in a specified space. From the last lesson we learnt that force is measured in Newtons. Since pressure is the amount of force over a given area, it is measured in Newtons per a meter squared (N/m2). Rather than referring to pressure as force over area we shorted the unit to pascals (Pa), named after the French physicist Blaise Pascal. A Pa and N/m2 are the same unit. Below you can see the formula for calculating pressure:

For larger pressures we often use kilopascals (kPa), which is 1000 Pa.

Compression of Gas

Think about the following situation, if you were a stunt double and had to jump from a 3 storey building what would you rather land on? A asphalt road or air mattress? The air mattress would act like a giant shock absorber, cushioning your fall. The reason the air mattress acts like a shock absorber is because air, like other gases, can compress. For a gas like air to compress three things are required:

  1. The games must be enclosed in a seal container that allows the gas to evenly distribute.
  2. There is enough space between the particles that, even after being compressed, they are still far enough apart to behave like a gas.
  3. An external, or outside, force is applied to the enclosed gas to push the particles together.

The reason gases can compress is because the vast space between individual particles is large enough that it can be compressed closer together. Unlike gases, while liquids and solids do have a vast amount of empty space between particles they are often as close together as they can possibly be. When you apply more pressure to a liquid or solid the particles cannot move any closer together or be squeezed into a smaller volume; because solids and liquids cannot be compressed they are referred to as almost incompressible. You may be asking what happens when a solid or liquid is compressed? Well since the particles cannot be compressed they transfer the force to the outer walls of their container.

We use compressed gases a lot in our everyday life. Think about the tires on your car or bike, a gas is often filled into the tire allowing it to slightly compress and create a smoother ride. Another important property of compressed gas is that it likes to reform its original shape after the external force is removed. This force is known as counterforce. Staying with the car analogy, the airbag is a safety feature that can save someone’s life. The airbag works by compressing and then applying counterpressure to slow down the velocity of the person in a safer manner.

Measuring Air Pressure

It may be hard to believe that the Earth is already an enclosed container and because of that we experience the pressure of the enclosed atmosphere. This is referred to as atmospheric pressure. Atmospheric pressure can be measured using a barometer.

A barometer works by allowing the application of atmospheric pressure on a liquid forcing the excess pressure up a small tube. See picture below for more information.

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