Fluids in Physics: Definition and Characteristics

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  • 0:01 What Is a Fluid?
  • 1:05 Properties of Fluids
  • 5:48 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Fluids really know how to move! As flowing materials, fluids have some unique properties that help us understand and describe them. In this lesson you'll explore what fluids are and how they are defined in the world of physics.

What Is a Fluid?

Let's take a trip to the beach. It's quite relaxing here, isn't it? The white sand, the blue water, the fresh air - just thinking about it makes me want to do some physics! Okay, that was a mean trick, but while we're here, we might as well use this great location and learn a little bit about the different phases of matter.

You are probably quite familiar with these already because they are solids (the sand), liquids (the water) and gases (the fresh air). Solids are pretty different from liquids and gases because they hold their shape. You can't put a square block into a round hole because the block is solid and won't conform to the shape of the hole.

Liquids and gases are more similar to each other because, unlike solids, they both conform to the shape of their container. They also both flow when moved around, and any material that flows is called a fluid. When you put air in your car tires, they stay inflated because the gas particles fill the entire space inside. Likewise, when you fill your bathtub, you can't keep the water on just one side because it flows to cover all the space of the tub.

Properties of Fluids

There are certain properties that fluids share, though the specifics of these may be slightly different for each type of fluid. The main difference between the two fluids mentioned here is that gas particles are much farther apart than the particles of a liquid. Both will spread out to fill their container, but a liquid only does so beneath its surface.

This important difference helps us understand that a gas is compressible, which means its volume can easily be increased or decreased, while a liquid is incompressible, meaning its volume cannot easily be changed. In other words, you can more easily press gas particles together than you can the particles of a liquid. This is because there is more space between those gas particles, while the liquid particles are already about as close together as they can get.

One property that all fluids do share is that they have density. This is simply the amount of matter in a given space for that substance. Another way of saying this is that the density is the amount of matter per unit volume, or in equation form: density = mass/volume.

Pressure is another important characteristic of fluids. This is the force exerted over a given area. There are many different units that can be used to express pressure, like pounds per square inch (psi), millimeters of mercury (mmHg) and Newton per meter squared (N/m^2). It all depends on the force and the area you're measuring.

In a liquid, the pressure comes from the weight of the fluid and the weight of the air above it, which we call the atmosphere. You feel this pressure when you swim under water. The deeper you go, the more pressure you feel. This is because there is more weight pressing down on you from above as you increase your depth.

The same principle is true for atmospheric gases. The lower in elevation you are, the more weight and pressure you experience. But even though these gases are constantly putting pressure on you, you don't feel it because your body is the same pressure as the surrounding air. Fish experience the same thing underwater - their bodies are the same pressure as the surrounding fluid, so they don't notice the constant pressure the water exerts on them.

Buoyancy is another common characteristic of fluids. This is the upward force from a fluid, which is usually felt by some object in that fluid. You float in a pool because of buoyancy, and a hot air balloon rises into the sky - also because of buoyancy.

Buoyancy in a fluid comes from the pressure of the fluid itself. An object submerged in a fluid experiences pressure from all sides. But since the pressure from below is greater (because that part of the object is deeper), it creates an unbalanced force on the object and it's pushed upward, lifting it in the fluid and against the force of gravity.

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