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.
Viscosity is yet another important property of fluids. Remember how I said that all fluids flow? Well, viscosity describes the resistance of a fluid to flow. Thicker fluids are more viscous than thinner ones because they don't flow as easily. For example, honey is a very viscous fluid because it flows very slowly, whereas water is not as viscous because it flows more easily.
The viscosity of a fluid changes with temperature, and this is where things get really interesting. Increasing the temperature decreases the viscosity of liquids (making them flow more easily), but increases the viscosity of gases (making them flow less easily). This is because the particles in a gas are already moving around with a lot of space between them, making it easy for them to flow.
When the temperature of the gas is increased, the molecules move around even more, but this also means they bump into each other more, which makes it more difficult for the particles to flow. In contrast, the particles of a liquid are already pretty tightly packed together, but when the temperature is increased, the molecules spread out a bit and are able to move around and flow more easily.
One property of liquids that is quite special is surface tension. This forms a thin layer where a liquid and another fluid meet. A thin film forms on the surface of the liquid (hence the name surface tension), and you can definitely try this one at home! Take a penny and an eyedropper and slowly put one drop of water at a time on top of the penny. You'll notice that the water builds up in a dome instead of spilling right off of the penny, and this is because of the surface tension of the water against the air. This same property allows for other fascinating events, such as insects walking across a pond or a leaf floating on top of a lake.
Like the objects we find on a tropical beach, most things on Earth can be easily categorized into one of three phases of matter: solid, liquid or gas. While each phase has unique properties, they also share some properties. This is how we can categorize both liquids and gases as fluids because they are both materials that flow.
Fluids have common properties that they share, such as compressibility, density, pressure, buoyancy and viscosity. However, just because fluids share similar characteristics doesn't mean the specifics of those characteristics are the same for each material. For example, different fluids will have different densities and viscosities, and liquids and gases respond differently to changes in temperature. Similarly, gases fill their entire container, which puts pressure on the container itself, but liquids can only fill their container under their surface.
Once you've finished reviewing this online video lesson and transcript, you might:
- Recall the three forms of matter and determine what characterizes a fluid
- Compare and contrast the properties of fluids