Fluid Statics
75 What Is a Fluid?
Learning Objectives
-
- State the common phases of matter.
- Explain the physical characteristics of solids, liquids, and gases.
- Describe the arrangement of atoms in solids, liquids, and gases.
Matter most commonly exists as a solid, liquid, or gas; these are the three common states of matter. Solids have a definite shape and volume. Liquids maintain a definite volume but take the shape of their container. Gases have neither definite shape nor volume and expand to fill their container. (See Figure 75.1.) Liquids and gases are classified as fluids because they yield to shearing forces, unlike solids which resist such deformation. The extent to which fluids flow in response to these forces depends on a property called viscosity, which we’ll explore further in Viscosity and Laminar Flow: Poiseuille’s Law.
In solids, atoms are tightly packed in a regular structure. They vibrate but cannot easily move past one another. The forces between atoms act like stiff springs, resisting compression and shearing forces. This rigidity explains why solids maintain their shape and volume.
Making Connections: Submicroscopic Explanation of Solids and Liquids
The atomic and molecular structures of solids and liquids explain their macroscopic behaviors. This approach—linking submicroscopic structure to physical behavior—is a recurring theme in this course and is explored further in Conservation of Momentum.
By contrast, in liquids, atoms are still in close contact but are free to slide past one another. Liquids resist compression like solids, but they do not resist shearing forces and thus can flow. Liquids stay in an open container due to interatomic attraction, unless the container leaks. In gases, atoms are spaced far apart and interact weakly. They move freely and rapidly, colliding with one another and the container walls. Because of the large spacing, gases are easily compressed and will escape from an open container. Both liquids and gases are considered fluids, but gases are distinct in being highly compressible.
Interactive Exploration: States of Matter
The following interactive simulation allows you to visualize what is happening at the atomic level inside solids, liquids, and gases. Instead of simply reading about how particles behave, you can observe their motion, change the temperature, and see how these microscopic changes produce the macroscopic properties we observe every day. Spend a few minutes exploring the simulation before continuing. There is no single correct path; experiment with the controls and pay attention to how the particles respond.
Guided Exploration
As you interact with the simulation, try to answer the following questions:
- Select each state of matter (solid, liquid, and gas). How does the arrangement of the particles differ in each case?
- Compare the motion of the particles. In which state do particles mainly vibrate in place? In which states are they free to move?
- Increase the temperature. What happens to the speed of the particles? How does this relate to the concept of thermal energy?
- Lower the temperature. Can you observe changes between the different states of matter? Describe what happens to the particles during each transition.
- Which state has the greatest spacing between particles? Which is the most difficult to compress? Explain your observations using the particle model.
- Based on what you observed, explain why liquids and gases are considered fluids while solids are not.
After completing the exploration, return to the text and compare your observations with the explanations presented in this section.
Healthcare Connection
The human body relies on specialized fluids to keep its moving parts working smoothly. Synovial fluid, a thick, slippery liquid found inside joints like the knee and shoulder, behaves much like the fluids described in this section: it flows to fill the space between the bones and cartilage, lubricating the joint and dramatically reducing the friction that would otherwise occur as the bones move against each other. In conditions such as osteoarthritis, this fluid can break down or lose its normal properties, which is part of why the joint becomes stiffer and more painful to move.
Section Summary
- A fluid is a substance that flows and yields to shearing forces. Both liquids and gases are fluids.
- Solids resist deformation because their atoms are fixed in a lattice structure and can only vibrate.
- Liquids flow and take the shape of their container, though they resist compression due to close atomic spacing.
- Gases flow and are easily compressed due to large spacing between atoms and weak intermolecular forces.
Conceptual Questions
- What physical characteristic distinguishes a fluid from a solid?
- Which of the following substances are fluids at room temperature: air, mercury, water, glass?
- Why are gases easier to compress than liquids and solids?
- How do gases differ from liquids?
Glossary
- fluids
- liquids and gases; a fluid is a state of matter that yields to shearing forces
liquids and gases; a fluid is a state of matter that yields to shearing forces
