8.1 Internal Structure and Density

Learning objectives

8.1A—Describe the properties of a fluidDescribe the properties of a fluid.• Distinguishing properties of solids, liquids, and gases stem from the varying interactions between atoms and molecules.• A fluid is a substance that has no fixed shape.• Fluids can be characterized by their density. Density is defined as a ratio of mass to volume. Relevant equation:• An ideal fluid is incompressible and has no viscosity. TOPIC 8.1 Internal Structure and Density

Identify a fluid by shape behavior

A fluid is a substance with no fixed shape, so it flows and takes the shape allowed by its container. Both liquids and gases are fluids; a solid retains a fixed shape.

Connect particles to macroscopic properties

State	Particle interaction and arrangement	Shape behavior
Solid	Strong enough to maintain an organized structure	Fixed shape
Liquid	Particles remain close but rearrange	No fixed shape; nearly fixed volume
Gas	Particles are widely separated and interact weakly except during collisions	No fixed shape; expands to fill available volume

Calculate mass per volume

$\rho=\frac{m}{V}$

Example: a fluid sample has mass $1.20\,\text{kg}$ and volume $5.00\times10^{-4}\,\text{m}^3$ .

$\rho=\dfrac{1.20}{5.00\times10^{-4}}=2.40\times10^3\,\text{kg}\,\text{m}^{-3}$ .

Density is mass per unit volume; doubling both mass and volume leaves the density unchanged.

State the ideal-fluid assumptions

An ideal fluid is modeled as incompressible, so its density does not change, and as having no viscosity, so there is no internal resistance to flow.

Use the model as an approximation

Ideal-fluid assumptions simplify later pressure and flow models. They are not claims that every real fluid is perfectly incompressible or frictionless; check whether the model is appropriate for the situation.

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