密度

Density is a physical property defined as the mass of an object per unit volume. It is calculated by dividing an object's total mass by its total volume. The SI unit for density is kg/m³, while the CGS unit is g/cm³. Density helps determine whether an object will float or sink in a fluid, connecting to density and archimedes principle.

Objects sink or float based on their density relative to the fluid. The stone sinks because its density is greater than water's density, causing it to be denser and heavier per unit volume. The wooden log floats because its density is lower than water's density. This principle applies regardless of the objects' masses.

A homogeneous substance has constant density throughout its volume, such as a solid iron bar where any sample has the same density as the average. A heterogeneous substance has varying density, like Swiss cheese containing both solid material and gas-filled voids. For heterogeneous materials, local density describes the density at specific locations.

Gases have densities that vary considerably with temperature because they expand and contract freely. Liquids, however, have densities that vary little with temperature, so their densities are often treated as constant and equal to their average density. This difference reflects the structural differences between gases and liquids.

Gases have much lower densities than liquids and solids because their atoms are separated by large amounts of empty space. In contrast, the atoms in liquids and solids are in close contact, making their densities roughly comparable. This structural difference explains the significant density variation between phases of matter.

Specific gravity is a dimensionless quantity defined as the ratio of a material's density to water's density at 4.0°C and one atmosphere of pressure, which is 1000 kg/m³. It provides a convenient, unit-free way to compare densities of different substances without worrying about unit conversions, making material comparisons straightforward.

No, objects of the same material with different masses have the same density because density is a material property independent of mass. However, objects made of different materials can have the same mass but different densities. For example, a small stone and a large piece of wood might weigh the same but have different densities.