Lecture notes in Medical and Biological Physics

Abstract

Matter most commonly exists as a solid, liquid, or gas; these states are known as the three common phases of matter. Solids have a de nite shape and a speci c volume, liquids have a de nite volume but their shape changes depending on the container in which they are held, and gases have neither a de nite shape nor a speci c volume as their molecules move to ll the container in which they are held. Liquids and gases are considered to be uids because they yield to shearing forces, whereas solids resist them. Note that the extent to which uids yield to shearing forces (and hence ow easily and quickly) depends on a quantity called the viscosity. Liquids deform easily when stressed and do not spring back to their original shape once the force is removed because the atoms are free to slide about and change neighbors that is, they ow (so they are a type of uid) with the molecules held together by their mutual attraction. Because the atoms are closely packed, liquids, like solids, resist compression.