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Unformatted text preview: Physics 53 Fluids An optimist is a fellow who thinks a houseFy is looking for a way out. G. J. Nathan Phases of matter Until now we have analyzed mostly systems in which the particles have a fxed average spatial relation to each other, i.e., are bound. This kind oF system is a solid . IF the particles are not bound, but still are on average close enough together to interact continuously with nearest neighbors, we have a liquid . When the particles are on average Far apart and only interact occasionally when they collide with each other, we have a gas . These situations defne the three commonly occurring phases oF matter. In the liquid and gas phases, the particles move about (ow) until the system takes the shape oF the solid container surrounding them. Such systems are called uids . They are the object oF our study here. or the most part we will discuss liquids frst, and later discuss gases in the context oF thermal physics. Macroscopic variables It would be hopeless to try to describe a uid by listing the detailed behavior oF every particle, because there are so many. We must seek a simpler description, in which the main Focus is on average behavior oF the particles. The variables describing the average (or bulk) behavior are called macroscopic variables . They are distinguished From microscopic variables , which describe the detailed behavior oF individual particles. Ordinary measuring devices tell us only the values oF the macroscopic variables. or uids, the most useFul macroscopic variables are the volume occupied by the uid, the pressure in the uid, and the (average) velocity oF the particles in a small region oF the uid (which nevertheless contains a very large number oF particles). Since pressure and velocity generally vary From place to place in the uid, they are elds ; pressure is a scalar feld and velocity is a vector feld. PHY 53 1 luids Stresses in a fuid Earlier we discussed elastic properties of solids in terms of stresses and strains. There are two important differences when Fuids are considered: luids do not resist shear stresses, but simply Fow if forces parallel to the surface are applied. Indeed, a Fuid is often dened as a system that cannot offer resistance to shear stresses. Compressive or tensile stresses may result in a change in volume of the Fuid, although for many liquids (such as water) the change in volume may be quite small. A tensile stress at the Fuid surface, tending to pull the particles at the surface back into the Fuid, is called surFace tension . The compressive stress within the Fuid, pushing the particles outward toward the surface, is called the pressure . The shape of a volume of liquid not constrained by walls of a container a water drop, for example is determined by equilibrium between these two stresses....
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