11-14-08LectPHY2048

11-14-08LectPHY2048 - Fluids In solids the atoms of the...

Info iconThis preview shows pages 1–7. Sign up to view the full content.

View Full Document Right Arrow Icon
Fluids In solids the atoms of the material have rigid bonds with neighboring atoms. propagate in all directions These can be crystalline as shown here or amorphous , (having little crystalline order). But in either case the neighbors for each atom are fixed . These support a shear stress meaning that if we glue the bottom side of the material to the floor and push laterally on the top end the material resists with a restoring force. Our push elastically deforms bonds making some longer and others shorter but because they don’t break, when we stop pushing the bonds between atoms return to their equilibrium lengths and the solid springs back to its original shape .
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
A fluid in contrast has no fixed nearest neighbors . The atoms or molecules making up a fluid are completely promiscuous, frequently changing partners . We can classify fluids as liquids or gasses with the distinction that in a liquid the forces between the molecules keeps them “touching” (but free to slide past each other) while for a gas the intermolecular forces are so weak the molecules spontaneously separate. Neither of these can support a shear stress. small force F This means that fluids will adjust their shape to conform to the shape of any container in which they are placed.
Background image of page 2
If we try to compress a a gas in a container with a piston the volume will change with the applied force. The gas is compressible . piston For a liquid , because its molecules are already in “contact”, it’s volume will change little, so to reasonable approximation (and certainly compared to gasses) liquids can be treated as incompressible . The inability of fluids to support a shear stress makes it difficult to talk about the force on a fluid. If your hand pushes on a fluid it yields, flowing around you. This shape changing nature and ability to flow makes it difficult to to talk of a particular chunk or mass of fluid. If two chunks of fluid meet they merge and mix making them no longer distinguishable. F
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Parameters that are more useful in (quantitatively) describing fluids are the density and pressure . For a uniform material the density is defined as its total mass divided by its total volume m V ρ= (a scalar) For a non-uniform material we can still define the local density as that of the mass Δ m per very small volume element Δ V m (x) V Δ Δ Δ V Δ m ρ( x) x
Background image of page 4
Table 14-1 lists some densities over a very broad range. Interstellar space Best lab vacuum Air: 20 o C, 1 atm. pressure 20 o C, 50 atm. Ice Water: 20 o C, 1 atm. 20 o C, 50 atm. Seawater: 20 o C, 1 atm. Iron Mercury (element) Earth (crust) Sun core (average) Neutron star (core) Material Density (kg/m 3 ) 10 -20 10 -17 1.21 60.5 0.917x10 3 0.998x10 3 1.000x10 3 1.060x10 3 7.9x10 3 13.6x10 3 2.8x10 3 1.6x10 5 10 18
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
To explore forces in a fluid we resort to a type of gauge described in the text consisting of piston cylinder arrangement evacuated behind the piston but containing a calibrated spring.
Background image of page 6
Image of page 7
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 08/25/2011 for the course PHY 2048 taught by Professor Field during the Fall '08 term at University of Florida.

Page1 / 31

11-14-08LectPHY2048 - Fluids In solids the atoms of the...

This preview shows document pages 1 - 7. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online