# Chapter 13 States of Matter

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Chapter 13 States of Matter
CO2 is dissolved in liquid. When cork is removed, pressure in bottle drops & results in a display of the states of matter.

13.1 The Fluid States Fluid - any material that flows & offers little resistance to change. Liquids & gases - fluids. Properties of fluids allow airplanes to fly, boats to float, and submarines either to float or submerge. Plasma provides us with northern lights.

Pressure Kinetic-molecular theory explains properties of gases, based on following: Gases are made up of large number of small particles. Particles are in constant motion. They are widely separated & make only elastic collisions. Particles make perfectly elastic collisions with walls of container that holds them.

Pressure = result of collisions of particles with walls of container.
Pressure - force per unit area of a gas. P = F / A. SI unit - pascal, which is 1 N per m2. kPa is used more often. Ex Prob 302 Prac Pr 303

Fluids at Rest - Hydrostatics
Blaise Pascal – 1st to discover that any change in pressure applied to a confined fluid at any point is transmitted undiminished throughout fluid. Discovery known as Pascal's principle. Tube of toothpaste - Pascal's principle.

Pascal's principle-applied in operation of machines that use fluids to multiply forces, as in hydraulic lifts. Hydraulic system, a fluid is confined in two connecting chambers, Figure 13-4. Each chamber has a piston that is free to move.

P1 = F1 / A1 & P2 = F2 / A2 F1 / A1 = F2 / A2 F2 = F1 A2 / A1. A hydraulic lift can increase force exerted, but not work done.

Fluids at Rest - Hydrostatics

Pressure of fluid on horizontal surface is weight per unit area, A, of fluid above surface.
P = Fg/A =Ahg/A = hg Pressure is proportional only to depth of fluid & its density. Shape of container has no effect.

When an object of height l is placed in a fluid, force is exerted on all sides, Figure 13-6.
Forces on four sides are balanced. Forces on top & bottom, however, are given by …

Ftop = Ptop A = hgA Fbottom = Pbottom A = (h + l)Ag. Fbottom - Ftop = (h + l)Ag - hAg = Alg = Vg Upward force - buoyant force. Fbouyant = Vg Buoyant force, Vg, has a magnitude = weight of fluid displaced by immersed object.

This relationship was discovered by Archimedes & is called Archimedes' principle.
An object immersed in a fluid is buoyed up by force = weight of fluid displaced by object.

Hydrostatics Buoyant force does not depend on weight of submerged object, only weight of displaced fluid. A solid cube of Fe, a solid cube of Al, & a hollow cube of Fe, all of same volume, would experience same buoyant force.

If density of object > than that of fluid, upward buoyant force will be < weight of object, object will sink. If density of object = density of fluid, buoyant force & W will be equal. Net force will be zero & object will float.

Wearing a life jacket filled with material of very low density has effect of decreasing body's average density. Ship loaded with cargo rides lower in water than ship with empty cargo hold. Submarines pump water into or out of special chambers to regulate vertical force, causing sub to rise or sink. Ex Prob 307 Prac Pr 309

Fluids in Motion Hydrodynamics
Relationship between velocity & pressure exerted by a moving fluid is described by Bernoulli’s principle: As velocity of a fluid increases, pressure exerted by that fluid decreases. Airplane wings are designed so that top is curved more than bottom.

So, air traveling over top has a greater velocity & less pressure than bottom.
This provides lift. Race cars also use Bernoulli’s principle. Spoilers have a greater curve on bottom. So, air traveling over spoiler has a greater velocity & less pressure on bottom.

Top of spoiler has less velocity & greater downward pressure.
This keeps car from flying off road. Streamlines – to ensure efficiency. If streamlines are diffused, flow is turbulent & Bernoulli’s does not apply. Fig

Fluids in Motion Hydrodynamics

Liquids vs Gases Particles of an ideal liquid are free to move around one another. In real liquids, particles do exert forces of attraction called cohesive forces, which affects behavior of the liquid.

Liquid - definite volume; gas takes volume of its container.
Liquid - incompressible; gas is easily compressed. Liquid - particles are close together; gas particles are spread out.

Surface Tension Surface tension - result of cohesive forces among particles of liquid. Surface of liquid tends to contract to smallest surface area. Adhesion - attractive force that acts between particles of different substances, which are EM in nature.

Capillary action – water rises because adhesive forces between glass & water molecules are stronger than cohesive forces. Molten wax rises in candle wick for this reason.

Evaporation and Condensation
If fast-moving particle is near surface of liquid, it can break through surface layers & escape from liquid. Since there is net downward cohesive force at surface, only more energetic particles escape. Escape of particles - evaporation.

Evaporation cools remaining liquid.
Each time particle with higher than avg KE escapes from liquid, avg KE of remaining particles decreases. Decrease in KE is decrease in temperature.

Liquids such as alcohol & ether evaporate quickly because forces between their molecules are weak.
Liquid that evaporates quickly is called volatile.

outside of glass on hot day soon becomes coated with water.
Water molecules moving randomly in air surrounding glass may strike cold surface. If molecule loses enough energy, cohesive force will be strong enough to prevent particle's escape. This is called condensation.

Air above water contains evaporated water vapor.
If temp is reduced, water vapor condenses around tiny dust particles in air, producing droplets only 0.01 mm in diameter. Cloud of these droplets - fog. Fog often forms when moist air is chilled by cold ground.

Works Cited Physics:Principles and Problems, Glencoe.
Microsoft Clipart

Air Flow on Wing

Down Force

Surface Tension

Condensation