Chapter 14 Gasses & Plasmas Weight of air is small but not negligible. For example, weight of air in this room is comparable to your weight (volume of room about 1000x your volume).

Figure 13.5

Once believed that you had to move water at a slow drop in gradient to move it.

Pressure does not depend on the amount of liquid Volume is not the key- depth is

The force exerted by a fluid on a smooth surface is always at right angles to the surface

18-Nov-15 Physics 1 (Garcia) SJSU Pressure in Liquids High Medium Low High Medium Low

18-Nov-15 Physics 1 (Garcia) SJSU Pressure Pressure is defined as Pressure = Metric unit of pressure is Pascal. 1 Pascal = 1 Newtons per square meter Atmospheric pressure is about 100,000 Pascals ( Force ) ( Area )

18-Nov-15 Physics 1 (Garcia) SJSU Demo: Bed of Nails

Sample Problem Note: Atmospheric pressure is about 100,000 Pascals so much more than pressure due to the “gold” brick.

18-Nov-15 Physics 1 (Garcia) SJSU Check Yourself In which case is the pressure greatest? A B A B

18-Nov-15 Physics 1 (Garcia) SJSU Check Yourself In which case is the pressure greatest? A B A B

Crush the Rail Tanker Car

Buoyancy Since pressure depends on depth, a submerged object has more force due to pressure below it than above it. Net effect is to have a net upward force, which we call buoyancy. Buoyancy Weight If weight exceeds buoyancy force then object sinks, otherwise it floats. Pressure

18-Nov-15 Physics 1 (Garcia) SJSU Buoyancy & Depth For a fully submerged object the buoyancy force does not depend on depth, even though pressure depends on depth Buoyancy

Only in the special case of floating does the buoyant force acting on an object equal the object’s weight. A floating object displaces a weight of fluid equal to its own weight. Ice cubes in water?

Figure To control how high they float in the water Crocs swallow stones Fish use air bladders

Figure 13.17

18-Nov-15 Physics 1 (Garcia) SJSU Check Yourself Did the designers of this “water bridge” have to account for the weight of ships or just the water?

Figure 13.20

18-Nov-15 Physics 1 (Garcia) SJSU Check Yourself ? 50 N 10 N Block 10 N Water 40 N Water A floating object displaces a weight of fluid equal to its own weight.

Archimedes’ Principle Weight of liquid displaced by floating or submerged object equals the buoyant force on the object.

Floating & Liquid Density Floating in Great Salt Lake, Utah is easy because the lake water is dense due to high concentration of salt.

Pascal’s Principle When force applied to a confined fluid, an increase in pressure is transmitted equally to all parts of the fluid A liquid completely filling a bottle exerts pressure in all directions

Figure 13.23

Surface Tension adhesion- attracted to unlike Cohesion- attracted to like contractive tendency of the surface of liquids

Surface tension causes

Buoyancy

18-Nov-15 Physics 1 (Garcia) SJSU Buoyancy in Air Objects can float in air, just as they float in water, if the objects’ average density is less than the density of air. Hot Air Balloon Helium-filled Blimp

Zeppelins Hindenburg was 10 times longer than today’s blimps

Atmosphere The thickness is determined by kinetic energy From sun) -tends to spread the molecules apart; tend to fly away: if disappeared- molecules moved too slowly our “atmosphere” would be a liquid or solid layer gravity, which tends to hold molecules near the Earth. shut off= dissipate and disappear. height of the atmosphere gets thinner and thins out to emptiness in interplanetary space.

Atmosphere We live at the bottom of an ocean of fluid—the fluid is air & “ocean” is the atmosphere.

18-Nov-15 Physics 1 (Garcia) SJSU Demo: Stop the Funnel Water will not enter an air-tight container. H A Weight A A A Block exit hole

18-Nov-15 Physics 1 (Garcia) SJSU Atmospheric Pressure

Demo: Magdeburg Hemispheres A A A A L A L - Atmospheric pressure - Low pressure

Barometer Any device that measures atmospheric pressure. A simple mercury barometer: The vertical height of the mercury column remains constant even when the tube is tilted, unless the top of the tube is less than 76 centimeters above the level in the dish—in which case the mercury completely fills the tube.

Demo: Drinking Straw L AA Weight

18-Nov-15 Physics 1 (Garcia) SJSU Prairie Dog Vacuum Prairie dogs captured by giant vacuum truck with a padded bin. L A Force

There is a 10.3-meter limit on the height that water can be lifted with vacuum pumps.  At sea level, however strong your lungs may be, or whatever device you use to make a vacuum in the straw, the water cannot be pushed up by the atmosphere higher than 10.3 m.

Boyle’s Law Density of a gas increases as it is compressed (volume decreased) As the density of a gas increases, the pressure in the gas also increases. P 1 V 1 = P 2 V 2 Compress the gas by pushing in the syringe. Dial gauge shows increase in pressure. Syringe Gauge Tank

Boyle’s Law A flat tire registers zero pressure on the gauge, but a pressure of about one atmosphere exists there. Gauges read “gauge” pressure— pressure greater than atmospheric pressure. Double volume & ½ pressure Dec speed of partaicles Dec. temp Decrease volume Density and pressure are increased P 1 V 1 = P 2 V 2 applies to ideal gases.

Archimedes’ principle holds for air just as it does for water: An object surrounded by air is buoyed up by a force equal to the weight of the air displaced.

Bernoulli’s principle

Blow the Roof If wind blows hard enough the low pressure above can create a large enough force to lift the roof off. New Orlean’s Superdome after hurricane Katrina L A Even a small pressure difference over a large roof area can produce a large upward “lifting” force

18-Nov-15 Physics 1 (Garcia) SJSU Check Yourself Wind blowing over the ocean causes waves to build due to Bernoulli’s principle. Where is the pressure lowered?

Airfoil - shape of wing Lift- upward force which results when the pressure on upper surface is less Drag- opposing force to forward motion Thrust - force which pushes the plane forward provided by plane’s propeller/jet

Airplane Wing L A Wing LIFT FORCE

Curving may be increased by threads or fuzz, which help to drag a thin layer of air with the ball and to produce further crowding of streamlines on one side.

Bernoulli’s principle How does this relate to fireplaces? On a windy day, waves in a lake are higher than normal. Why?

Plasma a plasma (different from a gas) readily conducts electric current, it absorbs certain kinds of radiation that pass unhindered through a gas, it can be shaped, molded, and moved by electric and magnetic fields.

Plasma Power MHD power, the magnetohydrodynamic interaction between a plasma and a magnetic field. Low-pollution MHD power is in operation at a few places in the world already. operate at high temperatures without moving partsmoving parts Fusion power may not only make electrical energy abundant, but it may also provide the energy and means to recycle and even synthesize elements

Pascal’s Principle Explains hydraulic systems Pressure is exerted equally throughout a closed container Hydraulic systems multiply force(over a greater distance)

Bernoulli’s Principle Pressure exerted on a moving steam of fluid is less than the pressure of the surrounding fluid Explains Flight

Archimedes’ Principle (Sinks) Buoyant force on an object is equal to the weight of the fluid displaced If an object floats- the volume of displaced water = volume of the portion of the object that is submerged.