Name Block Date Ch. 20 Gases Notes

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1 Name Block Date Ch. 20 Gases Notes Mrs. Peck Objectives: 1. Explain why the molecules in Earth s atmosphere neither escape nor settle to the ground Describe the source of atmospheric pressure Describe the effect of atmospheric pressure on everyday life (drinking from a straw) 4. Explain how humans breathe 5. Describe the relationship between pressure & density for a given amount of a gas at a constant temperatures. 6. Explain Archimedes principle 7. Explain what determines whether an object will float in air Describe relationship between the speed of a fluid at any point and the pressure at that point, for steady flow. 9. Describe some applications of Bernoulli s principle Explain how flight occurs using Bernoulli s principle & the airfoil shape. Fluid- anything that flows and takes the shape of its container liquid or gas gas-phase in wh/ molecules move about at higher rates, collide, & break away from each other primary difference b/w liquid& gas: is distance b/w molecules due to the E that the molecules have. LIQUID GAS SIMILAR DIFFERENT Gas molecules energized and kept in continual motion by the other wise they would fall to the ground Earth s keeps gas molecules from flying into outer space Density with increased altitude. There are molecules higher up in the atmosphere. Air is compressed at sea level than at higher altitudes Atmosphere divided into layers: & 99% of atmosphere is below an altitude of Top of atmosphere (stratosphere) is the layer

2 2 Atmospheric pressure is caused by weight of air molecules. Atmospheric pressure greater closer to surface of earth (sea level) Atmospheric pressure decrease as altitude increases At sea level: 1m 3 air = 1.2 kg = 12N volume mass weight Pressure of atmosphere is not uniform Variations at any one locality occur because of & Barometer- an instrument used for measuring the pressure of the atmosphere fill a tube with a liquid and invert it into a dish of the same liquid, atmospheric pressure pushes onto the surface of the liquid in the dish and in turn pushes the liquid up into the tube. The weight of the liquid in tube exerts the same pressure as the atmosphere outside & the barometer is balanced. Atmospheric pressure can push mercury up to 76 cm in height water up to 10.3 m in height Drinking through a straw: When you suck on a straw, you the air pressure in your mouth & straw that is placed in the drink. Atmospheric pressure on liquid s surface pushes the liquid up into the reduced pressure region. The liquid is not up: it is up by the pressure of the atmosphere. It is to drink with a straw in a deep mine shaft bcs of air pressure. It is to drink with a straw on top of a mountain bcs of air pressure. Straw into liquid...put finger on top of straw and lift straw out of liquid:

3 Breathing: Inhaling Exhaling 3 Striking a wooden stick covered by 2 sheets of newspaper: aneroid barometer- an instrument used to measure atmospheric pressure: based on the movement of the lid of a metal box uses a small sealed metal box that has lower air pressure inside that bends as air pressure outside(atmospheric pressure) changes. Can be used as an altimeter to indicate changes in elevation. Pressure is directly proportional to density of air Pressure is indirectly proportional to volume of air if double pressure: density if double pressure: volume if double volume: pressure if halve volume: density is Boyle s Law- for a constant number of molecules of gas at constant temperature, the product of pressure and volume is a constant (does not change) P1V1 = P2V2 initial final

4 Archimedes principle: An immersed obj. is buoyed up by a force equal to the weight of the volume of fluid it displaces. 4 *obj. surrounded by air is buoyed up by a force equal to the weight of the vol. of air displaced. *there is greater force on bottom of an object than against the top of object Fnet is upward *horizontal forces are = at given depth...cancel out Fnet is Fb therefore no sideways movement buoyant force does not change with depth or altitude in atmosphere Fb-the difference in the upward and downward forces is the buoyant force as an object increases it s depth, the difference between the upward and downward forces on the object is the same (buoyant force) at any given depth. buoyant force for an object does not change with depth or altitude If weight of object is greater than buoyant force it will sink Fg > Fb If weight of object is equal to buoyant force it has neutral buoyancy Fg = Fb If weight of object is less than buoyant force it will float Fg < Fb *which has more buoyant force: semi truck or a helium balloon? it is volume of air displaced that det. buoyant force not the weight of the object

5 5 Bernoulli s principle - pressure in a fluid (gas or liq) decreases as the speed of the fluid increases (only for steady horizontal flow) streamlines: the smooth path of a small region of fluid in steady flow lines are closer together in narrower regions where flow is faster and pressure is decreased lines are farther apart in wider regions where flow is slower and pressure is increased fig steady flow- one small bit of fluid follows along the same path as a bit of fluid in front of it eddy- changing curling paths in turbulent flow of a fluid atmospheric pressure decreases during a hurricane and tornado water flowing through pipe: wide parts it slows down and pressure increases narrow parts is speeds up and pressure decreases bernoulli s principle is a consequence of the conservation of energy water in a fireman s hose or any hosewhen water is in hose, not flowing or flowing slowly...increase pressure...increase diameter when water is in hose flowing fast...decrease pressure...decrease diameter of hose Applications of Bernoulli s Principle lift- the net upward force produced by the difference between upward and downward pressures. When lift equals weight, horizontal flight is possible if air moves faster over a surface the air pressure decreases on top of the surface and the air pressure below the surface is higher: the difference between these pressures produces a net upward force called LIFT Flight: even a small pressure difference multiplied by a large wing area can produce a lifting force Airfoil shape:

6 Applications of Bernoulli s Principle 6 Newton s 3rd law also explains flight. The wing forces air downward (action) which forces the win upward (reaction) In a moving car: place your hand outside window, tip it up slightly so air is forced downward. your hand goes. 1. Hold a piece of paper to your mouth and blow over the top of the paper: what occurs? 2. Firefighter hose: describe how diameter can change and why? 3. Frisbee flight: how does it occur? 4. 2 suspended balls close together: blow a steady flow of air between them. what occurs? why?

7 5. What is drafting? 7 6. High winds will tear a roof off of a building. How? 7. High winds causes an umbrella to invert. How? 8. Curved path of spinning balls. What occurs? 9. Bathroom shower curtain pushed inward during shower. Why?