Presentation on theme: "Atmospheric Stability Hot air and buoyancy. Outline Pressure in fluids –Pascal’s principle Buoyancy –Archimedes’ principle –Density and Temperature."— Presentation transcript:
Outline Pressure in fluids –Pascal’s principle Buoyancy –Archimedes’ principle –Density and Temperature –Adiabatic lapse rate and atmospheric stability
Atmospheric Pressure Pressure in a fluid increases with depth because of the weight of the fluid above. –Demonstration (water in column). Air pressure is a result of the weight of air above us. That pressure is strong enough to: –Hold up water in a cup –Hold together evacuated spheres –Crush cans
Pascal’s principle Pressure that is applied at one point in an enclosed fluid is communicated to all other points in the fluid. Currens Pressure Outside = P inside
Atmospheric Stability: Part I A stable atmosphere is one in which the pressure at the same height is the same, everywhere. The sun’s heating and the earth’s cooling make that an unreachable goal. Winds and the jet stream are all evidence that the earth’s atmosphere is seeking horizontal stability, but never finding it. This is the basic reason for all air movements and weather systems.
Hydraulic systems An important application of Pascal’s principle is in hydraulic controls. P P A 5A
Archimedes’ Principle An object submerged in a fluid experiences an upward, “buoyant” force. –Objects which are denser than the fluid SINK. –Objects less dense than the fluid FLOAT. ? Wood Metal
Vertical equilibrium in fluids The pressure below must be greater than the pressure above, to keep the fluid in place. The difference is just equal to the weight of the fluid in between, per area. P P Weight = D*V*g P + W/Area P
Buoyant Force The extra pressure from below produces a “buoyant” force which is just enough to keep each volume of fluid in place. F b = D w x V x g. – e.g. The buoyant force on 10 m 3 of water is: F b = D w x V x g = 1000 kg/m 3 x 10 m 3 x 9.8 m/s 2 F b = 98,000 N. This force balances that of gravity and maintains vertical equilibrium.
Floating or Sinking? I take an object of the same volume V as the water from the previous problem, only having a different density, and submerge it. –The buoyant force would be exactly the same! –But the weight of the object would be different. F net = W – F b. –If F net is positive, gravity wins, and it sinks. –If F net is negative, buoyancy wins, and it floats.
Density Ice is less dense than liquid water. –D ice = 917 kg/m 3 –So, the weight of a 10m 3 chunk of ICE is just W = 917 x 10 x 9.8 = 89,900 N. F net = W – F b = 89,900 – 98,000 = -8,100 N The water pushes the ice up out of the water, until the volume of water displaced corresponds to a buoyant force of 89,900N. Salt water is more dense, so actually, about 20% of an iceberg is above the ocean surface.
Buoyancy in air The density of air is quite low (1.3 kg/m 3 ), so most things sink. What can float in air? –Helium, –Hydrogen, –Hot Air
Gas law In a gas, Density is both temperature and pressure dependent. When pressure is constant (at a constant height) Density is inversely related to temperature. e.g. D 2 = 273/373 (1.3 kg/m 3 ) =.94 kg/m 3 Hot air is less dense, and it rises!