9 1 Fluids Under Pressure Fluids always move

9. 1 Fluids Under Pressure • Fluids always move from high pressure to low pressure w Fluids under pressure and compressed gases are used for a variety of everyday tasks • Air molecules pulled by gravity = atmospheric pressure w Air pressure increases as altitude decreases § The more air there is above, the more it compresses the air molecules below § Air pressure is lower at high altitudes § When humans change altitude, our bodies try to equalize the pressure differences by having our ears “pop” See pages 314 - 316 (c) Mc. Graw Hill Ryerson 2007

Pressure Differences • Fluids will always attempt to move from high pressure to low pressure w When we drink with a straw, we first remove air from the closed straw, which lowers the pressure inside. The atmosphere, having a higher pressure, then tries to get into the straw, and pushes the fluid up and out of the ny way to try to get into the straw! r ma d fo cs, e s u a is hydrauli. e d i e m ing tal tools a d s u l s i c w Th poses, in and den ts e pur k c o rr wate See page 316 (c) Mc. Graw Hill Ryerson 2007

Liquid Pressure and Buoyancy • The pressure of fluids increases with depth w When you dive deep, you can feel more pressure w In Earth’s atmosphere and oceans, pressure also increases with depth (air behaves like as fluid as well!) § Sea level is about the “deepest” the atmosphere gets • Sea level = 1 atmosphere = 101. 3 k. Pa (kilopascals) • Top of Mount Everest = 1/3 atm = 330 k. Pa • From sea level, every 10 m in water depth = +1 atm • A submarine at a depth of 500 m has the equivalent of a 500 000 kg object resting on every square metre! • Buoyancy refers to low density floating on high density w The amount a fluid allows objects to float = buoyant force See page 317 (c) Mc. Graw Hill Ryerson 2007

Rising and Sinking • Many vehicles, including submarines, airplanes and space shuttles all must consider pressure changes w Submarine designers must § ensure the sub is safe § design a way to change depths w When water is pumped in, density increases = sink w When water is pumped out, density decreases = rise w Compressed air, kept onboard, pushes out water • Convection refers to the movement of low density over top of high density fluids Take the Section 9. 1 Quiz See page 318 (c) Mc. Graw Hill Ryerson 2007