9.1 Fluids Under Pressure Fluids always move from high pressure to low pressure Fluids under pressure and compressed gases are used for a variety of everyday tasks Air molecules pulled by gravity = atmospheric pressure 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) McGraw Hill Ryerson 2007

Pressure Differences Fluids will always attempt to move from high pressure to low pressure 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 way to try to get into the straw! This same idea is used for many purposes, including hydraulics, water rockets and dental tools. See page 316 (c) McGraw Hill Ryerson 2007

Liquid Pressure and Buoyancy The pressure of fluids increases with depth When you dive deep, you can feel more pressure 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 kPa (kilopascals) Top of Mount Everest = 1/3 atm = 330 kPa 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 The amount a fluid allows objects to float = buoyant force See page 317 (c) McGraw Hill Ryerson 2007

Rising and Sinking Many vehicles, including submarines, airplanes and space shuttles all must consider pressure changes Submarine designers must ensure the sub is safe design a way to change depths When water is pumped in, density increases = sink When water is pumped out, density decreases = rise Compressed air, kept onboard, pushes out water Convection refers to the movement of low density over top of high density fluids See page 318 Take the Section 9.1 Quiz (c) McGraw Hill Ryerson 2007