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Under Pressure

Under Pressure. 9.4 Effects of External Pressure on Fluids. Compression is Pressure. We know that we can force a certain volume of gas into a smaller space by putting pressure on it Pressure – force applied to a unit of surface area

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Under Pressure

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  1. Under Pressure 9.4 Effects of External Pressure on Fluids

  2. Compression is Pressure • We know that we can force a certain volume of gas into a smaller space by putting pressure on it • Pressure – force applied to a unit of surface area • We can increase or decrease pressure depending on whether the force is applied to a large or small surface area • Pressure (p) = force/unit area • P = F / A • Measured in pascals (Pa), where 1 Pa = 1 N/m2

  3. Air and Water Pressure • Fluids also exert pressure • Earth’s atmosphere – 160km thick • Newspaper Example • Gravity pulls on every particle of the atmosphere – exerting pressure • Atmospheric pressure (air pressure) – force exerted by the atmosphere on a unit of surface area

  4. Air and Water Pressure • A newspaper sitting on a table exerts about 1N/m2 of force • The atmospheric pressure exerted on the newspaper is about 100,000 times greater • Flip the cup • How can we use atmospheric pressure to explain what just happened?

  5. Air and Water Pressure • Water also exerts pressure • When swimming underwater, the water exerts pressure over your entire body • Since water is much heavier than air, it exerts more pressure than air does • This explains why divers can only go so deep before it becomes dangerous • Also explains why the hulls of submarines are so thick

  6. Pressure and Pascal’s Law • Blaise Pascal (1623-1662) – French mathematician and physicist • Found that when fluids in a container are put under pressure, they push in all directions • Remember the balloon challenge (you could never have done it…..) • Pascal’s Law – a force is applied to a fluid is distributed equally through all parts of the fluid

  7. Pressure and Pascal’s Law • Connecting two different sized syringes • Plunger on smaller syringe moves farther than plunger on large syringe • Smaller plunger is easier to move as well • Remember – we did this in class

  8. Applying Pascal’s Law • When fluid in the small chamber is pushed into the larger one, it is spread throughout a larger volume • That’s the reason the smaller plunger moves much more • However, the force applied by the small plunger is transferred to every part of the fluid equally • Since the larger piston has a greater surface area, the force that the larger piston can apply is much greater than the force applied to the smaller piston

  9. Applying Pascal’s Law • Hydraulic brakes and heavy machinery rely on Pascal’s Law • Using fluid’s ability to transfer force, we can control the amount of force applied, as well as the distances moved by the parts of a mechanism • Remember liquids are virtually uncompressible • When you apply force to one part of a hydraulic system, the force is immediately transferred to the other parts • Gasses on the other hand are used when a cushioning effect is desired (e.g., bus doors)

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