27: Gases and Incompressible Liquids

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Topics covered: Concepts covered in this lecture include gases and incompressible liquids, Pascal's Principle, hydrostatic and barometric pressure.

Instructor/speaker: Prof. Walter Lewin

Date recorded: November 15, 1999

Video Index

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  • Pressure and Pascal's Principle
    Pressure is a scalar. Pascal's Principle is explained. In hydraulic jacks, a small force is applied to move a large mass a small distance.

  • Gravity and Hydrostatic Pressure
    Because of gravity, pressure increases with depth in a fluid. This is called hydrostatic pressure.

  • Compressibility of Gases vs. Liquids
    Unlike liquids, gases are compressible, so they cushion impacts. Liquids do not act like cushions. This is demonstrated in a very dramatic way by firing a bullet in a sealed can filled with air and one filled to the brim with liquid.

  • Pressure Difference and a Column of Liquid
    The pressure difference between the bottom and top of a vessel of liquid depends only on the height and density of the liquid, not on the area or weight of the column. This is rather non-intuitive.

  • Atmospheric or Barometric Pressure
    The pressure at sea level due to the air above determines the atmospheric or barometric pressure. It can be measured by raising (sucking up) a column of liquid from an open reservoir with a tube sealed at the end where we pump the air out. For every 10 meters depth in water, the hydrostatic pressure increases by about one atmosphere.

  • Submarines and Overpressure
    Cornelis Drebbel is credited with inventing the first submarine operating at a depth up to 5 meters. At this depth, the hydrostatic pressure is about half an atmosphere. A sealed paint can was evacuated to demonstrate the enormous forces acting upon it with an over pressure of about one atmosphere. The can imploded.

  • Overpressure in our Lungs
    The lung capacity, our ability to overcome hydrostatic pressure, is measured with a manometer. This is related to how deep snorkeling works, and why scuba-divers use pressurized air tanks. Professor Lewin demonstrates that by blowing on a manometer, or by sucking on it, we can raise or lower a column of water by about 1 meter (0.1 atmosphere). So why then was he able to suck fluid up a straw of several meters long?

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