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Pressure at elevation (from vacuum to liquids)

Pressure is the force exerted by a gas or a liquid. We live at the bottom of a vast ocean of air, the atmosphere. The weight of air above our heads presses on us with a pressure of about 14 pounds on every square inch of our bodies.

This pressure is less if we go to the top of a mountain because there is less air above our heads. Higher up in the atmosphere, where jet aircraft fly, the pressure is very low and insufficient for men to breathe. For this reason, all high-flying aircraft have pressurized cabins, in which the pressure of the air is kept the same as that near the ground. Air pressure is measured with a barometer (See Atmospheric pressure barometer – for kids’ facts). Because changes in the weather occur when the air pressure varies, barometers are used in weather forecasting.

multiplies forces fourfold

The barograph - Pressure at elevation

Pressure at elevation...

A force of one pound moving through 12 inches does the same work as a force of four pounds moving through three inches, so the jack in this case multiplies forces fourfold

Pressure at elevation...

The barograph records the pressure of the air

Pressure exists in the sea as well as in the atmosphere. At the deepest point in the ocean, the water above exerts a pressure, in all directions, of eight tons per square inch. Men can travel to this depth in a bathysphere, a craft with strong steel walls that can resist this pressure and enable the crew to breathe air at normal pressure. But divers have to breathe air at the same pressure as the water pressure at the depth they have reached. At such high pressures, nitrogen gas in the air dissolves in the blood and causes ill effects. As a result, men have to breathe a special mixture of gases to dive to depths of more than 300 feet.

Many other gases dissolve in liquids at high pressure. Fizzy drinks contain carbon dioxide gas dissolved at high pressure. When you take the cap off the bottle, the drink bubbles as the gas leaves the solution.

The bathysphere

Pressure is greater at the bottom of a full tank

Pressure at elevation...

The bathysphere has strong steel walls which resist the terrific pressures encountered deep under the sea. Lights illuminate the darkness on the sea bed

Pressure at elevation...

Pressure is greater at the bottom of a full tank than near the top.

Hydraulic machines work by liquid pressure. When you put your foot on the brake pedal in a car, it pushes a piston that creates high pressure in a pipe of oily fluid. At the other end of the pipe, the pressure of the fluid operates the brakes. Hydraulics can be used to operate powerful mechanisms, such as the undercarriage of an aircraft or a mechanical digger, or a jack.

A vacuum is a complete lack of air pressure. Vacuums are used in cathode-ray tubes in television sets, and in vacuum bottles for keeping hot liquids hot, or cold liquids cold. High pressure is necessary to carry out many manufacturing processes, including the refining of petroleum and the making of plastics. Tires contain air at high pressure.

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