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| Tutorial ID | Title | Tutorial | Video Tutorial | Revision Notes | Revision Questions | |
|---|---|---|---|---|---|---|
| 9.2 | Gas Pressure |
In these revision notes for Gas Pressure, we cover the following key points:
Atmosphere - as the layer that surrounds the Earth - is composed by a mixture of gases, which we call "air". Air is a fluid, and as such, it possesses all properties a fluid has, where one of the most important is the pressure it exerts on objects in contact.
Air pressure is produced when air molecules hit a given surface by a certain force, i.e. it is proportional to the number of air molecules available.
We use an air pressure gauge called "mercury barometer" or simply "barometer" to measure air pressure, a gauge which consists of an open vessel with a closed vertical tube at it middle. If we remove the air from the upper part of the tube, mercury will start to raise up inside the tube, as there is no air above to hamper it from raising up. In this case, mercury acts as a capillary liquid, i.e. as a liquid that rises up at a height h in narrow tubes because of the change in pressure.
In normal atmospheric conditions, the mercury column h in the barometer rises up to 76.0 cm. It corresponds to 101 325 Pa. This value is written as P0. Itcan also be written as 1 atm (atmosphere). Thus, we have:
To facilitate calculations, another unit known as "bar" is often used. The conversion factor between bar and Pa is
Mercury is used as a capillary liquid for practical purposes. If we used in barometers another liquid such as water instead of mercury, we would need a 11 + meter high device.
We use a combination of U-shaped tube and barometer as an equipment to measure gas pressure if the value of atmospheric pressure is known. This equipment is called "open-tube manometer" or "pressure gauge" which consists in a closed deposit at one side, in which there is the gas whose pressure needs to be measured, and an open end at the other side of a U-shaped tube, which is in contact with the air (atmospheric pressure). The separation between these two gases (air and the specific gas to be measured) is ensured by a liquid, such as water, mercury etc.
When air pressure is equal to the gas pressure (both gases push equally the capillary liquid at bottom of the U-shaped tube), the liquid level is the same in both sides of the tube.
If the capillary liquid inside the manometer is mercury, we can write
When gas pressure is greater than atmospheric pressure, the gas pushes the capillary liquid more than air. As a result, there will be a disparity h in the liquid level in both sides of the U-shaped tube, which helps us calculate the gas pressure through the formula
When air pressure is greater than gas pressure, the air will push the capillary liquid more than the gas. As a result, we will obtain the following formula for gas pressure
Water pressure calculated by the equation
represents only a part of total pressure exerted on object immersed in water. It must add to the atmospheric pressure to obtain the total pressure on objects in water, i.e.
The pressure at any level in the atmosphere represents the total weight of the air above a unit area. At higher altitudes, there are fewer air molecules above a given surface than on a similar surface at lower levels.
Since most of the atmosphere's molecules are near the earth's surface because of the attracting force of gravity, air pressure decreases rapidly at first (from sea level to h = 12 km), then more slowly at higher altitudes (from h = 12 km to h = 50 km) as air becomes very rare.
There is a mathematical formula, which expresses the relationship between air pressure and altitude. A simplified version of this formula is.
where P0 = 101 325 Pa is the standard atmospheric pressure in normal conditions and h is the altitude in metres. The other numbers are values obtained from operations with various constants.
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