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§ 31. Pressure (textbook)

§ 31. Pressure


Remember the course of science, called atmospheric pressure.


Concept about atmospheric pressure. Air is an invisible and easy. However, it also, like every substance that has mass and weight. So it puts pressure on the earth surface and all body that it is. This pressure is determined by weight of air column height of the whole atmosphere - from the earth's surface to its upper limit itself. Established that such a pillar of air pressure per 1 cm2 surface force of 1 kg 33 kg (under 1 m2 - Over 10 tons!). Thus, atmospheric pressure - A force with which air pressure on Earth's surface and all the items on it.

The surface of the human body averages 1.5 m2. Under air pressure on her weight was 15 tons Such pressure can crush all living things. Why do we not feel? This is due to the fact that within human body as a pressure - internal and it is atmospheric. If this balance is disturbed, the person feels bad.

Measurement atmospheric pressure. Pressure is measured by special device - the barometer. Translated from the Greek word means "Gravity meter.

At the weather station used mercury barometer. The main part of it - a long glass tube m, sealed at one end. She loaded mercury - a heavy liquid metal. Open end of the tube immersed in a wide bowl, also filled with mercury. In a rollover mercury from the tube resulted only to a certain level and stop. Why is it stopped, turned and not all? Because the air puts pressure on the mercury in the bowl and produces all of its tube. If the atmospheric pressure decreases, the mercury in the tube falls and vice versa. For high column of mercury in the tube, which marked scale determine the amount of atmospheric pressure in mm.

In parallel 450 at sea level at temperature 0 0C in air pressure mercury column in the tube rises to a height of 760 mm. This air pressure is normal atmospheric pressure. If column of mercury in the tube rises above 760 mm, the pressure increasedBelow - reduced. Thus, the pressure of the atmosphere of the whole air column urivnovazhuyetsya weight column of mercury height 760 mm.

In the campaigns and expeditions using more convenient device - aneroid barometer. "Aneroid" in Greek means "bezridynnyy: There was no mercury. The main part of it is metallic elastic capsule from which downloaded air. This makes it very sensitive to changes in external pressure. At elevated pressure is compressed to a lower - is expanding. These fluctuations transmitted through a special mechanism arrow that indicates the scale value atmospheric pressure in millimeters of mercury.

Dependence pressure from the ground elevation and air temperature. Pressure depends on the height area. The higher the level of sea, the air pressure is less. It decreases because of reduced lifting height of air column that presses on the earth's surface. In addition, high pressure falls also because it decreases the density of the air itself. At an altitude of 5 km atmospheric pressure is reduced by half compared with the normal pressure at Sea. In the troposphere on a show for every 100 m pressure decreases by about 10 mm рт. Art.

Knowing how the pressure can be calculated and absolute and relative height of location. There is a special barometer - altimeter, Which together with the scale atmospheric pressure, is and scale heights. So for each area will characteristic of its normal pressure: at sea level - 760 mm рт. Art. in the mountains, depending on height - below. For example, in Kyiv, which is at altitudes of 140-200 m above sea level, the normal average pressure is 746 mm рт. Art.

Pressure depends on air temperature. When heated air volume increases, it becomes less dense and easier. Because of this reduced and atmospheric pressure. When cooled place back phenomenon. However, as changes in temperature and pressure changes continuously. Throughout the day he rises twice (morning and evening) and double-declining (After noon and after midnight). In winter when the air is cold and heavy, the pressure higher  than in summer when it is warmer and easy. Thus, the change of pressure can predict weather changes. Decompression indicates precipitation, increase - in dry weather. Atmospheric pressure affects and wellbeing of people.

Distribution atmospheric pressure on Earth. Atmospheric pressure as the air temperature, the Earth is divided into strips: distinguish zones of low and high pressure. Their formation is connected with heating and moving air. 

Air above the equator well warmed up. From this it expands, becomes less dense and therefore easier. Lighter than air rises up - is upward movement air. So there near the surface of the Earth year established zone low pressure. Above the poles, where low temperatures throughout the year, air cools, it becomes more dense and difficult. It therefore falls - is downward movement air - and increased pressure. So near poles formed Zones high pressure. The air that has risen above the equator, spreads to the poles. But, reaching them at the height it cools, it becomes harder and falls on comparisons 30-350 in both hemispheres. As a result - there formed Zones High pressure. In temperate latitudes, the comparisons 60-650both hemispheres are formed belt of low pressure.

Thus, there is a close dependence of atmospheric pressure on the distribution of heat and air temperatures on Earth when ascending and descending air movements cause uneven heating of the earth's surface.



Questions and Tasks

1. Determine how much weight of air, which is in class, if its length is 8 m, width 6 m, height 3 m.

2. Why atmospheric pressure decreases with height?

3. Why change the pressure in the same place? How does this change in temperature?

4. Determine the relative height of the mountain about top if the mountain near the sole barometer shows 720 mm, and on top - 420 mm.

5. How is the atmospheric pressure on Earth?

6. Remember that your altitude area. Calculate that atmospheric pressure can be considered normal for your area.