Atmosphere is a gas cloud that surrounds the Earth. The weight of air, the height of which exceeds 900 km, has a powerful effect on the inhabitants of our planet. We do not feel this, taking life at the bottom of the air ocean as a matter of course. A person feels discomfort when climbing high in the mountains. Lack of oxygen provokes rapid fatigue. At the same time, atmospheric pressure changes significantly.
Physics deals with atmospheric pressure, its changes and the impact on the Earth's surface.
In the course of high school physics, considerable attention is paid to the study of the action of the atmosphere. Features of the definition, dependence on height, influence on processes occurring in everyday life or in nature, are explained on the basis of knowledge about the action of the atmosphere.
When do people start studying atmospheric pressure? Grade 6 - time to get acquainted with the peculiarities of the atmosphere. This process continues in specialized high school classes.
Study history
The first attempts to establish atmospheric air pressure were made in 1643 at the suggestion of the Italian EvangelistTorricelli. A glass tube sealed at one end was filled with mercury. Having closed on the other side, it was lowered into mercury. In the upper part of the tube, due to the partial outflow of mercury, an empty space formed, which received the following name: “Torricellian void”.
By this time, the theory of Aristotle dominated in natural science, who believed that "nature is afraid of emptiness." According to his views, there can be no empty space not filled with matter. Therefore, for a long time they tried to explain the presence of emptiness in a glass tube with other matters.
There is no doubt that this is an empty space, it cannot be filled with anything, because by the beginning of the experiment the mercury completely filled the cylinder. And, flowing out, did not allow other substances to fill the vacant place. But why didn’t all the mercury pour into the vessel, because there are no obstacles to this either? The conclusion suggests itself: mercury in the tube, as in communicating vessels, creates the same pressure on the mercury in the vessel as something from the outside. At the same level, only the atmosphere comes into contact with the mercury surface. It is her pressure that keeps the substance from pouring out under the influence of gravity. Gas is known to create the same action in all directions. It affects the mercury surface in the vessel.
The height of the mercury cylinder is approximately 76 cm. It can be measured in cm of mercury.column (or in millimeters).
Which units to use?
The international system of units is international, so it does not involve the use of millimeters of mercury. Art. when determining pressure. The unit of atmospheric pressure is set in the same way as it happens in solids and liquids. Measurement of pressure in pascals is accepted in SI.
For 1 Pa, such a pressure is taken that is created by a force of 1 N per area of 1 m2.
Determine how units of measure are related. The pressure of the liquid column is set according to the following formula: p=ρgh. Mercury density ρ=13600 kg/m3. Let's take a column of mercury 760 millimeters long as a reference point. From here:
r=13600 kg/m3×9.83 N/kg×0.76 m=101292.8 Pa
To record the atmospheric pressure in pascals, consider: 1 mm Hg.=133.3 Pa.
Example of problem solving
Determine the force with which the atmosphere acts on the surface of a roof measuring 10x20 m. Consider the pressure of the atmosphere to be 740 mm Hg.
p=740 mm Hg, a=10 m, b=20 m.
Analysis
To determine the force of action, you must set the atmospheric pressure in pascals. Taking into account the fact that 1 millimeter Hg. equal to 133.3 Pa, we have the following: p=98642 Pa.
Decision
Use the formula for determining pressure:
p=F/s, Since the area of the roof is not given, let's assume it is a rectangle. The area of this figure is determined by the formula:
s=ab.
Substitute the value of the area incalculation formula:
p=F/(ab), from where:
F=pab.
Calculate: F=98642 Pa×10 m×20 m=19728400 N=1.97 MN.
Answer: the pressure force of the atmosphere on the roof of the house is 1.97 MN.
Measurement methods
Experimental determination of atmospheric pressure can be performed using a column of mercury. If you fix the scale next to it, then it becomes possible to fix the changes. This is the simplest mercury barometer.
It was Evangelista Torricelli who was surprised to note the changes in the action of the atmosphere, linking this process with heat and cold.
The atmospheric pressure at sea surface level at 0 degrees Celsius was called optimal. This value is 760 mmHg. Normal atmospheric pressure in pascals is considered to be equal to 105 Pa.
It is known that mercury is quite harmful to human he alth. As a result, open mercury barometers cannot be used. Other liquids are much less dense, so the tube filled with liquid must be long enough.
For example, the water column created by Blaise Pascal should be about 10 m high. The inconvenience is obvious.
Liquidless barometer
A remarkable step forward is the idea of moving away from liquid when making barometers. The ability to manufacture a device for determining the pressure of the atmosphere is implemented in aneroid barometers.
The main part of this meter is flatbox from which the air is pumped out. So that it is not squeezed by the atmosphere, the surface is made corrugated. The box is connected by a system of springs to an arrow indicating the pressure value on the scale. The latter can be graduated in any units. Atmospheric pressure can be measured in pascals with the appropriate measuring scale.
Lift height and atmospheric pressure
The change in the density of the atmosphere as you rise up leads to a decrease in pressure. The inhomogeneity of the gas envelope does not allow introducing a linear law of change, since the degree of pressure decrease decreases with increasing height. At the surface of the Earth, as it rises, for every 12 meters, the effect of the atmosphere falls by 1 mm Hg. Art. In the troposphere, a similar change occurs every 10.5 meters.
Near the surface of the Earth, at the altitude of an aircraft, an aneroid equipped with a special scale can determine the altitude by atmospheric pressure. This device is called an altimeter.
A special device on the surface of the Earth allows you to set the altimeter to zero, so that you can later use it to determine the height of the ascent.
Example of problem solving
At the foot of the mountain, the barometer showed an atmospheric pressure of 756 millimeters of mercury. What will be the value at an altitude of 2500 meters above sea level? It is required to record atmospheric pressure in pascals.
r1 =756 mm Hg, H=2500 m, r2 - ?
Decision
To determine the barometer reading at height H, we take into account thatpressure drops by 1 mm Hg. every 12 meters. Therefore:
(p1 – p2)×12 m=H×1 mmHg, from:
p2=p1 - H×1 mmHg/12m=756 mmHg - 2500 m×1 mmHg/12 m=546 mmHg
To record the obtained atmospheric pressure in pascals, do the following:
p2=546×133, 3 Pa=72619 Pa
Answer: 72619 Pa.
Atmospheric pressure and weather
The movement of air atmospheric layers near the Earth's surface and the non-uniform heating of air in different areas lead to changes in weather conditions in all parts of the planet.
Pressure can vary by 20-35mmHg. in the long term and by 2-4 millimeters of mercury. during the day. A he althy person does not perceive changes in this indicator.
Atmospheric pressure, the value of which is below normal and often changes, indicates a cyclone that has covered a certain one. Often this phenomenon is accompanied by cloudiness and precipitation.
Low pressure is not always a sign of rainy weather. Bad weather depends more on the gradual decrease in the indicator in question.
A sharp drop in pressure to 74 centimeters Hg. and below it threatens with a storm, showers that will continue even when the indicator is already starting to rise.
Change in the weather for the better can be identified by the following signs:
- after a long period of bad weather, there is a gradual and steady increase in atmospheric pressure;
- pressure rises in foggy slushy weather;
- during the period of southerly winds, the indicator in question rises for several days in a row;
- an increase in atmospheric pressure during windy weather is a sign of comfortable weather.
