Atmosphere - a gas cloud surrounding the Earth. The weight of the air, whose column height exceeds 900 km, has a powerful effect on the inhabitants of our planet. We do not feel this, perceiving life at the bottom of the air ocean as a matter of course. A person feels discomfort, rising high in the mountains. Lack of oxygen provokes fatigue. At the same time, atmospheric pressure changes significantly.
Physics considers atmospheric pressure, its changes and its effect on the Earth's surface.
In a high school physics course, considerable attention is paid to studying the effects of the atmosphere. The features of the determination, the dependence on height, the effect on the processes occurring in everyday life or in nature, are explained on the basis of knowledge about the effect of the atmosphere.
When do they begin to study atmospheric pressure? Grade 6 - time to get acquainted with the atmosphere. This process continues in specialized classes of high school.
Study history
The first attempts to establish atmospheric air pressure were made in 1643 at the suggestion of the Italian Evangelista Torricelli. A glass tube sealed at one end was filled with mercury. Having closed on the other hand, it was lowered into mercury. In the upper part of the tube due to partial leakage of mercury, an empty space formed, which received the following name: "Torricellian void."
By this time, the theory of Aristotle dominated in natural science, believing 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 a void in a glass tube with other materials.
There is no doubt that this is an empty space, it cannot be filled with anything, because mercury completely filled the cylinder by the beginning of the experiment. And, emerging, did not allow other substances to fill the vacant seat. But why all the mercury did not spill into the vessel, because there are no obstacles to this either? The conclusion suggests itself: the 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 is in contact with the surface of the mercury. It is her pressure that keeps the substance from pouring out under the action of gravity. Gas, as you know, creates the same effect in all directions. It affects the mercury surface in the vessel.
The height of the mercury cylinder is approximately equal to 76 cm. It is noted that this indicator varies over time, therefore, the atmospheric pressure changes. It can be measured in cm of mercury (or in millimeters).
Which units to use?
The international system of units is international, therefore, does not involve the use of millimeters of mercury. Art. in determining the pressure. The atmospheric pressure unit is set in the same way as it does in solids and liquids. Pressure measurement in pascals is accepted in SI.
For 1 Pa, a pressure is taken which is created by a force of 1 N per 1 m 2 area .
Define how the units are related . The liquid column pressure is set according to the following formula: p = ρgh. The density of mercury is ρ = 13600 kg / m 3 . For the reference point, take a column of mercury with a length of 760 millimeters. From here:
p = 13600 kg / m 3 × 9.83 N / kg × 0.76 m = 101292.8 Pa
To record atmospheric pressure in pascals, we take into account: 1 mm Hg = 133.3 Pa.
Problem Solving Example
Determine the force with which the atmosphere acts on the roof surface with dimensions of 10x20 m. Consider the atmospheric pressure equal to 740 mm Hg.
p = 740 mmHg, a = 10 m, b = 20 m.
Analysis
To determine the force of action, it is necessary to establish the atmospheric pressure in pascals. Given the fact that 1 mmHg equal to 133.3 Pa, we have the following: p = 98642 Pa.
Decision
We use the pressure determination formula:
p = F / s,
Since the roof area is not given, suppose that it has the shape of a rectangle. The area of this figure is determined by the formula:
s = ab.
Substitute the value of the area in the calculation formula:
p = F / (ab), whence:
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
Atmospheric pressure can be experimentally determined using a column of mercury. If a scale is fixed next to it, then it becomes possible to record changes. This is the simplest mercury barometer.
With surprise, it was Evangelista Torricelli who noted the changes in atmospheric action, connecting this process with heat and cold.
The optimal pressure was called atmospheric pressure at sea level at 0 degrees Celsius. This value is 760 mmHg.
Normal atmospheric pressure in pascals is considered to be equal to 10
5 Pa.
Mercury is known to be harmful enough to human health. As a result, open mercury barometers cannot be used. Other liquids have a much lower density, so the tube filled with the liquid should be long enough.
For example, the water column created by Blaise Pascal should be about 10 m in height. The inconvenience is obvious.
Liquidless Barometer
A remarkable step forward is the idea of moving away from the liquid when creating barometers. The ability to manufacture a device for determining atmospheric pressure is implemented in aneroid barometers.
The main part of this meter is a flat box from which air is pumped out. To prevent it from being compressed by the atmosphere, the surface is made corrugated. The box system is connected by a spring system to an arrow indicating the pressure value on the scale. The latter can be graduated in any units. You can measure atmospheric pressure in pascals with the appropriate measuring scale.
Lifting Height and Atmospheric Pressure
A change in the density of the atmosphere as it rises leads to a decrease in pressure. The inhomogeneity of the gas shell does not allow us to introduce 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 drops by 1 mm Hg. Art. In the troposphere, a similar change occurs for every 10.5 m.
Near the surface of the Earth, at the flight altitude of the aircraft, an aneroid equipped with a special scale can determine the height by atmospheric pressure. This device is called an altimeter.
A special device on the surface of the Earth allows you to set the altimeter readings at zero, in order to further use it to determine the height of rise.
Problem solving example
At the foot of the mountain, the barometer showed 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.
p 1 = 756 mm Hg, H = 2500 m, p 2 -?
Decision
To determine the readings of the barometer at a height of H, we take into account that the pressure drops by 1 mmHg. every 12 meters. Hence:
(p 1 - p 2 ) × 12 m = N × 1 mm Hg, where:
p 2 = p 1 - N × 1 mm Hg / 12 m = 756 mm Hg - 2500 m × 1 mm Hg / 12 m = 546 mm Hg
To record the received atmospheric pressure in pascals, we perform the following steps:
p 2 = 546 × 133.3 Pa = 72,619 Pa
Answer: 72619 Pa.
Atmospheric pressure and weather
The movement of atmospheric air layers near the Earth's surface and the inhomogeneous heating of air in different areas lead to changes in weather conditions in all parts of the planet.
Pressure may vary by 20-35 mmHg. in the long term and by 2-4 millimeters of mercury. during the day. A healthy 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 is more dependent on a gradual decrease in the indicator in question.
A sharp decrease in pressure to 74 centimeters of mercury. and threatens below with a storm, showers, which will continue even when the indicator is already starting to rise.
Change in weather for the better can be determined by the following criteria:
- after a long period of bad weather, a gradual and steady increase in atmospheric pressure is observed;
- in foggy slushy weather, pressure rises;
- in 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 establishing comfortable weather.