Physical atmospheric pressure in the second day of junior high school

The air on the ground has a wide range and is often called the "atmosphere". More than 200 kilometers from the ground, and there is air. Although its density is very small, the pressure of such a high atmospheric column on the ground is still extremely high. The human body can't feel the pressure in the atmosphere, because the inside and outside of the human body are affected by the pressure at the same time, and they are completely equal. Otto gehrig, the mayor of Madeborg, Germany, made a hemispheric experiment in Madeborg, which confirmed the existence of atmospheric pressure.

The weight of air extending from the earth's surface to the sky makes the force on an object close to the earth's surface per unit area called "atmospheric pressure". The measurement of atmospheric pressure is usually expressed by the height of mercury column in mercury barometer. The standard atmospheric pressure on the ground is approximately equal to the pressure generated by a mercury column 76 cm high. Due to the influence of survey area and other conditions, the measured values are different. According to the formula of liquid pressure P=ρgh, the density of mercury is 13.6× 103 kg/m3, so the pressure generated by a 76 cm high mercury column is:

p？ = 13.6× 103 kg/m3× 9.8 Newton/kg× 0.76 m.

=1.013x105n/m2

=1.013x100000 Pascal

= 1 amount

= 76 mm Hg

=76 Torr

1 mmhg = 10 1300 pa/76 = 1332.895 pa

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1. Atmospheric pressure

The pressure produced by the atmosphere on objects immersed in it is called atmospheric pressure, or atmospheric pressure for short.

(1) Atmospheric pressure refers to the pressure generated by air in a certain position on the earth, and the air on the surface of the earth is subjected to gravity to generate atmospheric pressure. The density of the air layer above the earth is unequal. The air density near the earth's surface is higher, while the air at the top is thinner and less dense. Because atmospheric pressure is generated by air gravity, the air column above it is small in height and density, so the higher it is from the ground, the smaller the atmospheric pressure is. Usually at 2 o'clock.

(2) both gas and liquid have fluidity and similar pressure. Atmospheric pressure exists in all directions, and the atmospheric pressure in all directions in the same position is equal. However, due to the uneven density of the atmosphere, the formula of liquid pressure cannot be applied to the calculation of atmospheric pressure.

(3) The pressure of the gas sealed in the container is caused by the collision of a large number of irregularly moving gas molecules with the container wall. Its size is not determined by the gravity of the sealing gas.

④ Atmospheric pressure formula: P=ρ gas gh

2. The cause of atmospheric pressure

The earth is surrounded by a thick layer of air, which is mainly composed of nitrogen, oxygen, carbon dioxide, water vapor, helium, neon and argon. This layer of air is usually called the atmosphere as a whole. It is distributed around the earth from top to bottom, with a total thickness of 1000 km. All objects immersed in the atmosphere are subjected to atmospheric pressure, just as objects immersed in water are subjected to water pressure.

The causes of atmospheric pressure can be explained from different angles. The main point mentioned in the textbook is that air is affected by gravity, and air is flowing, so there is pressure in all directions. More specifically, due to the attraction of the earth to air, air is pressed on the ground, and it is supported by the ground or other objects on the ground. Both these objects and the ground supporting the atmosphere are subjected to atmospheric pressure. The atmospheric pressure per unit area is atmospheric pressure; Second, it can be explained by the viewpoint of molecular movement (the knowledge of molecular movement theory will be learned in the third grade). Because gas is composed of a large number of irregularly moving molecules, and these molecules will inevitably collide with objects immersed in the air. Every collision, air molecules will give an impact force to the surface of the object, and the result of the continuous collision of a large number of air molecules will be reflected in the atmospheric pressure on the surface of the object, thus forming atmospheric pressure. If there are more molecules per unit volume, the molecules in the air will be in the same time.

Using the viewpoint of molecular motion theory, we can explain why the uneven distribution of the atmosphere will cause the phenomenon of high and low atmospheric pressure.

3. Torricelli experiment

The ratio of atmospheric pressure was measured by Torricelli experiment. Fill a glass tube with a length of about 1m and a closed end with mercury, plug the nozzle, and then insert it backwards in the mercury tank. When the finger blocking the nozzle is released, the mercury column in the tube drops a little and stops falling. At this time, the height difference of mercury column inside and outside the tube is 760 mm. .

The reason why there is a 760mm high mercury column in the tube is because of the existence of atmospheric pressure. According to the characteristics of liquid pressure, the pressure on the liquid surface in the mercury tank should be equal to the pressure under the mercury column with a height of 760 mm in the glass tube. The pressure on the surface of the liquid in the mercury tank is atmospheric pressure, and the pressure in the tube can only be generated by the mercury column with a height of 760 mm. Therefore, the atmospheric pressure is generated by the mercury column with a height of 760 mm..

The commonly used units for expressing gas pressure are Pascal, Torr, Millimeter Mercury (mmhg), Centimeter Mercury (cmhg) and standard atmospheric pressure, and their symbols are pa, MMHg, CMHg and atm respectively.

Watch the animation demonstration and visit www.a-sky.com/kj/tl.swf..

4. The relationship between gas pressure and volume

The gas pressure mentioned here does not refer to atmospheric pressure, but refers to the pressure of a certain mass of gas.

Because the pressure of gas is essentially caused by the collision of a large number of irregularly moving gas molecules with the container wall, the reduction of gas volume will increase the number of collisions between gas molecules and the container wall and increase the pressure under other conditions unchanged.

When the temperature is constant, the smaller the volume of a certain mass of gas, the greater the pressure; The larger the volume, the smaller the pressure.

5. Relationship between boiling point and atmospheric pressure

Experiments show that the boiling points of all liquids decrease when the air pressure decreases and increase when the air pressure increases, and the boiling points of the same liquid are not fixed. It must be emphasized that the boiling point of water at standard atmospheric pressure is 100℃.

Because the air pressure increases with height, the boiling point of water decreases with height. For example, the boiling point of water is about 105℃ at an altitude of 1000 m, and about 97℃ at a distance of 3 km. At the top of Mount Everest, which is 8848 meters above sea level, it is difficult for water to boil. Therefore, cooking in high mountains should use a closed pressure cooker. The air pressure in the pot can be higher than the standard atmospheric pressure, so that the boiling point of water can be higher than 65438.

6. Working principle of piston pump

Piston-type water pump uses the movement of piston to discharge air, resulting in the pressure difference between inside and outside, which makes water rise and be pumped out under the action of air pressure. When the piston is pressed, the water inlet valve is closed and the exhaust valve is opened. When the piston rises, the exhaust valve closes and the water inlet valve opens. Under the action of external atmospheric pressure, water flows from the water inlet pipe to the upper water outlet through the water inlet valve. Therefore, the piston reciprocates up and down in the cylinder, continuously pumping out water.

7. Working principle of centrifugal water pump

Before the water pump starts, the pump shell is filled with water to exhaust the air in the pump shell, so that the pressure in the central part of the pump is less than the external atmospheric pressure. When starting, the impeller is driven by the motor to rotate at high speed, and the water in the pump casing also rotates at high speed with the impeller and is thrown into the outlet pipe at the same time. At this time, the pressure near the impeller is reduced, and the atmospheric pressure makes the water at the lower part push the bottom valve, and the incoming water is thrown into the outlet pipe by the impeller along the inlet pipe pump casing, and so on.

Both piston pump and centrifugal pump use atmospheric pressure to pump water up. Because the atmospheric pressure is limited, the pumping height of the pump is also limited, not exceeding10.3m. 。

8.？ standard atmospheric pressure

Atmospheric pressure not only varies with height, but also is not fixed in one place. Is it usually 1.05438+0325× 10? The atmospheric pressure of /5 (power) Pa is called standard atmospheric pressure. Equivalent to the pressure generated by 760 mm Hg.

The value of standard atmospheric pressure is often taken as 1.0 1× 10 in general calculation. /5 (power) Pa( 10 1Pa), a rough calculation can also be taken as 10? /5 (power) Pa( 100KPa). ?

9. Atmospheric pressure and altitude

Within 3000m above sea level, the atmospheric pressure drops 100Pa for every rise100m, and 1mmHg for every rise120m.

[Edit this paragraph] Madeborg Hemisphere Experiment

At that time, in the 7th century, there was a German mayor named Gehrig who loved science. He is a well-read soldier and likes to listen to Galileo's stories since he was a child. Love reading and science; I went to Leipzig University. 162 1 year, went to the University of Jena to study law. 1623 went to Leiden University to study mathematics and mechanics. He studied in three universities, with a wide range of knowledge, including astronomy and geography. Mathematics, law, philosophical engineering and so on. , are omniscient, omniscient. Therefore, he can live in the army; We must also gain a foothold in politics; 163 1 enlisted as an ordnance engineer in the army. Later, he joined the political circles, and 1646 was elected as the mayor of Fort Madre. No matter in the army or in the municipal government, he never stopped his scientific exploration.

1654, he heard about Torricelli, and heard that there are still many people who don't believe in atmospheric pressure. I also heard some people laugh at Torricelli; It is said that the two sides have a fierce debate, and they are not willing to give way to each other. Therefore, although Gehrig was far away from Italian and German, he felt aggrieved and angry.

He quickly found the glass tube and mercury, redone Torricelli's experiment, and concluded that the experiment was accurate. Then the air in the sealed barrel is pumped away, and the barrel "bang!" The sound was crushed by atmospheric pressure!

One day, he and his assistant made two hemispheres with a diameter of 14 inch, which is more than 30 centimeters, and invited a large group of people to the suburbs to start a "large-scale experiment."

On May 8th this year, the beautiful city of Fort Madre was sunny, with clear skies and bright Wan Li. A large number of people gathered around the experimental field, which was very lively. Some people say this, others say that; Some support Gehrig and hope that the experiment will be successful; Some people assert that the experiment will fail; People are talking and arguing; Forecast; Others ran to the proving ground in the streets and shouted:

"The mayor in the circus! The mayor is at the circus-"

Gehrig and his assistant put a rubber band in the middle of the brass hemispherical shell in public; Then fill the two hemispheres with water and put them together; Then all the water is pumped out, so that a vacuum is formed in the ball; Finally, tighten the faucet on the gas nozzle. At this time, the surrounding atmosphere pressed the two hemispheres tightly together.

With a wave of Gehrig's hand, four grooms brought eight tall and burly horses, and tied four horses on each side of the ball. Under the command of Gehrig, four grooms whipped the horses and walked back! It's like a tug of war.

"come on Come on! " The dark crowd on the experimental field shouted neatly and beat the time.

Four grooms and eight big horses are sweating like a pig. However, the copper ball is still intact. Gehrig had to shake his hand and pause.

Then, the left and right teams play each other. The grooms drank some boiled water, wiped the sweat on their heads and foreheads, and were preparing for the second performance.

Gehrig waved again, and the proving ground became more lively. 16 several big horses are pulling hard, and eight grooms are shouting loudly and waving whips to urge the horses. ...

The people in the experiment craned their necks, looked carefully and said, "Wow! Hey! " Sound.

Suddenly, "pa!" There was a loud noise and the copper ball split in half. Gehrig raised these two heavy hemispheres and proudly announced to you:

"gentlemen! Ladies! Citizens! You should believe it! There is atmospheric pressure, atmospheric pressure is so great! It's amazing! ……"

After the experiment, some people still didn't understand why the two hemispheres couldn't be pulled apart, so they asked him noisily. He patiently explained in detail: "Usually, we close the two hemispheres tightly, and we will separate if we don't force them. This is because there is atmospheric pressure inside and outside the ball; Balance each other. There seems to be no atmospheric effect. Today, after I vacuumed it, there was no outward atmospheric pressure inside the ball, only the atmosphere outside the ball pressed tightly against the two hemispheres ... ".

Through this "large-scale experiment", people finally believe that there is a vacuum; Have an atmosphere; The atmosphere is under pressure; The atmospheric pressure is amazing, but the mayor of Gehrig actually spent 4000 pounds for this experiment.