At one time in chemistryStandard temperatureAnd pressure (STP) are defined as 0 ° C (273.15K) and 101.325kPa (1atm), but since 1982 IUPAC has redefined "standard pressure" as 100 kPa.
1 StandardsAtmospheric pressure=760mm Hg=76cmmercury=10.336m water column=1.01325 × 10fivePa=101325 N/㎡[1]In calculation, it is usually 1 standard atmospheric pressure=1.01 × 10fiveN/㎡。
At the 10th International Metrology Conference in 1954, scientists set a "standard" for atmospheric pressure: at sea level with latitude of 45 °, when the temperature is 0 ℃, the pressure generated by 760mm high mercury column (equivalent to 1013.25 hPa) is called standard atmospheric pressure.This standard is widely accepted and used to compare atmospheric pressure.
Atmospheric pressure
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The earth is surrounded by thick air, which is calledatmosphere。Air can flow freely like water, and it is also affected by gravity.Therefore, there is pressure in all directions inside the air, which is called atmospheric pressure.In 1643, Italian scientistTorricelliFill a 80cm long thin glass tube with mercury and place it upside down in the water tank containing mercury. It is found that the mercury in the glass tube has dropped by about 4cm, and then it no longer drops.[2]There is no air entering the 4 cm space, which is a vacuum.Torricelli deduced that the atmospheric pressure is equal to the length of the mercury column.According to the pressure formula, scientists accurately calculated that the atmospheric pressure is 1.01 × 10 under the standard statefiveIn 1654, Grick made the famousMagdeburg hemispheres test , which makes people have a profound understanding of atmospheric pressure.
The standard atmospheric pressure is not fixed.Since it is "standard", we should pay attention to the accuracy of each physical quantity value when calculating according to the liquid pressure formula.According to relevant data, the density of mercury at 0 ℃ is 13.595 × 10threekg/m³，The g value at sea level with latitude of 45 ° is 9.80672N/kg.So the pressure generated by 760mm high mercury column is:
This is the value of 1 standard atmospheric pressure.
National standard GB1920-80 standard atmosphere (below 30km): 1976American StandardFor atmosphere, the part below 30km is regarded as the national standard of China, and the part above 30km can be used for reference.standardGravitational accelerationg=9.80665 N/kg, sea levelAbsolute temperatureT=288.150 K, sea level air densityρ=1.2250 kg/mthree。
In recent scientific work, for convenience, 1 standard atmospheric pressure is defined as 100 kPa, which is recorded as 1 bar.Therefore, the standard atmospheric pressure can also refer to100kPa。
Atmospheric pressure conversion
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The pressure of 1kg is about 0.1Mpa, about one standard atmospheric pressure;One kilogram pressure is the abbreviation of one kilogram force per square centimeter. One kilogram force equals 9.8 Newton, so one kilogram force per square centimeter=(9.8/0.0001) Pa=0.098 MPa, about 0.1 MPa.And a standard atmospheric pressure=1.01325X1000000Pa, equivalent to 0.1MPa=tenfivePa。
Therefore: 1MPa=10kg pressure=10kg f/c ㎡
1MPa=10 standard atmospheric pressure
Vacuum degree concept
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As the name suggests, vacuum degree is the degree of vacuum.It is a vacuum pumpMicro vacuum pump, micro air pump, micro air pump, micro air pumping pump and other vacuum pumping equipment.[3]If the pressure in the measured equipment is lower than atmospheric pressure, a vacuum gauge is required for pressure measurement.The value read from the vacuum gauge is called vacuum degree.The vacuum degree value indicates that the actual value of the system pressure is lower than the atmospheric pressure, that is, vacuum degree=atmospheric pressure - absolute pressure, absolute pressure=atmospheric pressure+gauge pressure (- vacuum degree).
The so-called "vacuum" refers to the gas state with a pressure lower than 101325 Pa (i.e. a standard atmospheric pressure is about 101 kPa) in a given space.
In the vacuum state, the rareness of the gas is usually expressed by the pressure value of the gas. Obviously, the smaller the pressure value is, the thinner the gas is.[3]
Measurement related experiments
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Torricelli experiment
In the old days, the academic community had no clear understanding of whether air had weight and whether vacuum might exist, mainly becauseAristotleThe legacy of thought holds that "all things in the world have their own weight except fire and air".And insist on the "fear" of naturevacuum”The statement of.GalileoHe doubted this statement and said: "We can't believe what Aristotle said. We just think that something is light and something is heavy. We should realize that all objects have their own weight, but each has its own weight and density."."If people cannot recognize the existence of vacuum by feeling and understanding, how can they deny the existence of vacuum by feeling and understanding?" Galileo once found that,Water pumpWhen working, he could not pump water to a height of more than 10 meters. He attributed this phenomenon to the fact that the water column could not bear its own weight, and could not find a reasonable and satisfactory explanation.
TorricelliI firmly agree with Galileo's statement that air has weight and vacuum.On the basis of summarizing previous theories and experiments, Torricelli carried out a lot ofexperiment, realized vacuum, verified the fact that air has weight, and denied Aristotle's statement about vacuum force.
Torricelli experimental steps
Around 1641, a famous mathematician and astronomerBellTizeng made a vacuum experiment with a 10 meter long lead pipe.Torricelli was inspired by this experiment and thought of using a higher densityseawater、Honey、MercuryAnd so on.The mercury experiment he selected achieved the most successful results.He filled a 1m long glass tube with mercury, and then pressed his finger against the tube mouth, and inverted it into the mercury tank containing mercury. After releasing his finger, it can be seen that the mercury on the top of the tube inside has fallen, leaving room, while the lower part is still full of mercury.To further prove that the upper part of the mercury surface in the tube isvacuumTorricelli improved the experiment.He filled the tank with clean water from above the mercury surface in the mercury tank, and then slowly lifted the glass tube upward. When the glass tube mouth was raised above the interface between mercury and water, the mercury in the tube would quickly leak out, and at the same time, the water suddenly rushed upward into the tube until it reached the top of the tube.It can be seen that the part above the mercury column in the original tube is really empty.The original mercury column and the current water column are not attracted by any vacuum force, but are held by the pressure generated by the air weight on the mercury surface outside the tube.Torricelli's experiment was directed at AristotleDynamicsAs a result, some people tried to deny Torricelli's research results and proposed that the upper end of the glass tube was filled with "pure air", not vacuum.Everyone expressed their own views and opinions, which caused a heated debate.The controversy lasted until Pascal's experiment successfully confirmed Torricelli's theory.
Torricelli also found in his experiment that the vertical height of the mercury column always kept the same height (760mm) regardless of the length of the glass tube and the inclination of the glass tube. He also cooperated with Viviani in 1644 to make the first mercury column in the worldMercury barometer。
Understand the principle of Torricelli experiment, and understand the method, operation process and steps of the experiment.
Instruments and equipment
Torricelli tester (J2116 type), mercury, more than 1m longglass tube(or two glass tubes are connected by rubber tubes),Beaker, red water.
Experimental methods
1. Hold the middle of the glass tube with one hand, fill the tube with mercury to remove air, tightly block the open end of the glass tube with the index finger of the other hand, carefully insert the glass tube into the tank containing mercury, release the finger when the open end is completely immersed in the mercury tank, vertically fix the tube, and read the vertical height of the mercury column.
2. Gradually tilt the glass tube, and the vertical height of the mercury column in the tube remains unchanged.
3. Continue to tilt the glass tube. When the tilt reaches a certain degree, the tube is full of mercury, indicating that there is no air in the tube.
4. Repeat the experiment with glass tubes with different inner diameters and different lengths (or do it at the same time and compare them side by side), and it can be found that the vertical height of the mercury column remains unchanged.explainAtmospheric pressureIt has nothing to do with the thickness and length of the glass tube.
5. Close one end of the long glass tube with a rubber stopper, fill the tube with red water, block the other end with a finger, invert the glass tube into the water, and release the finger.Observe the phenomenon and ask the students: "If the top rubber plug is pulled out, will the water column eject from the top of the pipe under the external atmospheric pressure?" Then demonstrate and verify, so as to eliminate some one-sided understanding and deepen understanding.
matters needing attention
1. DescriptionTorricellian vacuumThe existence of is the key to the experiment. Only if this is true, can we conclude that "the pressure of mercury column is equal to atmospheric pressure".
2. Mercury is highly toxic and harmful to human body. Special attention should be paid to safe operation.Glass tube with thick wall and small inner diameter shall be selected.Collision and shaking shall be avoided during operation, and pipe breakage shall be strictly prevented.Never lose mercury in the classroom.When pouring mercury, it is better to use an injection syringe or a funnel with a narrow neck and a sharp mouth, and pad it with a large plastic box (enamel basin) below to prevent silver from splashing on the ground.If there is a wound on the index finger of the right hand, it is absolutely not allowed to operate with the wound. The rubber finger cover can be used to protect the index finger.If mercury splashes on the ground, try to collect it. If it cannot be collected, it can be scatteredSulfur Vulcanize it, collect and bury it, and open the window for ventilation.
3. To reduce the experimental error, attention must be paid to: ① The glass tube and mercury tank must be clean and dry, free of dust, impurities and moisture. ②Mercury must be clean. If it is mixed with dust and impurities, or dissolved with other metals to form amalgam and attached to the inner surface of the glass tube, the reading will be inaccurate and the effect will be affected. ③Air shall not be mixed into the mercury tank when pouring mercury or after pouring.Can use a piece of wrapped yarn orEnameled wireIt goes all the way to the bottom of the glass tube, and when filling with mercury, the gauze wrapping line is pulled up and down from time to time, so that bubbles can be discharged with it.
4. Torricelli experiment is an important experiment for quantitative measurement of atmospheric pressure. Teachers should prepare carefully, operate normatively and set an example for students.Teach students how to read and observe: such asMeter gaugeThe starting line of is on the same horizontal line with the convex surface of the mercury surface in the groove;When reading, the line of sight should be the same as the mercury surface and scale linelevelSuperior.
Magdeburg hemispheres test
Magdeburg hemispheres test
Magdeburg hemispheres test (German: Magdeburger Halbkugeln), who also conducted the Magdeburg Hemisphere ExperimentMagdeburg mayorOtto Von Guericke toImperium RomanumOfRegensburg(today's Regensburg, Germany)scientific experimentThe purpose is to prove the existence of vacuum.And this experiment is also called“Madburg Hemisphere”Experiment.The two hemispheres of the experiment that year are still preserved in MunichDeutsches Museum Medium.At present, there are also imitations for teaching purposes, which are used to demonstrate the principle of air pressure. Their volume is much smaller than that of the hemisphere at that time. To vacuum the space of the hemisphere, it is not necessary to use more than 10 horses to pull it apart.
One day, Glick and his assistant made two hemispheres with a diameter of 14 inches, or more than 30 centimeters, and invited a large group of people to do a "large-scale experiment" in the suburbs.On May 8 of that year, the beautiful city of Madelberg was sunny, sunny and cheerful. A large number of people crowded around the experimental field.Some say this, some say that;Some support Grick and hope that the experiment will be successful;Some assert that the experiment will fail;People are talking and arguing;Is predicting;Others ran to the experimental field in the streets and alleys, shouting, "The mayor is playing a circus! The mayor is playing a circus..."
Glick and his assistant put a rubber band on the middle of the brass hemispherical shell in public, filled the two hemispherical shells with water and then put them together, and then pumped out all the water to form a vacuum in the ball.Finally, tighten the faucet on the air nozzle and close it. At this time, the surrounding atmosphere tightly presses the two hemispheres together.
With a wave of his hand, four grooms brought 16 horsestall and strong, tie four horses on both sides of the ball. At the order of Glek, the four grooms raised their whip to urge the horses and pulled away!It seems to be tug of war."Come on! Come on!" the dark crowd on the experimental field shouted in order, beating time at the same time.The four grooms and 16 horses were sweating all over. However, the bronze ball was still intact, and Glick had to shake his hand to pause.
Then, the left and right teams multiplied.The grooms drank some water, wiped the sweat on their foreheads, and were preparing for a second performance.Grick waved again, and the experimental field was even more lively.16 horses[4]The eight grooms were yelling and whipping their horses... The crowd on the experimental field stretched their necks and looked at them, making a sound of "wow! Wow!" from time to time.Suddenly, there was a loud sound of "Pa!", and the copper ball split into two original halves. Glick lifted the two heavy hemispheres and proudly announced to everyone, "Gentlemen! Ladies! Citizens! You should believe it! There is atmospheric pressure, and the atmospheric pressure is so great! So amazing!......"
principle
After the experiment, some people still did not understand why the two hemispheres could not be pulled apart. They asked him repeatedly, and he patiently explained in detail: "At ordinary times, we close the two hemispheres together, without using force, they will separate. This is because there are atmospheric pressure forces inside and outside the ball, which offset and balance each other, as if there were no atmospheric effect.But after I vacuumized it, there was no outward atmospheric pressure inside the ball, only the atmosphere outside the ball tightly pressed the two hemispheres... "Through this" large-scale experiment ", people finally believed that there was a vacuum, atmosphere, atmosphere pressure, and atmospheric pressure was amazing.
Social evaluation
In today's society, people can finallyMunichOfDeutsches Museum See the original "equipment" of this experiment, that is, the two hemispheres.At the turn of the century, the city of Madburg built a "Millennium Tower" on the site of the German Garden Expo at that time, which also placed two hemispheres in memory of Glick, but it is a replica.
In memory of the old mayor, Madburg people erected his statue on the small square next to the old city hall and named the young Madburg University with him.
conclusion
The Madelberg hemispherical experiment proved that the atmospheric pressure is very strong.In the experiment, the air in the two hemispheres is pumped out, so that the number of air particles in the ball decreases.The atmosphere outside the ball then compresses the two hemispheres together, so it is not easy to separate them.The more pumping, the greater the pressure.
Influencing factors of atmospheric pressure
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The size of atmospheric pressure is related to altitude, temperature, humidity, atmospheric circulation, geographical location and other factors.
1. Altitude: The atmospheric pressure is generated by the gravity of the atmosphere. The higher from the ground, the thinner the atmosphere is, and the lower the atmospheric pressure there should be.With the increase of altitude, the atmospheric pressure gradually decreases。However, because the air density related to the gravity of the atmosphere changes unevenly with height, the atmospheric pressure decreases unevenly with height.Therefore, at the same time and place, the higher the altitude, the lower the pressure.
2. Temperature: at the same height,The higher the temperature, the lower the air pressure。This is because the speed of gas molecules increases at high temperature, the rebound force after collision decreases, and the pressure of air also decreases.However, in a closed environment (with the same volume), the higher the temperature, the higher the pressure.
3. Humidity: at the same height,The higher the humidity, the lower the air pressure。This is because the molecular weight of water vapor is smaller than that of air molecule, which will occupy a part of the space, thus making the average free path of gas molecule larger and the pressure of air smaller.
4. Atmospheric circulation: atmospheric circulation refers to the pressure difference between different regions of the earth, which will cause the movement and flow of gas.For example, the tropical low pressure zone over the equator will cause the air to flow to the poles, resulting in a high pressure zone in high latitudes, thus affecting the size of the pressure.
5. Geographic location: The pressure is also affected by different geographical locations on the earth.For example, the low pressure zone near the equator and the high pressure zone near the poles will affect the global atmospheric circulation, thus affecting the pressure.
In conclusion, the atmospheric pressure is affected by many factors, which interact and influence each other to form a complex atmospheric environment.The value of atmospheric pressure above the earth is different in different seasons, climate conditions and geographical locations.The following five main changes of atmospheric pressure are selected for analysis and discussion for reference.
Topographic change
From a microscopic perspective, there are two main factors that determine the gas pressure: one is the density of the gasn；The second is gasThermodynamic temperatureT。On the earth's surface, with the rise of the terrain, the gravity of the earth on atmospheric gas molecules gradually decreases, and the density of air molecules decreases;The temperature of the atmosphere also decreases.thereforeOn the earth's surface, the value of atmospheric pressure gradually decreases with the increase of terrain height。If the air in the atmosphere is regarded asideal gasThe formula that approximately reflects the change of atmospheric pressure with height can be derived as follows:
μ=pzerogh/RT
μAiryAverage molar mass，pzeroIs the atmospheric pressure at the earth's surface,gIs the gravitational acceleration at the earth's surface,RIs the universal gas constant,TbyAtmospheric thermodynamic temperature，hIs the height of the air column.It can be seen from the above formula that when the influence of the secondary factor of atmospheric temperature change is not considered, the atmospheric pressure value varies with the geographical heighthThe increase of decreases exponentially.Within 2 km, the atmospheric pressure value can be approximately considered to decrease linearly with the increase of geographical height;Beyond 2km, the atmospheric pressure decreases gradually with the increase of geographical height.So it was introduced in junior high school physics textbooks in the past: within 2km above sea level, it can be approximately considered that the atmospheric pressure will decrease by 1mm Hg for every 12m increase.
Latitudinal variation
The composition of the atmosphere on the earth's surface is water vapor.People call the air with more water vapor "wet air" and the air with less water vapor "dry air".Some people intuitively think that wet air is heavier than dry air, which is incorrect.Dry airAverage molecular weightIt is 28.966, while the molecular weight of water vapor is only 18.106, so the density of wet air containing more water vapor is smaller than that of dry air.That is, under the same physical conditions, the pressure of dry air is greater than that of wet air.On the surface of the earth, from the equator to the polesGeographic latitudeOn the one hand, due to the rotation of the earth and the reduction of the polar radius, the attraction of the earth to the atmosphere gradually increases, and the air density increases;On the other hand, due to the low temperature in the polar regions, there is less water vapor in the air, which can be approximately seen as dry airFrom the equator to the poles, with the increase of geographical latitude, the general change law of atmospheric pressure is gradually increasing(Due to the influence of climate and other factors, the change of atmospheric pressure value at a local place may not follow this rule).
daily variation
For the same area, the atmospheric pressure on the ground will be different at different times of the day, which is called the daily variation of atmospheric pressure.In a day, the atmospheric pressure on the earth's surface has a maximum and a minimum.The highest value occurs at 9~10.The lowest value occurs at 15~16。
There are three main reasons for the daily change of atmospheric pressure.One is the movement of the atmosphere;The second is the change of atmospheric temperature;ThirdAtmospheric humidityChanges.After sunrise, the ground begins to accumulate heat, and at the same time, part of the ground willHeat transferTo the atmosphere, the atmosphere also continuously accumulates heat, its temperature rises and humidity increases.When the temperature rises, the atmosphere gradually rises to the upper airDivergenceAt 15~16 p.m., the speed of atmospheric rising and divergent movement reached the maximum, and the atmospheric humidity also reached a larger value. Due to the influence of these two factors, the atmospheric pressure at this time of the day was the lowest.After 16:00, the atmospheric temperature gradually decreased, its humidity decreased, the upward divergent movement weakened, and the atmospheric pressure began to increase;Enter the night;The atmospheric cooling began to decline toward the ground convergence. At 9~10 a.m., the atmospheric convergence decreased and compressed to the maximum extent, and the air density was the largest. At this time, the atmospheric pressure was the highest value of the day.
Annual change
In the same area, the value of atmospheric pressure varies at different times of the year.This is called the annual change of atmospheric pressure.The annual variation of atmospheric pressure can be divided into three types, namely continental type, oceanic type and alpine type.Among them, the annual change of atmospheric pressure of ocean type is just opposite to that of continental type.so called"The atmospheric pressure is higher in winter than in summer", refers to the annual variation of continental atmospheric pressure.The following is a brief analysis (the other two cases will not be discussed).
Because the atmosphere is in an open space around the earth without specific boundaries, it is different from the gas in a closed container.In summer, the temperature in the mainland is higher than that in the ocean, and the atmospheric humidity is also higher (compared with that in winter). In this way, the air on the mainland continues to diffuse to the ocean, resulting in a decrease in its pressure.In winter, the temperature on the continent is lower than that on the ocean, and the air humidity on the continent is also lower than that in summer. In this way, the air on the ocean spreads to the continent, increasing the pressure on the continent.This is why the atmospheric pressure in winter on the mainland is higher than that in summer (the atmospheric temperature is also a factor affecting the atmospheric pressure, but the factors determining the atmospheric pressure change here are not the temperature, but the atmospheric flow and density).
climate change
The atmospheric pressure changes with the climate more often, but the most typical is the change of atmospheric pressure in sunny and cloudy days.There is a saying that“The atmospheric pressure is higher on sunny days than on cloudy days”It reflects the change rule of atmospheric pressure.Generally, the ground continuously sends long waves into the atmosphereEffective radiationAt the same time, the atmosphere is also continuously retroradiating to the ground.In sunny days, the heat on the ground can be transported outwards more smoothly through the upward divergent movement of effective radiation and convective gas layer.On cloudy days, clouds reduce the outward divergent movement of the tropospheric atmosphere.The effect of cloud layer on preserving the surface and heat of liquid layer is called“greenhouse effect”。In this way, the atmospheric expansion in cloudy areas is more severe, which leads to the horizontal outward diffusion of the atmosphere in cloudy areas, reducing the density of the air. At the same time, the atmospheric humidity in cloudy areas is relatively large, which also reduces the density of the atmosphere.Due to the influence of these two factors, the atmospheric pressure in cloudy days is lower than that in sunny days.
The air in the pressure cooker is sealed. When the air in the pressure cooker is heated, the pressure of the gas in the pressure cooker increases, making the water in the pressure cooker boiling at a higher temperature and easier to cook food.The pressure of domestic pressure cooker is generally 1.7 × 10five(114C), or with 1.5 × 10five(110C)、1.3×10five(106C)。
It can be pressed on the wall by the external atmospheric pressure, and can hang things.
Cupping therapy
3. Cupping therapy
There is a kind of glass jar in traditional Chinese medicine. When it is heated, it is quickly pressed on a part of the human body. When the air in the jar cools, it will be pressed on the skin by the external pressure. At this time, pulling out the glass jar forcefully will suck out harmful blood in the human body, which is conducive to recovery.
4. Aircraft flight
The upper part of the wing of the aircraft is streamlined. When air flows over the wing, part of the air flows over the wing and part of the air flows under the wing. Because the upper part of the wing is streamlined, if the air flows over different distances in the same time, the speed will be different. The air velocity above the wing is higher and the atmospheric pressure is lower;The lower part is flat, the air velocity is low, and the atmospheric pressure is high.When the aircraft is traveling at high speed, the atmospheric pressure below the wing is high, while the atmospheric pressure above the wing is low. The pressure difference between the top and bottom of the wing enables the aircraft to obtain lift.
Life experiment proves that atmospheric pressure exists
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Experiment 1: Simulating the Madelberg Hemisphere Experiment
The two leather cups are squeezed mouth to mouth, and then pulled out with both hands. It is found that it requires a greater force to pull out.
The common point of the Madelberg hemispherical experiment and the simulation experiment is that the air in the metal ball and the leather cup is pumped out or squeezed out, so that the pressure of the air in the metal ball and the leather cup is reduced, and the atmospheric pressure outside tightly presses them together, which requires a greater force to pull them apart, which strongly proves the existence of atmospheric pressure.
Experiment 2: "bottle swallowing eggs" experiment
Block the mouth of the wide mouth bottle with a boiled egg that has been peeled off. Before the experiment, gently use your hand to press the egg into the bottle.Then throw the ignited cotton ball into the bottle with fine sand (to prevent burning the bottom of the bottle), quickly plug the cooked egg into the mouth of the bottle, and after the fire goes out, observe that the egg falls into the bottle with a bang.In the above experiment, the air pressure in the bottle will rise due to the burning of cotton, and the sudden cooling will rapidly reduce the air pressure. When the pressure in the bottle is less than the atmospheric pressure outside the bottle, the eggs will be pressed into the bottle under the action of atmospheric pressure.
Experiment 3: "cup covering" experiment
Fill the glass with water, cover the mouth of the glass with a piece of hard paper, press it with your hand, and turn it upside down. After letting go, the whole glass of water is held by a piece of paper, and the paper does not fall off.The experimental glass is filled with water and air is discharged. The downward pressure of the water in the glass on the paper is less than the upward pressure of the atmosphere on the paper, so the paper does not fall off.
Analyzing the above three experiments, it is not difficult to understand the problem of atmospheric pressure.More in-depth research: "bottle swallowing eggs" indicates that the atmosphere has pressure vertically downward, and "cup covering experiment" indicates that the atmosphere has pressure upward.Therefore, it shows the characteristics of atmospheric pressure: the atmosphere has pressure in all directions.
revise
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According to the note on page 8 of New University Chemistry (Science Press, ISBN 978-7-03-0333880-3): "p The original international calibration is 101.325kPa,International Union of Pure and Applied Chemistry(IUPAC) and the new international calibration in 1993 was 100.000kPa."However, the domestic middle school textbooks, including the eight new junior high school physics curriculum standards and the elective module three of the new senior high school physics curriculum standards, have not seen any numerical changes.
