The atmosphere exerts pressure on objects in it from all directions. Atmospheric pressure is referred to as atmospheric pressure. The instrument that measures atmospheric pressure is called a barometer, and a common one is a mercury barometer. One standard atmospheric pressure (1atm) = 760 millimeters of mercury (mmHg).
Liquid pressure calculation formula: P=ρgh
The standard atmospheric pressure on the ground is approximately equal to the pressure produced by a 760 mm high mercury column. Due to the influence of conditions such as the measurement area, the measured values are different.
According to the formula of liquid pressure P=ρgh, the density of mercury is 13.6×10^3 kg/cubic meter, so the standard atmospheric pressure generated by a 76 cm high mercury column is:
P =13.6×10^3kg/cubic meter×9.8 Newton/kg×0.76m
1mmHg=1.01325*10^5Pa/760=133.32pa
The earth is surrounded by a thick layer of air, which is mainly composed of a mixture of nitrogen, oxygen, carbon dioxide, water vapor and helium, neon, argon and other gases. This entire layer of air is usually called the atmosphere. It is sparse The lower ground is densely distributed around the earth, with a total thickness of 1,000 kilometers. All objects immersed in the atmosphere are subject to the pressure of the atmosphere, just like objects immersed in water are subject to the pressure of water.
Drinking through a straw is because of the strong atmospheric pressure
The causes of atmospheric pressure can be explained from different perspectives. The main thing mentioned in the textbook is: air is affected by gravity, and air is fluid, so there is pressure in all directions. To be more detailed, due to the attraction of the earth to the air, the air pressure on the ground must be supported by the ground or other objects on the ground. These objects and the ground that support the atmosphere are affected by atmospheric pressure. The atmospheric pressure on a unit area is the atmospheric pressure; secondly, it can be explained from the perspective of molecular motion because gas is composed of a large number of molecules that move irregularly, and these molecules must constantly exert pressure on objects immersed in the air. A collision occurs. Every time a collision occurs, gas molecules give an impact force to the surface of the object. The result of the continuous collision of a large number of air molecules is reflected as the pressure of the atmosphere on the surface of the object, thus forming atmospheric pressure. If there are more molecules per unit volume, the number of collisions between air molecules per unit area of the surface of the object will be greater within the same period of time, and thus the greater the pressure generated.
The viewpoint of molecular kinetic theory can be used to explain why the uneven distribution of the atmosphere can cause high and low atmospheric pressure.
standard atmospheric pressure
Atmospheric pressure not only changes with altitude, but is not fixed at the same location. The atmospheric pressure of 1.01325×10^5 Pa is usually called the standard atmospheric pressure. It is approximately equivalent to the pressure generated by a 760mm mercury column. Standard atmospheric pressure can also be called 760mm mercury column atmospheric pressure. .
The value of standard atmospheric pressure is often taken as 1.013×10^5 Pa (101KPa) in general calculations, and can also be taken as 10^5Pa (100KPa) in rough calculations.
p=F/S (when both use the International System of Units, the unit is pa)
When the stress area is constant, the greater the pressure, the more obvious the effect of pressure. (At this time, the pressure is proportional to the pressure.) When the pressure remains unchanged, increasing the force-bearing area can reduce the pressure; decreasing the force-bearing area can increase the pressure. (At this time, the pressure is inversely proportional to the force-bearing area)
p=ρgh ( p liquid=F/S=G/S=mg/S=ρ liquid Vg/S=ρ liquid Shg/S=ρ liquid hg=ρ liquid gh)
(1) The liquid has pressure on the bottom and side walls of the container, and there is pressure inside the liquid in all directions.
(2) The pressure of a liquid increases with depth. At the same depth inside the same liquid, the pressure of the liquid in all directions is equal; for different liquids, the pressure generated at the same depth is related to the density of the liquid. The higher the density, the greater the pressure. The larger, the greater the pressure of the liquid.
Atmospheric pressure and altitude
The density of the air layers above the earth is not equal. The air near the surface is denser, while the air at upper levels is thinner and less dense. Since the atmospheric pressure is generated by the gravity of the air, where the height is high, the height of the air column above it is small. , the density is also small, so the higher the distance from the ground, the smaller the atmospheric pressure.
Within an altitude of 3000m, the atmospheric pressure decreases by approximately 100Pa for every 10m of elevation. Within an altitude of 2000m, the atmospheric pressure decreases by approximately 1mmHg for every 12m of elevation.
The air on the ground has a very wide range and is often called "atmosphere". More than 200 kilometers above the ground, there is still air. Although its density is very small, the pressure exerted on the ground by such a high atmospheric column is still extremely high. The human body does not feel any pressure from the air pressure in the atmosphere. This is because the inside and outside of the human body are affected by the air pressure at the same time and are exactly equal.
The relationship between gas pressure and volume
The gas pressure mentioned here does not refer to the atmospheric pressure, but refers to the pressure of a certain mass of gas.
Since the pressure of a gas is essentially caused by the continuous collision between a large number of gas molecules that move irregularly and the container wall, when other conditions remain unchanged, a decrease in gas volume will increase the number of collisions between gas molecules and the container wall. causing the pressure to increase.
When the temperature remains constant, the smaller the volume of a certain mass of gas, the greater the pressure; the larger the volume, the smaller the pressure.
The pump is made using this principle.
Factors affecting gas pressure in closed containers
The pressure of a certain amount of closed gas is related to its volume, temperature and other factors. It can be expressed specifically as: PV=nRT; where P represents the gas pressure, V represents the total volume of the gas, n represents the molecular weight of the gas, R is a constant, and T is The temperature of the gas. This can also be confirmed, "When the temperature remains constant, the smaller the volume of a certain mass of gas, the greater the pressure; the larger the volume, the smaller the pressure."
Relationship between boiling point and atmospheric pressure
Experiments have shown that the boiling point of all liquids decreases when the air pressure decreases and increases when the air pressure increases. The boiling points of the same liquid are not fixed. When saying that the boiling point of water is 100°C, it must be emphasized that it is under standard atmospheric pressure.
Since air pressure decreases with altitude, the boiling point of water decreases with altitude. For example, the boiling point of water at an altitude of 1,000 meters is about 97°C, and at an altitude of 3,000 meters, it is about 91°C. At the top of Mount Everest, which is 8,844.43 meters above sea level, water can boil at 72°C. Therefore, when cooking in high mountains, you need to use an airtight pressure cooker. The air pressure in the pot can be higher than the standard atmospheric pressure, so that the boiling point of water is higher than 100°C. Not only will the rice cook quickly, but it can also save fuel.
The relationship between fluid pressure and flow velocity
The relationship between fluid pressure and flow rate: In gases and liquids, the greater the flow rate, the smaller the pressure (i.e. Bernoulli's principle). The lift of the aircraft: the air flow speed above the wing is high and the pressure is low; the air flow speed below the wing is low and the pressure is high. This pressure difference creates a pressure difference, causing the aircraft to obtain vertical upward lift.
Collapse and edit this paragraph. Standard atmospheric pressure fitting formula
According to the "1964 ICAO standard atmospheric data adopted by the International Civil Aviation Organization, the fitting formula of the standard atmospheric pressure is as follows:
r is the distance from the location to the center of the earth, r is in kilometers, and the pressure unit is Pascal. The practical range of the formula is from 400 meters below sea level to 32 kilometers above sea level. The error is within 2 and -1 Pascal. .
The piston type water pump uses the movement of the piston to discharge air, causing the difference in internal and external air pressure to cause the water to rise and be pumped out under the action of air pressure. When the piston is pressed down, the water inlet valve closes and the exhaust valve opens; when the piston is raised, the exhaust valve opens. The valve is closed and the water inlet valve is opened. Under the action of external atmospheric pressure, water flows out from the water inlet pipe through the water inlet valve and from the outlet above. In this way, the piston reciprocates up and down in the cylinder, continuously pumping out the water.
How centrifugal water pumps work
Before starting the water pump, fill the pump casing with water and discharge the air in the pump casing. When started, the impeller rotates at high speed driven by the motor, and the water in the pump casing also rotates at high speed with the impeller, and is thrown into the outlet pipe due to centrifugal force. At this time, the pressure near the impeller decreases, and the atmospheric pressure causes the water in the lower part to push open the bottom valve. Along the pump casing of the water inlet pipe, the incoming water is thrown by the impeller into the outlet pipe. This cycle continues, and the water is continuously pumped to high places.
Piston pumps and centrifugal pumps both use atmospheric pressure to pump water up. Because atmospheric pressure has a certain limit, the pump's water pumping head - the height difference from the water surface to the pump - also has a certain limit, which does not exceed 10.334 meters. Of course, the actual lift is much greater than this height, because the water is pumped to the water pump and then "thrown" upward by the pump, which can reach a very high height.
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