What is standard atmospheric pressure?

With the development of science and technology, the standard atmospheric pressure has changed several times. At first, the atmospheric pressure at sea level of 0℃ and latitude of 45 in sunny days was defined as standard atmospheric pressure, which was equivalent to about 76 cm Hg. Later, it was found that the air pressure value under this condition was unstable and changed under the influence of wind, temperature and other conditions. Therefore, the height of 76 cm Hg is defined as the standard atmospheric pressure. However, it was later found that the pressure value of 76 cm Hg was also unstable, and the density of mercury changed under the influence of temperature. The value of g also varies with latitude.

In order to ensure that the standard atmospheric pressure is a constant value, 1954, the resolution of the tenth international metrology conference stipulates that the standard atmospheric pressure value is

1 standard atmospheric pressure = 10 1325 Newton/square meter.

"Teaching Resources" Junior Middle School Physics Volume 1

Standard atmospheric pressure (QNE): refers to the air pressure at sea level under standard atmospheric conditions. Its value is 10 13.2 hectopascals (or 760 mm Hg high or 29.92 inches Hg high).

The earth is surrounded by thick air, which is called the atmosphere. Air can flow freely like water, and it is also influenced by gravity. So there is pressure in all directions in the air, and this pressure is called atmospheric pressure. 1654, Gehrig made a famous experiment in the Madeborg hemisphere in Germany, which made people have a deep understanding of atmospheric pressure, but people still don't know how big it is. 1 1 years later, Italian scientist Torricelli filled an 80 cm long thin glass tube with mercury and put it in a sink filled with mercury. He found that the mercury in the glass tube dropped by about 4 cm, and then stopped falling. There is no air in this 4 cm space, it is a vacuum. Torricelli deduced from this that the pressure of the atmosphere is equal to the length of the mercury column. According to the pressure formula, scientists accurately calculated the atmospheric pressure in the standard state as1.01×105pa.

The change of atmospheric pressure is related to altitude. Atmospheric pressure is produced by gravity in the atmosphere. The higher the ground, the thinner the atmosphere, and the smaller the atmospheric pressure should be. However, because the air density related to the gravity received by the atmosphere varies unevenly with height, the atmospheric pressure decreases unevenly with height.

The change of atmospheric pressure is also related to the weather. At different times, the atmospheric pressure in the same place is not exactly the same. We know that the density of water vapor is less than that of air. When there is more water vapor in the air, the air density will become smaller and the atmospheric pressure will also decrease. Generally speaking, the air pressure in rainy days is smaller than that in sunny days, and it is found that the sudden drop of air pressure in sunny days is a precursor to rain; However, after several days of continuous rain, it is found that the air pressure has become larger, and it can be expected that it will clear up soon. In addition, the change of atmospheric pressure is also related to temperature. Because the air density becomes smaller at high temperature, the atmospheric pressure is smaller at high temperature than at low temperature.

Atmospheric pressure is not fixed. In order to compare atmospheric pressure, at the 10th International Metrology Conference in 1954, scientists stipulated a "standard" for atmospheric pressure: at the sea level of latitude 45 and the temperature of 0℃, the pressure generated by a mercury column with a height of 760mm is called the standard atmospheric pressure. Since it is a "standard", we should pay attention to the accuracy of each physical quantity according to the formula of liquid pressure. According to relevant data, the density of mercury at 0℃ is 13.595× 103 kg/m3, and the g value at sea level at latitude 45 is 9.80672 N/kg. Therefore, it can be concluded that the pressure generated by a 760mm high mercury column is

P mercury = ρ mercury gh

= 13.595× 103kg/m3×9.80672N/kg×0.76m

= 1.0 1325× 105 Pa。

This is the value of 1 standard atmospheric pressure, and it is recorded as 1atm.

In recent scientific work, for the sake of convenience, another standard atmospheric pressure 1 is defined as 100kPa and recorded as 1bar. Therefore, reference to standard atmospheric pressure can also refer to 100kPa.