Pressure measures force per unit area, with SI units of pascals (1 Pa = 1 N/m2). As elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Atmospheric pressure, atm Definition:ĭefinition of Atmospheric pressure, atm provided by WikiPedia:Ītmospheric pressure, sometimes also called barometric pressure, is the pressure within the atmosphere of Earth (or that of another planet). One millimeter of mercury is approximately 1 torr, which is 1/760 of standard atmospheric pressure (101325760 =133.322368421 pascals). Although not an SI unit, the millimeter of mercury is still routinely used in medicine, meteorology, aviation, and many other scientific fields. Millimeter of mercury, mmhg Definition:ĭefinition of Millimeter of mercury provided by wikipedia: Conversion formulas Kilopound per square inch , Pounds per square inch (ksi, psi): Thus, when some liquid evaporates, the molecules left behind have a lower average energy, and the liquid has a lower temperature.Online pressure converter. The highest-energy molecules are those that can escape from the intermolecular attractions of the liquid. Liquids, like gases, have a distribution of molecular energies. We can also now take another look at evaporative cooling, which we discussed in the chapter on temperature and heat. Thus, over the billions of years that Earth has existed, far more hydrogen and helium molecules have escaped from the atmosphere than other molecules, and hardly any of either is now present. Only a few exceed escape velocity, but far fewer heavier molecules do. Because of the lower mass of hydrogen and helium molecules, they move at higher speeds than other gas molecules, such as nitrogen and oxygen. Why are those elements so rare in our atmosphere? The answer is that gas molecules that reach speeds above Earth’s escape velocity, about 11 km/s, can escape from the atmosphere into space. Moreover, helium is constantly produced on Earth by radioactive decay. Hydrogen is by far the most common element in the universe, and helium is by far the second-most common. We can now gain a qualitative understanding of a puzzle about the composition of Earth’s atmosphere. Substitute the known quantities, along with their units, into the appropriate equation and obtain numerical solutions complete with units. Or if approximating an integral, use the method asked for in the problem. Note that if you are taking a ratio of values of the distribution function, the normalization factors divide out. Solve the appropriate equation for the ideal gas law for the quantity to be determined (the unknown quantity). Determine whether you need the distribution function for velocity or the one for energy, and whether you are using a formula for one of the characteristic speeds (average, most probably, or rms), finding a ratio of values of the distribution function, or approximating an integral. In many cases, though, using R and the molar mass will be more convenient than using and the molecular mass. Convert known values into proper SI units (K for temperature, Pa for pressure, for volume, molecules for N, and moles for n). Identify exactly what needs to be determined in the problem (identify the unknown quantities). Make a list of what quantities are given or can be inferred from the problem as stated (identify the known quantities). Examine the situation to determine that it relates to the distribution of molecular speeds. Problem-Solving Strategy: Speed Distribution
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