| Basic Thermodynamics ~ J. Pohl © | www.THERMOspokenhere.com (64-B100) |
A properly installed barometer can be used with the hydrostatic equation to quantify the pressure of the atmosphere. With the barometer, according to Torricelli's discovery, the pressure in the void above mercury is zero (the first quantified pressure).
Earth-surface atmospheric pressure varies with time (at a
location) and with location (at the same time). A spatial and
temporal average value is useful for calculations.
We let the hydrostatic principle start "in the void" and be applied along a path downward through liquid mercury to the atmosphere (at zero elevation and sea level). Thus the magnitude of term (1) (previous page) is zero and the result of the calculation will be the pressure of the atmosphere.
pvacuum(z* = 0.76m) + ρmercurygo(0.76m) = patmosphere
To proceed, the acceleration of gravity and the density of mercury must be entered and with this contrived example, these numbers need to be precise.
~ 0 + 13,595(kg/m3)(9.81m/s2) (0.76m) = 101,300 N/m2
Next to the assumed "zero pressure" of the mercury vapor in the "void" of a barometer, the most commonly assumed pressure is "Standard Atmosphere." Measurements of atmospheric pressure have been recorded at sea level all over Earth for centuries and have been found (except during storm conditions) to vary slightly from the average value, 101.3 kPa (or 14.7 psi).
| Standard Atmosphere | ||
| T = 25°C | p = 101.3 kPa | ρ = 1.2 kg/m3 |
| T = 80°F | p = 14.7 lbf/ft2 | ρ = 0.075 lbm/ft3 |
In the absence of a specified pressure, engineers assume the above values to be standard for use at the Earth surface. Our next illustration begins with the assumption of standard atmospheric pressure then uses the hydrostatic principle to approximate a storm surge.
A properly installed barometer can be used with the hydrostatic equation to quantify the pressure of the atmosphere. With the barometer, according to Torricelli's discovery, the pressure in the void above mercury is zero (the first quantified pressure).
We let the hydrostatic principle start "in the void" and be applied along a path downward through liquid mercury to the atmosphere (at zero elevation and sea level). Thus the magnitude of term (1) (previous page) is zero and the result of the calculation will be the pressure of the atmosphere.