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Physics For Scientists & Engineers
Found in: Page 447

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Short Answer

Show that the variation of atmospheric pressure with altitude is given byP=Poe-ay, where a α=ρogPo, Pois atmospheric pressure at some reference levely=0, and role="math" localid="1663776653925" ρo is the atmospheric density at this level. Assume the decrease in atmospheric pressure over an infinitesimal change in altitude (so that the density is approximately uniform over the infinitesimal change) can be expressed from Equation 14.4 asrole="math" localid="1663776670018" dp=-ρgdy. Also assume the density of air is proportional to the pressure, which, as we will see in Chapter 20, is equivalent to assuming the temperature of the air is the same at all altitudes.

It is proved that the variation of atmospheric pressure with altitude is given by P=Poe-ay .

See the step by step solution

Step by Step Solution

Step 1: Pressure:

Pascal's law states that when pressure is applied to a confined fluid, the pressure is transmitted undiminished to every point in the fluid and to every point on the walls of the container.

The pressure in a fluid at rest varies with depth h in the fluid according to the expression

P=Po+ρgh

Where, Pis the absolute pressure,Po is the atmospheric pressure and ρis the density of the fluid, assumed uniform, gis the gravitational acceleration constant, and h is the height.

Step 2: Prove that the variation of atmospheric pressure with altitude is given byP=Poe-ay :

The incremental version of P=Po+ρghis dP=-ρgdy.

Assume that the density of air is proportional to pressure, or Pρ=P0ρ0.

Combining these two equations you have,

dP=-Pρ0P0gdy

Integrating both the above sides,

popdPP=-gρ0P00ydylnPpop=-gρ0P0ylnPPo=-ρ0gyP0

Defining α=ρ0gP0 then gives P=Poe-ay.

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