Understanding Pressure Variation with Altitude in a Horizontal Slab of Air

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SUMMARY

The discussion focuses on deriving the expression for the variation of pressure with altitude in a horizontal slab of air. It establishes that the pressure difference, dP, is equal to the product of the air density (ρ), gravitational acceleration (g), and the differential height (dz). The correct approach involves applying Newton's second law to balance the forces acting on the slab, leading to the conclusion that dP/dz = -ρg, where the negative sign indicates that pressure decreases with increasing altitude.

PREREQUISITES
  • Understanding of basic fluid mechanics principles
  • Familiarity with Newton's second law of motion
  • Knowledge of air density variations with altitude
  • Basic calculus for differential equations
NEXT STEPS
  • Study the derivation of hydrostatic pressure equations in fluids
  • Learn about the ideal gas law and its implications for air density
  • Explore applications of pressure variation in meteorology
  • Investigate the effects of altitude on atmospheric pressure and density
USEFUL FOR

This discussion is beneficial for students and professionals in physics, meteorology, and engineering, particularly those interested in fluid dynamics and atmospheric science.

greencheeseca
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"consider a horizontal slab of air whose thickness(height) is dz. if this slab is at rest, the pressure holding it up from below must balance both the pressure from above and the weight of the slab. use this fact to find an expression for dP/dz, the variation of pressure with altitude, in terms of the density of air"

P = \rhogh

(h = dz)

dP = \rhog dz

dP = \rhog
dz
 
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You can't start with P= rho*g*h. That is basically the result the problem is asking you to derive. You need to express the balance of differential forces on the slab of air. In other words, write the force balance equation that corresponds to Newton's second law for the slab.
 

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