Air Pressure and Water Pressure

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SUMMARY

The discussion centers on calculating the depth at which water pressure reaches 4.5 atm in a freshwater lake, given an air pressure of 1.1 atm at the surface. The correct approach involves using the equation Pgauge = Dgh, where D is the density of water (1000 kg/m³) and g is the acceleration due to gravity (9.8 m/s²). The user initially calculated the depth as 45.92 meters but failed to account for the atmospheric pressure, which must be included in the total pressure calculation. The correct depth is derived from the absolute pressure, requiring an adjustment to the initial calculations.

PREREQUISITES
  • Understanding of fluid mechanics principles, specifically pressure calculations.
  • Familiarity with the equation Pgauge = Dgh for calculating gauge pressure.
  • Knowledge of unit conversions, particularly between atmospheres and Pascals.
  • Basic understanding of the properties of freshwater, including its density (1000 kg/m³).
NEXT STEPS
  • Review the concept of absolute pressure versus gauge pressure in fluid mechanics.
  • Learn about hydrostatic pressure and its applications in various fluid scenarios.
  • Practice unit conversions between different pressure units, such as atm and Pa.
  • Explore additional examples of pressure calculations in fluids, including varying densities.
USEFUL FOR

This discussion is beneficial for students studying physics or engineering, particularly those focusing on fluid mechanics, as well as educators looking for practical examples of pressure calculations in real-world scenarios.

vrobins1
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Homework Statement



"At the surface of a freshwater lake the air pressure is 1.1 atm. At what depth under water in the lake is the water pressure 4.5 atm?"



Homework Equations



I used the equation Pgauge = Dgh



The Attempt at a Solution



I converted my atm to Pascals--> (4.5)(100000) = 450000 Pa
Pguage = Dgh
Pguage = (1000)(9.8)(h)

-I used 1000, the density of water, but I'm not sure that is the right info to use for D!

Then I solved for h.

h = 450000/ (10000x9.8)
h = 45.92
I got it incorrect though. Can anyone offer any insight? Thanks!
 
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vrobins1 said:

Homework Statement



"At the surface of a freshwater lake the air pressure is 1.1 atm. At what depth under water in the lake is the water pressure 4.5 atm?"



Homework Equations



I used the equation Pgauge = Dgh



The Attempt at a Solution



I converted my atm to Pascals--> (4.5)(100000) = 450000 Pa
Pguage = Dgh
Pguage = (1000)(9.8)(h)

-I used 1000, the density of water, but I'm not sure that is the right info to use for D!

Then I solved for h.

h = 450000/ (10000x9.8)
h = 45.92
I got it incorrect though. Can anyone offer any insight? Thanks!
That would have been OK if the problem asked for the gauge pressure. I think it's looking for the depth at an absolute pressure of 4.5 atm, which includes the atmospheric pressure.
 

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