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Class 11 Physics
Mechanical Properties of Fluids
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Absolute Pressure

  • Absolute pressure is defined as the pressure above the zero value of pressure.
  • It is the actual pressure which a substance has.
  • It is measured against the vacuum.
  • Absolute pressure is measured relative to absolute zero pressure.
  • It is sum of atmospheric pressure and gauge pressure.
  • P =Pa+hρg where P = pressure at any point, Pa = atmospheric pressure and hρg= gauge pressure.
  • Therefore P =Pa + Gauge Pressure. Where P = absolute pressure.
  • It is measured with the help of barometer.

Problem:  The density of theatmosphere at sea level is 1.29 kg/m3.Assume that it does not change withaltitude. Then how high would theatmosphere extend?

Answer:

From equation: - P=Pa+ρgh

ρgh = 1.29 kg m–3 × 9.8 m s2 × h m = 1.01 × 105 Pa

∴ h = 7989 m ≈ 8 km

In reality the density of air decreases withheight. So does the value of g. The atmospheric

cover extends with decreasing pressure over100 km. We should also note that the sea level

atmospheric pressure is not always 760 mm ofHg. A drop in the Hg level by 10 mm or more isa sign of an approaching storm.

Problem:- At a depth of 1000 m in anocean (a) what is the absolute pressure?(b) What is the gauge pressure? (c) Findthe force acting on the window of 20 cm × 20 cm of a submarine at this

depth, the interior of which is maintainedat sea-level atmospheric pressure. (Thedensity of sea water is 1.03 × 103 kg m-3,g = 10m s–2.)

Answer:

Here h = 1000 m and ρ = 1.03 × 103 kg m-3.

(a) From Eq. P2 − P1= ρgh, absolute pressure

P = Pa + ρgh

= 1.01 × 105 Pa+ 1.03 × 103 kg m–3 × 10 m s–2 × 1000 m

= 104.01 × 105 Pa

≈ 104 atm

(b) Gauge pressure is P − Pa = ρgh = Pg

Pg = 1.03 × 103 kg m–3 × 10 ms2 × 1000 m

= 103 × 105 Pa

≈ 103 atm

(c) The pressure outside the submarine isP = Pa + ρgh and the pressure inside it isPa. Hence, the net pressure acting on thewindow is gauge pressure, Pg = ρgh. Sincethe area of the window is A = 0.04 m2, theforce acting on it is

F = Pg A = (103 × 105 Pa) × 0.04 m2 = 4.12 × 105 N

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