Measuring Pressure. What does zero pressure look like? There is no force applied to an area If atmospheric pressure, there are no particles pushing on

Measuring Pressure

What does zero pressure look like?•There is no force applied to an area

•If atmospheric pressure, there are no particles pushing on the surface

•We call this a vacuum

Two types of pressure measurements

1)Gauge Pressure• Assumes that atmospheric

pressure is zero pressure• Measures the difference

between atmospheric pressure and an object with additional pressure

Calculating Gauge Pressure

•Gauge pressure = Absolute pressure – atmospheric pressure

•pg = pabs - patm

Two types of pressure measurements

2) Absolute Pressure• Assumes that zero

pressure is zero pressure• Measures the total

pressure relative to a vacuum

Calculating Absolute Pressure

•Absolute pressure = Gauge pressure + atmospheric pressure

•pabs = pg + patm

Absolute Pressure is zero

Gauge Pressure is zero

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Ex 1 – Note: patm = 101kPa

You come out in the morning and your car’s tire is flat. You read the side of the tire and it says “Maximum capacity 80kPa”

a) What is the initial gauge pressure in the flat tire?

b) What will the gauge read after you fill the tire?

c) What is the absolute pressure in the tire before and after the tire is filled?

Instruments•Barometer – Measures atmospheric

pressure•Aneroid barometer –Does not use a liquid

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Instruments

•Altimeter – Measures altitude using the value of atmospheric pressure

Static Pressure Head

•Height of a column of fluid that produces a specific pressure

Calculating Pressure From Height of a Liquid

pg = Dhg

D – Density of the liquid (kg/m3)Dwater = 1000kg/m3

h – height of the liquid (m)g – Acceleration due to gravity (g = 9.81N/kg)

Ex 2:

•If you dive to the bottom of a swimming pool that is 10m deep

a) What will be the gauge pressure be on your measuring instrument?

b) What is the absolute pressure acting on you?

Hydroelectric Dam•Greater the depth of a liquid, the greater

the pressure•Gravity pulls down the water, increasing

the pressure on the bottom of the dam

Thickest where pressure is greatest

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‘Water seeks it’s own level’

•In an open container, all water will settle at the same level

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Work

•Pg 235 # 21, 23•Pg 236 # 3,7,10,11, 14•Pg 244 # 5-7,9,10

Documents

Measuring Pressure. What does zero pressure look like? There is no force applied to an area If atmospheric pressure, there are no particles pushing on