101 Pressure What is atmospheric pressure Where measured

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101 Pressure

101 Pressure

What is atmospheric pressure? Where measured? Why important?

What is atmospheric pressure? Where measured? Why important?

When T> 0 K, atoms and molecules are moving • When they hit an

When T> 0 K, atoms and molecules are moving • When they hit an object they provide a small push and apply a small force to the object

Convenient to talk in terms of pressure rather than forces Pressure = force area

Convenient to talk in terms of pressure rather than forces Pressure = force area P= F A

Pressure is isotropic (independent of direction)

Pressure is isotropic (independent of direction)

Atmospheric pressure is due to the weight of the atmosphere above the point in

Atmospheric pressure is due to the weight of the atmosphere above the point in question So, is there atmospheric pressure under a table? ?

What about a tank of water with a hole in the side? Lower p

What about a tank of water with a hole in the side? Lower p Higher p

Pressure • Units of force per unit area • E. g. , pounds (lb)

Pressure • Units of force per unit area • E. g. , pounds (lb) per square inch, newtons (N) per square meter. • So, a 150 lb man standing on shoes with an area of 30 in 2 puts a pressure on the floor of P= 150 lb/(30 in 2) = 5 lb in-2

How to use pressure concepts to avoid this

How to use pressure concepts to avoid this

Ski Example • Two cross country skis, each 125 in 2 • Total area

Ski Example • Two cross country skis, each 125 in 2 • Total area 250 in 2, 150 lb skier • P=F/A= 150 lb/250 in 2 or 3/5 lb in 2 • Less pressure! Doesn’t fall in.

Collapsing Can Demo

Collapsing Can Demo

Collapsing Barrel Collapsing Tanker Car

Collapsing Barrel Collapsing Tanker Car

Atmospheric pressure at any point is dependent of the weight of air above that

Atmospheric pressure at any point is dependent of the weight of air above that point P= F/A = W/A W= weight of air in the column above the point

Pressure decreases with height because there is less air above, so less weight P

Pressure decreases with height because there is less air above, so less weight P 3 P 2 P 1 > P 2 > P 3

Pressure Does NOT Decrease at the Same Rate with Elevation Falls off more quickly

Pressure Does NOT Decrease at the Same Rate with Elevation Falls off more quickly near the surface

Why? Because the atmosphere is compressible and the density of air molecules is greater

Why? Because the atmosphere is compressible and the density of air molecules is greater near the surface

How does this fit with our discussion of pressure caused by the forces of

How does this fit with our discussion of pressure caused by the forces of molecules impacting on a surface? • Near the surface, there is more density, so more molecules are available to impact a surface • The temperature is also warmer near the surface, so they are going faster.

How do we measure pressures? With barometers. Three types • Mercury • Aneroid •

How do we measure pressures? With barometers. Three types • Mercury • Aneroid • Electronic pressure sensor

Mercury • Invented by Evangelista Torricelli, a student of Galileo, around 1645 AD •

Mercury • Invented by Evangelista Torricelli, a student of Galileo, around 1645 AD • To make one you need a long tube around 40 inches long filled with mercury, and a dish of mercury

Mercury Barometer Why doesn’t the mercury all flow out of the tube? Atmospheric pressure

Mercury Barometer Why doesn’t the mercury all flow out of the tube? Atmospheric pressure

Mercury Barometer • If atmospheric pressure increases, the height of the column of mercury

Mercury Barometer • If atmospheric pressure increases, the height of the column of mercury increases, and vice versa • How high? At sea level, on average about 29. 92 inches or 76 cm (one inch=2. 54 cm) • So when a TV weatherperson says the pressure is 30. 06 inches…that is how tall a mercury column atmospheric pressure can support

Why not use water in a barometer?

Why not use water in a barometer?

It would have to be roughly 30 ft tall! • Water is much less

It would have to be roughly 30 ft tall! • Water is much less dense than mercury, so would need a much higher barometer. • Related question: what is the size of the largest straw that would work?

Straws use atmospheric pressure to work, thus the straw can’t be more than 30

Straws use atmospheric pressure to work, thus the straw can’t be more than 30 ft long

Aneroid Barometer

Aneroid Barometer

No mercury, pressure changes the thickness of an air tight chamber/diaphragm

No mercury, pressure changes the thickness of an air tight chamber/diaphragm

Barograph—another form of aneroid barometer

Barograph—another form of aneroid barometer

barogram

barogram

Altimeter: Aneroid Barometer Inside

Altimeter: Aneroid Barometer Inside

Solid State Pressure Sensor

Solid State Pressure Sensor

Smartphone Pressure Sensor

Smartphone Pressure Sensor

Pressure Units • Remember P= F/A, so inches of mercury are not really units

Pressure Units • Remember P= F/A, so inches of mercury are not really units of pressure. • Needs to force per unit area • English units: lbs per square inch (psi). Average sea level pressure is 14. 7 lbs per inch 2. Car tires, 30 -35 psi. Bicycles 35 -80 psi • Meteorologists DON’T USE psi

Pressure Units • Metric units. Newton per meter squared (N/m 2). 1 Nm-2 is

Pressure Units • Metric units. Newton per meter squared (N/m 2). 1 Nm-2 is called a Pascal. • Newton is the unit of force in the metric system. • Meteorologists love to use hectopascals (h. Pa). h. Pa is 100 Nm-2 or 100 Pascals • Another favorite is millibar (mb), which is another names for hectopascals.

Sea Level is considered zero elevation in meteorology • Mean sea level pressure=1013. 25

Sea Level is considered zero elevation in meteorology • Mean sea level pressure=1013. 25 h. Pa= 1013. 25 millibars (mb) =14. 7 lbs in-2 • Same as 29. 92 inches of mercury or 76 cm of mercury or 760 mm of mercury • Meteorologists generally use h. Pa or mb

Surface weather maps plot sea level pressure-–the pressure at mean sea level (h. Pa/mb)

Surface weather maps plot sea level pressure-–the pressure at mean sea level (h. Pa/mb)

Pressure often used instead of height in meteorology • Instead of using physical height

Pressure often used instead of height in meteorology • Instead of using physical height (in meters or feet), meteorologists often use pressure instead • Pressure decreases with height – 1013 h. Pa – 850 h. Pa – 500 h. Pa – 250 h. Pa ~ 0 ft, 0 km ASL ~5000 ft, 1. 5 km ASL ~18, 000 ft, 5. 5 km ASL ~34, 000 ft, 10. 5 km ASL

Pressures • • • Snoqualmie Pass ~ 900 h. Pa Top of Mount Rainier

Pressures • • • Snoqualmie Pass ~ 900 h. Pa Top of Mount Rainier ~ 600 h. Pa Highest human settlement ~ 530 h. Pa Jet cruising altitude ~ 230 h. Pa (35 K ft) An altimeter is a device that uses pressure change to give elevation. Many smartphones have altimeter software using the on-board pressure sensor.