101 Atmospheric Pressure.

Slides:



Advertisements
Similar presentations
Chapter 11: Behavior of Gases
Advertisements

Under Pressure. What is Pressure?  Pressure (P) is defined as the amount of force (F) applied per unit area (A) or as the ratio of force to area: P=
L12 - FLUIDS-1 liquids gases sand, snow, or grain (granular materials) FLUIDS  STUFF THAT FLOWS FLUIDS 1.
Air Pressure.
AIR PRESSURE Keeping an Atmosphere Atmosphere is kept by the world’s gravity –Low mass (small) worlds= low gravity =almost no atm. –High mass (large)
L12- FLUIDS [1]  liquids  gases  sand Matter  Comes in three states – solid, liquid, gas  So far we have only dealt with solid objects  blocks,
Air Pressure - #1 element of weather prediction.
Bell Ringer Be sure to put the date on the outside of the page! Copy down the question below and answer it. 1.What is the composition of air?
Chapter 1, Section 3 Pages 25-30
Pressure. Remember, Gas Volume Changes…  Suppose you have a one liter bottle of air. How much air do you actually have?  A liter of air means little.
The Air Around You. Earth’s Atmosphere Earth’s atmosphere is the envelope of gasses that surround the planet. Nitrogen makes up 78% of our air Oxygen.
Chapter 12 Section 2 AIR Pressure.
Units of Pressure (interestingly, while the ‘Bowie’ isn’t one of them, Mercury is related)
Chapter 12: Gravity, Friction, and Pressure
Air Pressure.
Air Pressure 5.4 Ms. De Los Rios 6 th Grade Weather Factors.
Air Pressure Chapter 1, Section 2, p Properties of Air Air has mass – Air is made of atoms & molecules – Atoms & molecules have mass How can you.
AIR PRESSURE Keeping an Atmosphere Atmosphere is kept by the world’s gravity –Low mass (small) worlds= low gravity =almost no atm. –High mass (large)
AIR PRESSURE. What is pressure? A force that is applied over an area.
Air Pressure. Does Air Have Mass?  Air consists of atoms and molecules, which have mass.  Because air has mass, it also has other properties, including.
Air Pressure  The atmosphere is a blanket of gasses around the planet.  The 4 main ways the atmosphere helps us: 1. Provides Oxygen 2. Blocks Harmful.
Gases Chapter 20. Gases Gases and liquids are both fluids – they are substances that flow. Because of this, the behavior of gases is very similar to the.
13.1 Describing the Properties of Gases
Stuff that Flows L12- FLUIDS-1 liquids gases
Fluids & Pressure Objectives:
Chapter 6 Air Pressure. Gale Force Winds Create Waves that Batter Coast in UK in 2002.
15.2 Air Pressure Key Concepts: What are some of the properties of air? What instruments are used to measure air pressure? How does increasing altitude.
Pressure in solids, liquids Atmospheric pressure
Weather! Coulter. Air pressure 0 Because air has mass, it also has other properties, including density and pressure. 0 Density=mass/volume 0 Pressure-
Air Pressure The Force of the Air Above. What is Air Pressure? The force that is exerted on a surface by atoms and molecules. Surface Pressure.
L12 - FLUIDS-1 FLUIDS – liquids – gases sand, snow, or grain (granular materials) While kernels of corn are solid, they behave more like a liquid when.
Weather Part 2: Air Pressure. Air Pressure… is a measure of the force of the air pressing down on Earth’s surface. at any point on the Earth is equal.
Air Pressure Notes. Properties of Air Oxygen (O 2 ) Nitrogen (N 2 ) CO 2, H 2 O, and others.
Chapter 5 Air Pressure. Driving Question What is the significance of horizontal and vertical variations in air pressure?
The weight of Earth’s atmosphere above us exerts a pressure on us. In a way, we live at the bottom of an ocean of air.
Air Pressure.
The Air Around You Section 15:1.
L12- FLUIDS [1] Stuff that flows. liquids gases sand.
L12- FLUIDS [1] Stuff that flows liquids gases sand.
101 Pressure.
AIR PRESSURE.
Bill Nye on Atmosphere
Stuff that Flows L12- FLUIDS-1 liquids gases
L12- FLUIDS [1] Stuff that flows liquids gases sand.
Air and Air Pressure 6C.
Empirical Properties Of Gases
L12- FLUIDS-1 Stuff that flows liquids gases sand.
Air pressure.
Stuff that Flows L12- FLUIDS-1 liquids gases
Pressure in Fluid Systems
Air Pressure Notes.
L12- FLUIDS [1] Stuff that flows. liquids gases sand.
Air Pressure Notes.
Does air have mass?.
Pressure Chapter 43.
Air Pressure Notes.
Air Pressure.
Air Pressure.
At ppt Air Pressure Section 1.3 Pages
Air Pressure Notes.
The Nature of Gases Under Pressure!
Forces in Fluids.
Air Pressure.
Chapter 5 Air Pressure.
Unit: Water and the Atmosphere Lesson 2: Air Pressure
L12- FLUIDS [1] Stuff that flows liquids gases sand.
Aim: How can we explain air pressure?
Chapter 1.2 Air Pressure pgs 10-14
Chapter 11 Gases and Pressure Section 1.
Air Pressure.
Presentation transcript:

101 Atmospheric Pressure

What is atmospheric pressure? How measured? Why is it important?

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

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

Pressure is isotropic (independent of direction)

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

YES…because Pressure is isotropic

What about a tank of water with a hole in the side? Lower p Higher p A good illustration of isotropic nature of pressure

Pressure Units 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 in2 puts a pressure on the floor of P= 150 lb/(30 in2) = 5 lb in-2

Knowing about pressure can help you avoid this

Ski Example Two cross country skis, each 125 in2 Total area 250 in2, 150 lb skier P=F/A= 150lb/250 in2 or 3/5 lb in2 Skis result in less pressure! Less chance of sinking in snow!

You don’t notice atmospheric pressure but it is there.

Collapsing Can Demo (Done Live in Sections!)

Collapsing Can Collapsing Barrel Collapsing Tanker Car

Atmospheric pressure at any point is dependent on the weight of the 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 P1 > P2 > P3

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

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 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. The result: more pressure!

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

Mercury Barometers 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 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

Mercury is dangerous…

Why not use water in a barometer?

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 push liquid up the straw, thus the straw can’t be more than 30 ft long

Aneroid Barometer

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

Barograph—another form of aneroid barometer

barogram

Altimeter: Aneroid Barometer Inside

Solid State Pressure Sensor

Solid State Pressure Sensor The electrical properties of some materials are dependent on atmospheric pressure.

Smartphone Pressure Sensor

This app collects pressure

Pressure Units Review Remember P= F/A So inches of mercury are not really units of pressure. English units: lbs per square inch (psi). Average sea level pressure is 14.7 lbs per inch2 . Car tires, 30-35 psi. Bicycles 35-80 psi Meteorologists DON’T USE psi

Pressure Units They use metric units. Newton per meter squared (N/m2). 1 Nm-2 is called a Pascal. Newton is the unit of force in the metric system. Meteorologists frequently use hectopascals (hPa). hPa is 100 Pascals or 100 Nm-2 Another favorite is millibar (mb), which is another name for hectopascals.

What is the pressure typically at sea level?

Sea level is considered zero elevation in meteorology Mean sea level pressure=1013.25 hPa= 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 hPa or mb

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

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 hPa ~ 0 ft, 0 km ASL 850 hPa ~5000 ft, 1.5 km ASL 500 hPa ~18,000 ft, 5.5 km ASL 250 hPa ~34,000 ft, 10.5 km ASL

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