cahier physique - physics workbook

PART I: PRESSURE - MOLECULAR PERSPECTIVE - KINETIC THEORY

1) READ
AIr pressure or gas pressure in a container is due to the collision of the molecules between them and against the wall of the container. Molecules are constantly in motion, they move in straight line until they collide with an object or the wall of a container. The effect of the collisions is the pressure. We live in a large container filled with air molecules. We don't feel the pressure of air around us unless the air pressure changes.

The air pressure is enormous. about 1kg/cm² or about 15pounds/in².

How come we are not crushed by this pressure ? (remember pressure is a force per unit area).

Air pressure is equivalent to 65 players pile on top of you after being tackled.

clue: our body is permeable to air

2) Our senses are only sensible to pressure changes. THe only time we are aware of air pressure is when it changes. You may have noticed a change in air pressure in your ears when driving on a mountain road. See the picture. As you ascend the pressure outside decreases and is smaller than the pressure inside your body. Your ear drum is pushed toward the outside. This produces pain. To releave the pain you can yawn so the small Eustachien tuv\be opens allowing the air to rush out of the middle ear through your nose or mouth. The ears pop as the pressure is balanced again.

WE notice the effects of air pressure by noticing its effect only when a difference in air pressure (pressure differential) exists. You will find out about the effects during demonstrations and experiments.

watch this U-tube this video to understand how molecules behave in a container
and how collision explain pressure. See also this model from Berkeley college.

2) read then answer questions
A) THE ATMOSPHERE
EARTH ATMOSPHERE CONSISTS OF MOLECULES THAT OCCUPY SPACE AND EXTENDS MANY KILOMETER ABOVE EARTH'S SURFACE.
- The molecules that makes up Earth's atmosphere are energized by sunlight and kept in continual motion. Without Earth's gravity, the molecule would fly off into outer space.

- Earth's atmosphere has no definite surface. Its density decreases with altitude.

- In interplanetary space, there is a gas density of about one molecule per cubic centimeter. This is primarily hydrogen, the most plentiful element in the universe.

- Ninety nine percent of the atmosphere is below an altitude of about 30 kilometers. Compared with EArth's radius, EArth's atmosphere is just a thin shell.

B) ATMOSPHERIC PRESSURE
ATMOSPHERIC PRESSURE IS CAUSED BY THE WEIGHT OF AIR, JUST AS WATER PRESSURE IS CAUSED BY THE WEIGHT OF WATER/
-the density of air changed with temperature. At sea, level, 1 cubic meter of air at 20C has a mass of about 1.2 kg.
(remember the lab air/aluminum ? 1.29 kg/m³ = 1.29 10^-3 g/cm³, you divide by 1,000)
So if you redo the air lab with a huge balloon 1 meter x 1meter x 1meter you will find a mass of about 1kg
or about 2pounds. (only the air inside, not the rubber around)

- the weight of air that bears down on a 1-square meter surface ((1m x 1m) at sea level is about 100,000 N
(that is about 10,000 kg per m² or 10, 000 kg/m² or 10 tons /m² or about 15lbs per in² or about 18,000 pounds per yard square)

- Aside from variations with altitude, there are variations in atmospheric pressure at any one locality due to moving air currents and storms.

3) Answer the questions based on 2) (or fill the blanks)
A) THe _____________ energizes the molecules in EArth's atmosphere/

B) Why is gravity important to eArth's atmosphere ?

C) What would happen to EArth's atmosphere without the sun ?

D) IS the following sentence true or false ?
Like the ocean, Earth's atmosphere has a definite surface. ______

E) The density of the atmosphere ____________ with altitude.

F) How are the molecules that make up EArth's atmosphere like a huge pile of feathers?

G) What is the density of gas in interplanetary space?

H) Circle the letter of the most plentiful element in the universe.
oxygen nitrogen hydrogen carbon

I) Compare the thickness of Earth's atmosphere to EArth's radius

J) Describe how the temperature of the atmosphere changed with increasing altitude.
refer to the picture below:

to learn more about the atmosphere

4) answer based on 2)

A) Air pressure is equal to 10, 000 kg/m² = _____ kg/cm²
That means that for each area 1cm x 1cm (each square) on your desk, the mass of the air pushing on it =_____________. This push is due to EArth's ___________. Or for each cm x cm, there is _________ pounds pushing on it (1kg = 2.2 pounds).
convert kg/cm² to lbs/in². _______________________. so the atmospheric pressure is about _______ lbs/in²
So for each area (square) 1 in x 1 in, weigh of the air = _________ lbs. That's a lot !

C) Atmospheric pressure is caused by the _____________ of air.

D) What is the mass of air in a room that has a volume of 50m³ ? THe temperature of the room is 20C?
(use the density of air about 1.3kg/m³)

E) What is the volume of 10, 000 kg of air ? ____________(use density)
Now consider a column of air cut out from our atmosphere (from ground to space)
(prism shape, see image on the side), with a base of 1m x 1m . You learnt that this prism of air has a mass of about 10, 000 kg. (because the atmosphere exerts a force of 10,000kg per cubic meters.
Estimate the height of the column (using volume prism = WxLxH).
H= _______m.

F) IS the following sentence is true or false ?
Air doesn't weigh very much, no matter how much you have it . _______________

5) answer based on 2)
A) Consider a 1-square-meter of air that extends through the atmosphere.
- What is the mass of the air in the column ?
- What is the weight of the air in the column /
- What is the pressure does the air in the column produce ?

B) The average atmospheric pressure at sea level is __________.
(convert 10,000kg/m² to N/m² with 1 N = 10kg about)

C) Name three things that can cause variations in atmospheric pressure

D) Measurement of changing ____________ is important to meteorologists in predicting weather.

6) Air pressure can be measured in Kpa (kilo pascals, 1 Kpa = 1,000 pa)
or in bars. 1 bar = 101.3 kPa

For questions A... , refer to the table below

Changes in Air pressure with Altitude
Altitude above Sea Level (m)	Air Pressure (bars)	Air pressure (kPa)
0	1.000	___________
200	0.9971	_________
400	________	96.68
600	________	94.42
800	0.9103	92.21
1000	0.8888	____________
1200	0.8677	87.89

A) Complete the air pressure columns in the table by converting between units of air pressure.

B) How does air pressure change as a function of altitude ?

C) Suppose a hiker is on a mountain ridge 1200 meters above sea level. Approximately what air pressure will she experiences?

D) By how much does the air pressure decrease , in bars, from sea level to an altitude of 1200 meters?

E) Is the following sentence true or false ? Air exerts a force of more than 1000N on top of your head ?

F) What keeps a person from being crushed by air pressure ?

PART II: EXPERIMENTS

here is another experiment to do for extra credits :
Note that there is a vacuum in the elevated container.
so the water rises pushed . you need jam jars.

Here some experiments to do. We will do some of them:
http://www.youtube.com/watch?v=vGulvutZJpg&feature=related

1)
EXPERIMENTS DONE IN CLASS + TAKE HOME QUIZ

2) do LAB1 (is air pressure a lot ? magnitude of air pressure)

3) LAB2 (collapsing can)

4) LAB3 (revisiting Pressure equation P = F/A)

5)

source: http://www.uq.edu.au/_School_Science_Lessons/UNPhysics2.html

3) If you love Sciences what this video about kinetic theory and pressure.
streaming video. (ask me for the login and passwd)

PART III: BAROMETERS - THE MERCURY BAROMETER THE ANEROID BAROMETER

above: a mercury barometer. The air pressure forces the mercury to rise in the tube.
There is no air at the top of the tube (vacuum). The variations in the height is proportional
to the change in atmospheric pressure.
source: http://www.uq.edu.au/_School_Science_Lessons/UNPhysics2.html

An aneroid barometer uses a small metal box that is partially exhausted of air.
The box has a slightly flexible lid that bends in or out as atmospheric pressure changes
- An aneroid barometer is an instrument that measures variations in atmospheric pressure without a liquid. The barometer's lid bends in or out as atmospheric pressure changes.
- Since atmospheric pressure decreases with increasing altitude, a barometer can be used to determine elevation. An aneroid barometer calibrated for altitude is called an altimeter.

PART V:BERNOUILLI PRINCIPLE

0) READ
When you blow across the top of a single sheet of paper, the far end of the paper lifts upward. The Swiss scientist Daniel Bernoulli discovered the reason why the sheet of paper behaves as it does. According to Bernoulli's principle, as the speed of a fluid increases, the pressure within the fluid decreases. The air blowing across the top of the paper exerts less pressure than the stationary air underneath. Because the air below the paper is nearly motionless, it exerts a greater pressure. The difference in pressure forces the paper upward. do the experiment.
(find picture)

1) experiment to do
2 small foam balls hanging from strings at the same height with about three centimeters of space between them. What do you think will happen if you blow air through the space between them. Think about Bernoulli's principle.

2) Cricle the letter of the sentence that correctly states Bernouilli's principle:
A) As the speed of a fluid decreases, the pressure within the fluid decreases.
B) As the speed of a fluid increases, the pressure within the fluid increases.
C) As the speed of a fluid increases, the pressure within the fluid decrreases.

3) Because the air traveling over the top of an airplane wing moves faster that the air passing underneath the wing, the pressure above the wings is ______________ than the pressure below the wing.

4) What is lift, and how does it relates to an airplane's flight?

5) explain atomizer with Bernouill's principle

cool experiments
http://www.youtube.com/watch?v=QxZ6AWLpnUw&feature=related

source: McGraw Hill

EXTRA CREDITS: REVIEW
6)

A)Mathematician who discovered that the buoyant force equals the weight of the fluid displaced by the object.

B) The result of a force distributed over an area.

C) Type of substance that assumed the shape of its container

D) Ability of a fluid to exert an upward force on an object within it

E) Upward force that keeps an aircraft aloft

F)Device that uses pressurized fluids acting on pistons of different sizes to change a force.

G) Force that opposes the weight of an object floating in a fluid.

PART VI: GAS LAW

relationship between pressure and volume. If you decrease the volume
of a container of gas, the temperature increases because the kinetic energy increases.
we say pressure is inversely proportional to volume
u-tube video (from software to check)