Pressure is caused by the collision of particles with the walls of the container.

Remember you can increase or decrease the pressure within a container by changing one of three factors

1.) change the volume
2.) change the temperature
3.) add or remove gas

Can you tell what happens to the pressure in each case?  Remember this only works for rigid containers. 

For items like balloons (that are allowed to change their volume) what happens to the pressure inside the balloon when it is place in a freezer?  You might be inclined to say that it decreases because the molecules slow down and move closer together.  This would be true for a rigid container like you 2-liter pop bottle.  However, remember that a balloon can change size so as the gas molecules slow down and move closer together the balloon also shrinks and thus the pressure stays the SAME.

A region with no pressure is called a Perfect vacuum.  A region of very low pressure is called a partial vacuum.  It is hard to get a perfect vacuum but very possible to get partial vacuums

Boyles law: Is the relationship between the volume of a gas and the pressure of a gas if the if the temperature remains constant. It states that as you increase the volume of a container you will decrease the pressure of the gas.

   or 

V1 = original volume

V2 = new volume

P1 = original pressure

P2 = new pressure

Charles law: Is the relationship between the temperature and the volume of a gas if the pressure is constant. It states that as you increase the temperature you will increase the volume.

V1/V2=T1/T2

 V1 = original volume T1 = original absolute temperature

V2 = new volume T2 = new absolute temperature

Gay Lussacs law: Is the relationship between the pressure and temperature of a gas if the volume is held constant.  It states that as the temperature of a gas increases the pressure will increase.

P1/P2 = T1/T2

IMPORTANT:  Temperature must be in degrees K.  Kelvin = Celsius + 273.

Combined gas law

P1 = Initial Pressure

V1= Initial Volume

T1= Initial Temperature

P2= Final Pressure

V2= Final Volume

T2= Final Temperature

Practice problems for the gas laws

Our gas pressure practice problems

Atmospheric air pressure is caused by the column of air that sits above your head.  Remember gravity acts on the air so the air has weight. This weight (force) over an area causes pressure.  As you go up in altitude (to Denver for example) the column or air becomes shorter and the pressure decreases. Therefore the air pressure at Denver is lower that at sea level.

Normal atmospheric pressure is 14.7 pounds per square inch or PSI.  Remember to calculate pressure you need a force over an area.

Pressure can also be measured in pascals which is the force of 1 N over 1 square meter. (think about an apple spread about two black lab tables!) Since this is pretty small we usually talk about kilopascals which are 1000 pascals.

Pressure can also be measured by how high it can push a column of liquid.  This is 760 mm of Hg. 

Pressure is also measured in a unit called atmospheres.

Remember 14.7 psi=101.3kPa = 760 mm of Hg = 1 atm.

To change from one unit to another just use unit cancellation.  For example, how many PSI is 300 kPa?

300kPa (14.7 PSI /101.3 kPa=43.5 PSI

To Find the amount of force on a surface in pounds you first must calculate the area that the air is acting on. Look at the following problem

Your desk has a length of 36 inches and a width of 16 inches.  How many pounds of air are pushing down on the desk?

First find the area of the desk

36 inches X 16 inches = 576 square inches. 

Then use 14.7 pounds/square inch to get the amount of force in pounds

576 inch2 (14.7 pounds/inch2) = 8467.2 pounds or a little over 4 tons!

Why doesn't the desk collapse?  Remember air pressure acts in all directions so there is 14.7 PSI also pushing up on the desk!

Remember crushing the pop can on the hot plate?           

What caused the can to collapse? When you heated the can you caused the water in it to boil. The vapor from the boiling water pushed the air out of the can. When the can was filled with water vapor, you cooled it suddenly by inverting it is water. Cooling the can caused the water vapor in the can to condense, leaving the can empty. When the can was empty, the pressure of the air outside crushed it.When the water vapor inside the can condensed, the can was empty. You may have expected the water in the pan to fill the can through the hole in the can. Some water from the pan may do this. However, the water cannot flow into the can fast enough to fill the can before the air outside crushes it.

How about the egg in the bottle? ( Notice in this video they use a match and paper to heat the air inside the flask instead of a hot plate with some water)

What happens? The hot plate heats the water inside the bottle and changes it to steam. When the water is heated it expands (and takes up more room). As the heated steam expands, some of it escapes out the bottle. When you take the bottle off of the hot plate, the air inside the bottle cools and contracts (and takes up less room), thus creating a lower pressure inside the bottle than outside. The greater pressure outside the bottle forces the egg into the bottle.

How does a drinking straw work?

Here's the Challenge: what's the maximum effective length of a drinking straw? At what length can you no longer generate enough pressure difference to draw the liquid all the way through the straw?

Is it
a) about 20 cm, the length of a typical straw?
b) about 1 meter, or
c) about 3 meters?

Here’s another experiment
    1) put two straws in your mouth
    2) Place the other end of one of the straws in your drink, but leave the second straw in the air
    3) Try to drink!

    What happens when we inhale: When you inhale, your diaphragm contracts and bends down. This makes more space inside your ribcage and lowers the pressure inside your chest. Because the pressure in your mouth and outside is greater, air pushes its way inside to the lower pressure, goes through the trachea and into the lungs, which fill up with air. The air goes through the bronchial tubes and into the air sacs where the actual gas exchange takes place. This is a partial vacuum system... we work just like a vacuum cleaner.

    The air sacs and bronchial tubes are moist, just like the worms skin, but nothing else has to be. Why is our windpipe so hard? Its made of cartilage because if it were soft, the low pressure inside it would make it collapse.

    When we exhale: The diaphragm relaxes again: the space gets smaller and there is higher pressure inside the chest. The high pressure air moves to the lower pressure area outside the mouth. Breath comes out (now with more carbon dioxide than before and less oxygen). The larynx helps us talk.

    When we hiccup: Out diaphragm spasms, i.e. twitches against our will, so it contracts and relaxes randomly. The result is we breathe in and out suddenly and unexpectedly.

    Why do your ear's pop (contributed by Abby and Olivia, Thanks! Mr. Kuhlman)

    Compression and expansion of gases:

    Remember only gases can be compressed, NOT liquids!  When a gas is compressed, ( remember the plastic cylinder piston from class) its temperature increases.  This is because of the friction of the molecules as they get closer together.

    As a gas expands (comes out of a cylinder) it is cooled.  As the molecules spread out they absorb energy from the air and thus the air is cooled. (remember the tank of carbon dioxide gas that we felt in class?)

    Pascal's Principle:  Fluids exert pressure equally in all directions.  If I push on a confined liquid the force is transmitted through the liquid undiminished in all directions.  This is the principle behind hydraulic lifts.   If I push with a force of 50 pounds on a piston with an area of 1 inch2, what will be the force on the piston with an area of 10 inch2?  Assume the pistons are connected through a system that is filled with a hydraulic fluid.  (Remember the hose demo that we did in class)

    Pascal's principle and the bottomless bottle (load and watch movie)

    Bernoulli's principle:  The pressure of a fluid decreases as its velocity increases. Be able to relate this to the airfoils that we discuss in class. Remember wherever it is flat that is where the air is moving the SLOWEST and thus has the highest pressure of the two sides.  Here is a picture of a spoiler on a car.  How does Bernoulli's principle apply to this? This picture shows two cars drafting.  How does this work?

    siphoning:

    Here are some short movies of demonstrations done in class

    Toilet paper demo
    toilet paper II
    Leaf blower and water
    Toys
    Balloon on fan
    Ping pong balls

    Airfoil demos
    Candle one
    Candle two
    Candle three