Ideal Gas Law Simulation
Applet courtesy
Oklahoma State University.
Click here to launch
Java simulation of an ideal gas. Please follow the instructions below.
- Density: of a gas is a measure of the mass of molecules in a unit of volume. In the ideal gas simulation,
pumpng air into vessel increases the density of the gas. Keep on pumping until a pressure of 1 atmosphere is reached.
Density increases as molecules are squeezed closer together.
Release excess gas molecules by removing lid on vessel.
- Temperature: is a measure of the total kinetic energy (energy of motion) of the gas molecules. Heat the vessel.
In a gas, this heat goes into increasing the molecular speed of the gas molecules; the temperature of the gas increases.
- Pressure: is the force caused by gas molecule collisions with the container walls. Increasing number of molecules (density)
increases collisions with walls. So does increasing their average molecular speed (temperature). Pressure increases in both cases.
- Ideal gas law: pressure = density x constant x temperature;
expresses the above relationship mathematically. Increasing density and/or increasing temperature increase pressure.
If pressure stays constant, then increasing the temperature will lead to a decrease in density.
Case 1: A fixed volume
In a fixed volume, the volume is forced to stay constant.
Case 1.1 Isolated fixed volume vessel
- In an isolated fixed volume vessel, you cannot add any gas molecules to the vessel and the volume is constant.
That measn the density is constant. Increasing the temperature leads to an increase in pressure. A presure cooker is a
good example of such a system.
Case 1.2 Unisolated fixed volume vessel
- In a fixed volume vessel where you can add gas molecules, increasing the number of molecules will increase
the mass of the gas. The pressure will increase, while the temperature stays constant. A tire is a good example of such a system.
Case 2: Real atmosphere
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In the real atmosphere, we do not deal with fixed volumes. Air parcels (or small bubbles in the atmosphere) are free to expand
as they are heated, lifted, or advected. Air parcels will either expand or contract until the pressure inside the parcel is
equal to the ambient pressure (i.e. the pressure outside the parcel).
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Case 2.1: Heating an air parcel
- Heating an air parcel will lead to an increase in the temperature. The increased kinetic energy will
result in the heated gas expanding the parcel until pressure inside the parcel is qequal to the pressure outside.
This is what happens when the lower atmosphere is heated by sensible heating from the surface of the earth.
This is called diabatic heating becasue heat energy is added to the air parcel.
Case 2.2: Latent heating
- Evapouration at the surface of the earth cause by solar heating leads to lighter water molecules
being added to the air. The parcel would expand until the pressure inside the parcel would equal the
pressur outside the parcel. Becasue the water molecules are ligher, the humid air would actually
be ligher (less dense) than the air outside.
Case 2.3: Lifing a parcel
- When you lift an air parcel, the pressure outside the parcel decreases. The volume of the parcel expands
until the pressure inside an outside is equal. This leads to a cooling (adiabatic cooling) of the parcel.
This is called adiabatic because there is no heat energy taken away from the parcel.
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Column temprature and pressure
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Air density decreases with height as you go upward in the atmosphere. Because of the ideal gas law
so does pressure. This is due do gravity.
Air molecules are in constant motion. Gravity pulls all of them towards the Earth so that on average
most molecules. See this link for an example of gravity affecting the vertical profile of density
for an ideal gas.
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At any given height in the atmosphere, the pressure is proportional to the weight of the
air above. At 500 mb over an area of 1 square meter, there is approximately 5000 kg
of air above you. Doubling the pressure by
moving down to the 1000 mb level means you are also doubling the atmospheric mass above you. The average
sea-level pressure is 1013 mb, very close to 1000 mb.
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The temperature of the air affects the rate that pressure decreases as you
move upward in the atmosphere. Colder air is denser. The molecules of air are
more closely packed. As a result, pressure decreases more rapidly as you
ascend through cold air than in warmer temperatures.
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Because tropical and polar air masses have different temperatures and densities,
pressure surfaces at upper level have different heights. In the illustration at right, 250 mb occurs
at a lower height in the cold polar air mass than in the tropical air mass. An airplane using an altimeter
to fly at constant pressure will actually ascend if it flies from New England to Florida.
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Now imagine your airplane flying at a constrant height from a polar to a tropical region.
You start in cold polar air at 250 mb.
When you arrive over your warm destination you will notice that your pressure has to be somewhere between
250 and 500 mb (350 mb). You have moved into an area of low pressure aloft.
This indicates increasing heights on a constant pressure surface are equivalent to increasing pressure at a constant altitude.
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In the upper atmosphere, the height of various pressure levels is mapped.
Over cold polar air in the south, you usually find lower heights (associated with lower pressure);
over warm tropical air, you usually find higher heights (associated with higher pressure).
Meteorologists use these maps
to assess upper-level weather systems. The jetstream usually appears where height gradients
are strongest (i.e. where heigh contours are closer together). Click on map for current larger map.
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A result of these heigh variations is that the jetstream is a good inicator of
the border between cold polar and arm tropical air masses. TV meteorologists will often show
the jetstream instread of fronts to indicate that warm or cold air masses are moving into an area.
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The above reasoning for high and low pressure applies only to upper air pressure systems.
Near the surface, colder air is associated with high pressure and warmer air with low pressure. To make sense of this
imagine extending both columns from 1000 mb to the surface of the earth using equal volumes of cold and warm air.
Because the cold air is denser and heavier, it will exert a larger pressure (1030 mb) on the surface below it than the warm air (1020 mb).
As a result, the pattern of high and low pressure is reversed from the pattern aloft. 
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