Air Pressure (Lauren)

• Air Pressure can be defined as the pressure of air in the earth’s atmosphere. It reaches a maximum of about
100,000 Pa near sea level and decreases as altitude increases. More simply, it is the force exerted by the weight of the air.

• Even though air molecules are invisible, they still have mass and take up space. Since most people are accustomed
to the air pressure at sea level, they forget that they are being “pushed on” by the air all of the time! Because these air
molecules have a lot of “empty” space between them, they can be compressed in order to fit inside of a smaller volume.

• When the air is compressed, it is said to be under high pressure.

• Many people wonder…if air pressure is so strong, why doesn’t it squash humans? The answer is simple. People have
air inside their bodies too. The air inside their bodies balances with the air outside and keeps them from getting squished.
• In order to visualize air pressure, imagine a sealed container full of air, like the one above. When the molecules of
air collide with the walls, they exert a pressure on the container.

• The force of the air pressure depends on two things:
• The rate of particle collision and the force of the impact.
If more particles collide in a given amount of time, then more energy is transferred to an object.
• Also, if the particles crash into each other and the container with greater force, then more energy is transferred to the object.

• There are fewer air particles per unit of volume inside the balloon, but because those particles are moving faster,
the inside and outside air pressure is the same.

• A hot air balloon rises because it is filled with hot, less dense air and is surrounded by cool, denser air. When air
is heated, the air particles absorb the heat energy and become more excited. This makes them move faster, which means they
collide with a surface more often, and with greater force. For this reason, hot air exerts greater air pressure per particle
than cold air, so you don't need as many air particles to build to the same pressure level.

• In order for a hot air balloon to work, it has to exert an outward pressure on the envelope, or outer covering, that is
equal to the inward pressure of the surrounding air. That way, each portion of the envelope will experience zero net force
and the balloon will not be crushed. A balloon with a very thin outer shell wouldn’t work because it would be crushed by the
atmospheric pressure, with each square meter of its outer envelope experiencing a force of 100,000N. While a thick outer shell
could withstand the pressure, it would be too heavy to float in the air.

• To do this, you have to have a gas that has the same pressure as the surrounding air but a lower density, since filling the
balloon with outside air would make its average density too high. A hot air balloon contains fewer particles, has less mass,
and weighs less than it would if it contained cold air.

• Since the air pressure inside the balloon is the same as that outside, the air has no tendency to move in or out and the
balloon doesn’t have to be sealed.

• A hot air balloon won’t ascend forever because as it does, the air becomes thinner and the pressure decreases inside
and outside of the balloon. It will eventually reach the flight ceiling, or the point at which the air is too thin to lift
it any higher.

Sources:

http://kids.earth.nasa.gov/archive/air_pressure/
http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/circulation/air_pressure_p_1.html
http://science.howstuffworks.com/hot-air-balloon5.htm
http://www.hotairballoons.com/hotairballoon-air-science.asp

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