# Air Pressure - PowerPoint PPT Presentation

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Air Pressure. Pressure is force pushing on an area. pressure = force area. larger force = larger pressure smaller force = smaller pressure larger area = smaller pressure smaller area = larger pressure.

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Air Pressure

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## Pressure is force pushing on an area.pressure = force area

larger force = larger pressure

smaller force = smaller pressure

larger area = smaller pressure

smaller area = larger pressure

### Air Pressure = molecules pushing (exerting force) on other molecules

• At sea level a 300-mile high column of air molecules push down with a force of 14.7 pounds per square inch (psi), or 1 kg per square centimeter (kg/cm2).

### Try this:

• Lay a ruler on a table with about 3” hanging over the edge.

• Lay a sheet of printer paper on the part of the ruler in direct contact with the table.

• Press down on part of the ruler hanging over the edge. Observe.

• Repeat with an opened sheet of newspaper.

• Observe and explain any differences.

### Air pressure is measured with a barometer .

Air pressure is measured in millibars or

“inches of mercury”

29.92” Hg = 1,013 mb

Average height of barometric column at sea level

### Altitude affects air pressure

pressure = force

area

Increasing altitude means less and less air molecules per given area. Less molecules means less pressure (force exerted) because fewer molecules collide.

Also, it’s colder up there which causes molecules to slow down and collide with less force.

Altitude increases and amount of air molecules decrease. Less air molecules per given area also mean atmosphere is less DENSE at higher altitudes.D = M V

When amount of mass in a given volume decreases, the density also decreases.

altitude

density

### Temperature affects pressure

• Heating causes molecules to move faster.

• Moving faster increases the amount of collisions between individual molecules and the relative force of those collisions.

• More force results in more pressure.

• Faster moving molecules exert greater pressure on a rigid container. If there is no container, then faster molecules will move apart.

### What happened?

• Burning candle consumes oxygen from air inside glass.

• This reduces mass & density of air in glass and therefore, the air pressure.

• Candle goes out, heat source is gone causing gas molecules to slow down  further drop in air pressure.

• Outside pressure is now greater than inside.

Difference in air pressure pushes water

from pan (outside) up into the glass.

High pressure area pushes towards low pressure area.