Pressure, Winds, Storms, and Cyclones · Lesson 3 of 8
Air Pressure Around Us
“Make the invisible push of air visible through paper, balloons, and a rubber sucker.”
• Define the atmosphere and atmospheric pressure. • Interpret experiments that reveal air pressing on surfaces and in all directions. • Explain a sucker using the pressure difference inside and outside it. • Relate pascal, millibar, and hectopascal, and explain why people are not crushed by the atmosphere.
The atmosphere is not empty space
Air surrounds Earth as a deep envelope called the atmosphere. It contains mainly nitrogen and oxygen, with argon, carbon dioxide, and small amounts of other gases. We cannot usually see air, but it has mass and can exert a force on objects. The pressure made by surrounding air is atmospheric pressure.
The envelope of gases surrounding Earth, extending many kilometres above the surface.
The pressure exerted by the air around us on surfaces.
A balloon becomes round when you blow air into it because the air inside pushes on its rubber wall in many directions. If you release its untied neck, some air escapes: the pressure inside is initially greater than the surrounding pressure. These observations offer a small-scale picture of how a gas exerts pressure.
A sheet of paper feels the push
The chapter’s chart-paper experiment begins with an inverted paper plate and a stick used to lift it. Place a folded sheet of chart paper over the plate and try lifting; then unfold an identical sheet and repeat. The unfolded sheet covers a larger area. It is harder to lift, though the paper weight has not increased. Surrounding air acts on the larger sheet across more area, increasing the overall force resisting the lift.
Remember the relationship F = pressure × area. If the pressure acting over a sheet is similar in both trials, a larger area gives a larger total force. In an actual setup, how closely the paper seals to the surface and how air can enter underneath also affect the result. The observation nevertheless gives evidence that surrounding air presses on objects.
Problem
Why is an unfolded chart-paper cover harder to lift than a folded one of equal weight?
- 1.The unfolded cover presents a larger area to the surrounding air.
- 2.At similar pressure, total push grows with area: F = P × A.
- 3.The extra effort is due to air pressure across the larger area, not extra paper weight.
A sucker stays on a smooth surface
Press a rubber sucker firmly onto a smooth table. Much of the air beneath its cup is squeezed out. The pressure under the cup becomes smaller than the surrounding atmospheric pressure, so the outside air pushes the sucker firmly against the table. Pulling it off requires a force that overcomes this pressure difference. A rough surface lets air leak in, so a good seal is difficult to keep.
Problem
Identical suckers are pressed onto a smooth tile and a rough brick. Which one is more likely to stay attached?
- 1.A smooth tile allows the cup’s rim to seal after much of its inner air is pushed out.
- 2.Outside atmospheric pressure then exceeds the pressure below the cup.
- 3.The sucker on the smooth tile is more likely to stick; leaks through a rough contact weaken the effect.
A large pressure that is balanced
Atmospheric pressure can exert a surprisingly large force. The chapter compares its push on a 15 cm × 15 cm patch with a weight of about 2250 N, comparable to a 225 kg mass under the chapter’s approximate gravitational conversion. This is a total force on the patch, not a claim that a person feels a 225 kg object sitting there. Fluids and gases inside our bodies exert pressure outward, and pressure acts all around us, so these pushes are broadly balanced.
Air pressure can be written in pascals, but weather descriptions often use millibars (mb) or hectopascals (hPa). One millibar equals one hectopascal, and each equals 100 pascals. These are different-sized units for the same physical quantity.
Problem
Express 10 hPa in pascals and millibars.
- 1.Use 1 hPa = 100 Pa, so 10 hPa = 1000 Pa.
- 2.Use 1 hPa = 1 mb, so 10 hPa = 10 mb.
- 3.Both results describe the same pressure in different units.
The useful explanation is a pressure difference. Outside air pushes the cup toward the surface more strongly than the remaining air beneath it pushes outward.
Check your thinking
Ask where the air is, which way it pushes, and whether two sides of an object have the same pressure. A difference in pressure explains the sucker and escaping balloon air.
Quiz
What is the atmosphere?
Why does an inflated balloon expand in all directions?
What mainly holds a rubber sucker to a smooth tile?
How many pascals are in 2 hPa?
Why are we not crushed by atmospheric pressure?
Practice Problems
- Define atmospheric pressure in your own words and give one observation that reveals it.
- Describe the folded-versus-unfolded paper investigation and explain why sheet weight does not account for the difference.
- Explain why an inflated balloon becomes round and why air escapes through its open neck.
- Describe the steps and pressure difference that make a rubber sucker stick; predict the effect of a rough surface.
- Convert 15 mb to hPa and Pa.
- Explain the meaning of the chapter’s 2250 N comparison and why it does not imply that the atmosphere crushes us.
Key Takeaways
• Air presses on objects and container walls; this surrounding push is atmospheric pressure. • A larger area at similar pressure experiences a larger total force. • A sucker sticks when outside air pressure exceeds the reduced pressure underneath it. • Internal body pressure helps balance the atmosphere. • One millibar equals one hectopascal, which equals 100 pascals.