1. Idea

Atmospheric pressure is the pressure that air exerts on objects immersed in it.

2. Proof it exists

• Magdeburg hemispheres (1654, Otto von Guericke): after pumping, legend says 16 horses were needed to pull them apart — air pressure is real and huge.
• Inverted cup: atmospheric pressure holds the paper and the water.
• Suction-cup hooks: air pressure presses the cup onto the wall.
• Drinking with a straw: air pressure pushes the drink up the straw.

3. Torricelli — measuring P₀

1. Fill a ~1 m glass tube closed at one end with mercury.
2. Seal with a finger, invert, and dip into a mercury dish.
3. Release: the level falls until the height difference is about 760 mm.
4. Idea: air pressure supports the mercury column; P₀ equals that column's pressure.

• p₀ = ρ_Hg g h ≈ 1.013×10⁵ PaStandard ≈ 1.013×10⁵ Pa; often take 1.0×10⁵ Pa in rough work. Roughly equal to a 10 m water column.

4. Barometers

• Mercury barometer: accurate but bulky — weather stations and labs.
• Aneroid (metal-box) barometer: portable and common.

5. How pressure changes

• Pressure falls with altitude. Within about 2 km, roughly −111 Pa per +10 m.
• It also depends on the weather.
• Boiling point rises with pressure (pressure cooker).

6. Applications

• Piston pumps and centrifugal water pumps.
• Suction-cup hooks and drinking straws.
• Pressure cookers raise boiling point by raising pressure.

7. Torricelli pitfalls

• If the tube tilts, vertical Δh stays ~760 mm, but the mercury path lengthens.
• Air in the tube makes the measured value too small.
• A wider tube does not change the mercury height.
• Why not water? Lower density would need a tube over 10 m tall.