# Lec12 - Today Finish Chapter 13(Liquids Start Chapter...

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Today: Finish Chapter 13 (Liquids) Start Chapter 14 (Gases and Plasmas)

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Gases and plasmas: Preliminaries Will now apply concepts of fluid pressure, buoyancy, flotation of Ch.13, to the atmosphere. Main difference between a liquid like water and a gas like air is that in the gas, the density can vary hugely; our atmosphere’s density is depth dependent. Gases vs liquids: both are fluids but molecules in gas are far apart and can move much faster, free from cohesive forces. A gas will expand to fill all space available Note! An “empty” cup is not really empty – it’s filled with air. In fact a 1 m 3 “empty” cube of air has a mass of 1.25 kg (at sea level).
Example Before you go grocery shopping you check what’s in the refrigerator and find only a large orange. Which weighs more, the air in the fridge, or the orange? The fridge has a volume of about 0.75 m 3 . The air in the fridge! Note: We don’t notice the weight of air because we are submerged in air. If someone handed you a bag of water while you were submerged in water, you wouldn’t notice its weight either. A fish also “forgets” about the weight of water just like we don’t notice weight of air.

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The atmosphere What determines the thickness of our atmosphere? Balance between: kinetic energy of molecules vs gravity spreads molecules apart holds molecules near earth Consider extremes: (i) If very little gravity (eg on moon), then molecules would move, collide, and eventually disappear into space. So no atmosphere. (ii) If gravity very strong c.f. kinetic energy (eg on a remote planet), molecules move too slowly, and form a liquid or solid, like the planet itself – so again no atmosphere. Earth – balance between the two effects, so we do fortunately have an atmosphere! (we can breathe!!)
Exactly how tall is the atmosphere? Not a meaningful question, since it gets thinner and thinner as you go higher and higher. Even in interplanetary space, have about 1 gas molecule (mostly hydrogen) every cubic meter. and most dense here Air is least dense up here…

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Atmospheric Pressure Atmosphere exerts pressure, like water in a lake. We are at the bottom of an “ocean of air”. “Madeburg hemisphere” experiment (1654): Make sphere from 2 copper hemispheres, ½ m in diameter. Evacuate the sphere with vacuum pump. Two teams of 8 horses each couldn’t pull the spheres apart! What is holding the two hemispheres so tightly together? Atmospheric pressure outside, no pressure inside (vacuum). Rather than being “sucked together”, they are “ pushed together ” by air molecules. Same idea behind why the weight is lifted when air is pumped out here – no pressure inside cylinder, so unbalanced atmos pressure outside pushes piston upwards, raising the weight.
Atmospheric pressure cont. Unlike water, density of atmosphere varies with height, so pressure

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Lec12 - Today Finish Chapter 13(Liquids Start Chapter...

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