For an explanation, see the main splats page
SPLATS about air and pressure
The principles of air and pressure
- The air we breathe is made mainly of two gases. One is oxygen, which most living things need. There are also other gases in small amounts, and water vapour.
- Air has weight, and it exerts a pressure on us. This air pressure can be measured, and it decreases with altitude and as the weather changes.
- In a mixture of gases, each gas exerts a partial pressure, equal to the pressure it would exert on the container if it alone filled the container.
- Pressure measurement in the atmosphere can be done in different ways: with a pressure gauge or with a barometer, but each relies on air exerting pressure.
- Places on the map with the same (sea level-corrected) air pressure are linked by lines called isobars to reveal weather patterns that involve pressure changes.
- In 1632, Galileo Galilei said that he had been told by a workman that there was no way suction could raise water a hair's breadth more than eighteen cubits.
- You cannot suck air up more than 10 metres or siphon over a rise greater than 10 metres. This limit is imposed by the pressure of the atmosphere.
- Warm air is less dense than cooler air, so it rises. As it rises to areas of lower pressure, it expands and so gets cooler. This is called adiabatic cooling.
- In 1646, Blaise Pascal made a barometer using a mixture of water and wine, which rose under atmospheric pressure, to twenty cubits, more than Galileo's report.
- In 1648, Blaise Pascal took his wine and water barometer up a mountain and discovered that the atmospheric pressure varies with altitude in a systematic way.
- In 1660, Otto von Guericke used a barometer and the trends shown (whether it was rising, falling, and the rate) in order to develop forecasts of future weather.
- Flotation effects happen when a solid is placed in a more dense fluid and displaces some of it. The law of flotation depends on Archimedes' principle.
- When one object floats in a fluid it floats because the mass of fluid it displaces, pushes out of the way, is equal to the mass of the floating object.
- A hot air balloon floats in the air because the total mass of the balloon and the air in it is less than the mass of cool surrounding air displaced by it.
- An object only floats in a fluid when it is less dense. A steel ship floats in water because its overall average density is less than the density of water.
- Before the launch of the first iron ship, many people predicted that it would plummet to the bottom of the ocean, because iron always sinks in water.
- You can make a vacuum with a suitable air pump, or by boiling water to drive out air, then sealing and condensing the water vapour. Other ways exist as well.
- Getting a good vacuum requires a vacuum pump: the Magdeburg hemispheres were an early demonstration that a vacuum could exist, even if it was thought unnatural.
- It is very difficult to obtain a good vacuum, and gases could not really be discovered until a vacuum could be created after an effective pump was invented.
- In 1663 Blaise Pascal proposed isotropy of pressure: pressure acts equally in all directions, a rule which we know today better as Pascal's principle.
- Fluids exert pressure, and the pressure exerted obeys Pascal's principle that the pressure applied is transmitted equally and in all directions.
- Pressurized fluids can be used in many ways. The hydraulic press is an application of Pascal's principle with the advantage that the force direction changes.
- Gas bubbles appear in carbonated drinks when the seal is broken (opened), as the solubility of gases in the blood depends on pressure, which is eased.
- Divers can get the 'bends' as bubbles form if they come up from great depth too fast, as the solubility of gases like nitrogen in the blood depends on pressure.
- In 1738 Charles Dangeau de Labelye developed the caisson, a pressure cabinet to allow workers to operate beneath the water, to build a bridge at Westminster.
- In 1686, Edmond Halley worked out the theory of the trade winds, established the relationship between barometric pressure and height above sea level.
- Robert Boyle's investigations that led to what we now call Boyle's law relied on Robert Hooke constructing an effective air pump for the experiments.
- In 1727, in his Vegetable Staticks, Stephen Hales showed that air was an element which took part in chemical reactions, that it could be 'fixed' in some way.
- In 1771, Joseph Priestley showed by experiment that air in which a candle had burned could be restored by a sprig of mint to let another candle burn in it.
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