October 23, 2011

5.5 Equal Pressure

Understand that the pressure at a point in a gas or liquid which is at rest acts equally in all directions

Magdeburg Hemispheres

When the pressure inside the sphere is equal to the atmospheric pressure outside, the hemispheres can easily be pulled apart because there is no external force holding the hemispheres together, but if a vacuum is introduced inside the sphere, the pressure atmospheric pressure pushing against the sphere is much greater than the pressure pushing outwards, so the resultant force clamps the sphere closed…

If you’re confused… I tried my best to explain the pressure as a group of men.

1. No vacuum
There are 50 people inside the sphere trying to push the sphere open and 50 outside trying to keep it shut; obviously the sphere will remain in a stable state as neither side has more strength, but when 2 extra people are introduced (the people holding onto the handles) all of a sudden the side trying to open the sphere has more force than the opposing team. So the sphere comes loose and all the little people can escape.

2. Vacuum
Inside the vacuum there are no longer 50 people inside trying to break open the sphere, but the 50 people outside (representing the atmosphere) are still there so the sphere remains firmly closed, and when 2 people are introduced to attempt to open the sphere, the sphere remains closed because those two people alone don’t have enough strength to fight against the 50 people working against them.

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