Pressure II Segment 2: Where Does Pressure Come From? - a podcast by PSD Channel 10 and Little Shop of Physics

Grace and Ellis help Brian demonstrate the source of pressure. They start with a canister containing pellets -- it has a speaker at its base. When the power for the speaker is turned on, it bounces the pellets, which in turn push on the lid. This simulates what is happening at the microscopic level with pressure.
They then demonstrate how pressure effects liquids. Using a pitcher and a bottle with a tube attached, they build a barometer. When they increase the molecules, the pressure inside increases. They can also increase the pressure with heat.

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