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How To Debunk Perpetual Motion Machines

Remember that little drinking bird toy that fascinated us when we were young because it would "drink" all day long without batteries or an obvious power source?

Many of us think these perpetual motion machines run on magic when we're young, but then we become better educated on the laws of thermodynamics and discover the drinking bird is a thirsty little liar.

(YouTube Link)

Jon Farrow of the Royal Institution joined Tom Scott to discuss how to debunk perpetual motion machines by locating their hidden power source, starting with that basic water drinking bird:

There are two major systems at play here- the first is an external one to do with evaporation- the bird's head gets loaded with water, and then this water is allowed to evaporate. As you'll know if you've ever worked up a sweat when liquids evaporate they cool down their surroundings. The second one is an internal one to do with the red fluid here. This fluid is quite volatile, and has a boiling point close to room temperature- it's being pushed and pulled up the neck of the bird depending on the temperature of the head. When the head is cold the liquid is sucked up, and when it's warm the liquid drains into the base. This shifts the center of gravity of the bird allowing it to continue rocking so long as it has a ready supply of water.

-Via Laughing Squid


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I cannot remember the exact quote or the attribution (I thought Feynman, but can't find the story): A physicist was once asked to debunk a perpetual motion device, and responded, "You should ask a magician instead." Sometimes understanding such devices is about understanding misdirection and how people can be tricked (even if by themselves) more so than understanding the basic principles behind the device.
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