My coworker is telling me that perpetual motion exists and shows me a video of a guy using the siphoning effect on some water cans.
I’m telling him that perpetual motion, physically, cannot exist.
But why would it not work?
In: Physics
Because sooner or later the container you are siphoning from runs out, and then the siphon stops.
If you post a link to the video I can probably tell you why it is either fake or won’t work.
Siphoning only works if your target is at a lower elevation than your source. Or, more expressively presented, if the fluid column going *down* is heavier than the fluid column going *up*. It basically borrows a bit of potential energy from the fluid going down the bend and uses it to lift the intake fluid over the bend. It’s very much the same thing as throwing a long bit of rope over a pulley down into a basement and watching it spool up the pulley and down in the hole from its weight.
Which video? Without special effects the water level will just equalize between the tube and jug.
You can’t siphon upwards, the energy required to lift the liquid being siphoned would be greater than the gain of harnessing the theoretical siphon.
Siphoning works by moving water (or other fluid) from high potential energy to low potential energy, but by going through a *higher* state on the way. Gravitational potential energy is directly proportional to height, so we can just go with height = energy.
The core reason siphoning works is that at the highest point of the siphon, the energy gained by a unit of water going to the lower container is more than the energy required to lift another unit of water from the higher container up to the top.
Ok so the basics of the perpetual motion is defined as “a system which requires no external source of power to function once set in motion”. For example if in space you pushed a block and if there were no gravity affecting it and no space debris or anything hindering its motion, it would move in a straight line forever. The answer why siphoning systems are not perpetual motion is because it requires there to be a potential energy difference which adds power to the system.
So whilst in effect is a perpetual motion machine, in definition it is not.
But the more important definition for it which is beyond a curiosity and makes a perpetual motion machine useful is “a system which can provide power without requiring external forces once set in motion”. So for example the sun, whilst requiring a great deal of energy to get started provides energy for free. But the sun is not a perpetual motion machine since it will die off one day once all the initial energy is used (in very basic terms (advanced terms this is not wholly correct but that is beside the point)).
So whilst a siphoning system can run “forever” (Heavy use on the quotations marks there because it is not a true statement but in terms of human life span could maybe be considered valid in the same way the sun is not a perpetual motion machine). It requires an external power source which is gravity. And cannot provide more power than what is coming in. So even if it can be classed as a perpetual motion machine (which it cannot) it is a pointless one either way since it does not produce power.
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