Kind of a confusing title, Mods, feel free to clean it up if necessary.
I saw this drawing of a hypothetical perpetual motion machine that I think was a joke, but seems incredibly plausible.
The main issue I can see with the machine is that it requires a tank of water on one end, with a chain and string of balls running from the bottom to the top.
Is it possible to have a water tight seal that can allow things to move through it, but also keep the water inside? (Or at least make leakage/spillage very small)
The problem with that type of perpetual motion machine design is not the seal, but that as each ball passes through the seal into the bottom of the tank of water it will have to push a corresponding volume of water upwards out of the way. This will take exactly as much energy as you would then get back from that ball rising to the top of the tank of water.
First of all, as pointed out already, perpetual motion machines don’t work. There’s a picture of the device you describe in Orde-Hume’s book Perpetual Motion ( http://www.amazon.com/Perpetual-Motion-The-History-Obsession/dp/1931882517 ), along with a great many other devices. It doesn’t work – some unconsidered kink will inevitably eat up the extra force or energy you thought you had, and condemn the machine to idleness.
That said, there’s a classic case of a liquid that will penetrate seals – Superfluid helium has zero viscosity and zero entropy, and will flow through barriers that block normal fluid helium.
One hitch is that helium is only liquid within 4 degrees or so of absolute zero, and only superfluid within about 2 degrees.
The other is that, even with this advantage, you can’t build a perpetual motion machine using superfluid helium, evenutilizing the fact that heating it will convert it to normal fluid, and chilling it back down will return it to superfluid state.
You can, however, use it to construct a dilution refrigerator that will get you closer to absolute zero than a simple helium refrigerator will, so that’s something.
Friction is not what kills free-energy machines. If that were the only problem, then all you’d have to do would be to improve your engineering until the losses due to friction were less than your energy output. We can’t get friction all the way to zero, but we can get it arbitrarily low.
fuel filters for diesel engines have a hydrophobic coating on the filter element which lets fuel through but water beads up on the element and drops into a collection bowl.
As Chronos points out, friction isn’t the issue. The problem in this case is that when a ball enters the bottom of the tank through that seal, it’s fighting against the water pressure at the bottom of the tank; it has to lift up an entire column of water to make room for itself at the bottom of the tank, offsetting the energy that’s recovered as the ball floats up to the surface (and gradually lets the column of water settle back down at the bottom of the tank). Even if the whole device (including that amazing artificial anus at the bottom of the tank) is completely frictionless, it would grind to a halt as soon as you tried to extract any mechanical work from it.
Speaking of which, that nicely answers the OP’s question: an anus is pretty good at retaining liquid while allowing the admission of solid objects from the gas side to the liquid side.
But friction IS what killed perpetual motion machines.
Let’s say I have an upside down bicycle. If we could reduce friction to zero, I could spin a wheel and it would keep spinning forever. But we can’t, so the wheel will eventually stop.
Machines that generate more energy than they use are certainly a popular subclass of perpetual motion machine, but they aren’t the only kind, and ALL of them are impossible.
Martin Gardner once wrote about a similar machine invented by Irving Joshua Matrix. One of the clever elements to the device was the float components that were labeled “up” as they were going up, but on the down trip, “up” became “dn”.
Dr. Matrix, the noted numerologist, had an aboriginal-looking sidekick named Rhee, who was missing most of his choppers. They called him “One-Tooth Rhee.”
Thought experiment: suppose you invent a frictionless bicycle wheel that spins forever. Now you attach a frictionless generator to it and connect a load to the generator. Will the wheel continue to spin? Why or why not?
It will stop, because in extracting the power to run the light bulb, the generator will exert forces upon the wheel tending to slow it.
A current generated by a moving magnetic field generates its own magnetic field that opposes the change that caused it. Lenz’s Law - the Sod’s Law of Physics.
no, because a generator is only around 70% efficient, even w/o friction. heat loss in the windings, eddy current losses in the armature/stator laminations, etc. Let’s say you wanted to power a 100 watt light bulb with such a setup. Your light bulb needs 100 watts from the generator to reach full brightness. but its inefficiencies mean you need about 145 watts mechanical power to drive it.
You don’t even need friction to kill the proposed frictionless bicycle wheel. Wrap a rope around the rim so that the spinning of a wheel winds the rope and raises a weight. Even if the rope has no internal friction whatsoever, and there’s no aero drag on the weight as it is raised, the weight will receive mechanical work form the frictionless bicycle wheel, causing it to decelerate.
