Boyle’s perpetual motion device
Jearl Walker www.flyingcircusofphysics.com
March 2014 Robert Boyle (1627-1691) designed a perpetual motion device in which water continuously drains from a container into a tube and then flows through the tube and back above the container where it falls back into the container.
An immediate argument against the scheme is that the water should not rise any higher in the tube than in the container. Although the weight of the water in the container is much greater than the weight of the water in the tube, the water pressures are equal at any given height. Pressure is force divided by area. At the bottom of the container, the large weight of the water bears down on the large area of the bottom. At the same level in the tube, the small weight of the water above that level bears down on a small cross-sectional area of the tube. Although the weights and areas are different, the pressures (the ratio of force to area) are the same. Here is photographic evidence of the equilibrium:
http://www.oberlin.edu/physics/catalog/demonstrations/fluids/pascalvases.htm Pascal vases
A counter argument is that if the tube narrows considerably, a capillary force will pull the water up the tube. This force is really due to the attraction of the water molecules for themselves and for the tube molecules. You can see this electrical attraction in an ordinary glass of water. Fill the glass almost full and then examine the surface near the glass wall. That surface is curved upward along the wall. But also notice that the water climbs only a short distance up the wall. If the water is in a narrow tube, it can climb higher.
If the tube in Boyle’s arrangement is very narrow, the water can climb significantly higher than the water level in the container. However, that high climb means that there is a significant force holding the water to the tube wall. If the tube is bent over and down as in Boyle’s arrangement, that force is too strong to allow the water to leave the tube. So, the water might climb higher than the container but certainly cannot flow.
Other liquids behave just like water. However, here is a video that supposedly demonstrates how carbonated beverages might actually flow as envisioned by Boyle.
The argument supporting this video here is in two parts.
(1) The presence of foam in the tube means that the liquid density there is smaller than the liquid in the container. Thus, at any given level the height of the foamy liquid would have to be greater than the height of the non-foamy liquid in order to have the same pressure.
(2) Because the bubbles in the foam are pushed upward by buoyancy, they drive the liquid up the tube.
Even if what we see in the video were real, this still would not be a perpetual motion device because the bubbles would eventually break, equilibrium would be established, and the liquid would be stationary.
However, I and many others believe that the video has been faked. The flow is just too vigorous to be driven by buoyant bubbles. When I pour a beer into a glass too quickly, the foam can overflow the glass rim but not with the vigor I see in the video. Most likely a small motor is hidden in the tube in the region where it is supported by the wood arm.
Here is another video where no such motor is used:
When I wrote “The Amateur Scientist” department for Scientific American magazine, I received numerous designs for perpetual motion devices along with earnest letters explaining how they would work if built. Although disproving such designs can be interesting, convincing someone that a design cannot work can be extremely time consuming. Some people simply want to believe in perpetual motion regardless of the fact that not a single working perpetual motion device has ever been built. (I leave out continuous motions that are possible with superfluids and superconductors.) Here is a video about such beliefs:
Do you want the pub-trick stories here at the FCP site? Use the following links and then scroll down a page or search for "pub trick". Keep in mind my point --- anyone can do a pub trick but the real trick is to explain it without bluffing or just waving your hands in the air. Physics = the power to explain.
Chapter 1 archives
Water and the disappearing cigarette click this
Balancing a coin on a folded paper edge click this
Lifting a bottle with a thumb and one finger, click this
Hanging spoons from the nose, click this
Hanging bottle caps on your face, click this
Standing eggs on end click this
Removing a lighter from under a bottle click this
Removing a bill from between balanced bottles click this
Removing the cork from a wine bottle click this
Balancing a hammer and a lorry click this
Champagne cork as a morter round click this
Removing a coin from under a mug click this
Hanging a bottle on the wall click this
Matchstick rocket click this
Transferring a steel ball between beer mats click this
Tying a ring hitch click this
Chapter 2 archives
Reversing an egg in a tequila glass click this
Blowing out a candle click this
Escape from a cellophane pocket click this
Exchanging water and whiskey click this
1000 drops from an empty bottle click this
Yard of ale and beer boot click this
Collecting grains of black pepper click this
Tia Maria wormy action click this
Vortex in a bottle and the vortex beer bottle click this
Making straw paper stretch and crawl click this
Making a ketchup packet float and dive click this
Lifting rice with a rod click this
Inflating a long sandwich bag click this
Toothpick design trick click this
Using glug-glug to clear beer foam click this
Egg tricks click this
Raisin in champagne click this
Inverted can of Red Bull click this
Chapter 3 archives
Moving match sticks on a glass rim
Popping a plastic straw
Chapter 4 archives
Slam-freezing a beer or soda
Lighting a candle
Cooling beer on a hot day
Picking a shot glass with the palm
Rattling a coin
Chapter 5 archives
Rotating a matchstick balanced within a glass container