What if you have a need for liquid nitrogen, but you do not wish to simply order it from a local supplier? In that case you can build your very own pulse tube cryocooler, as [Hyperspace Pirate] is in the process of doing over at YouTube. You can catch part 1 using a linear motor and part 2 using a reciprocating piston-based version also after the break. Although still very much a work-in-progress, the second version of the cryocooler managed to reduce the temperature to a chilly -75°C.
The pulse tube cryocooler is one of many types of systems used for creating a cooling effect. Commercially available refrigerators and freezers tend to use Rankine cycle coolers due to their low cost and effectiveness at (relatively) warmer temperatures. For cryogenic temperatures, Stirling engines are commonly used, although they find some use in refrigeration as well. All three share common elements, but they differ in their efficiency over a larger temperature range.
In this video series, the viewer is taken through the physics behind these coolers and the bottlenecks which prevent them from simply cooling down to zero Kelvin. Despite the deceptive simplicity of pulse tube cryocoolers — with just a single piston, a regenerator mesh, and some tubing — making them work well is an exercise in patience. We’re definitely looking forward to the future videos in this series as it develops.
Leave most objects on top of a turntable, and set it spinning, and they’ll fly off in short order. Do the same with a ball, though, and it somehow manages to roll around on top for quite some time without falling off. [Steve Mould] set about unpacking this “Turntable Paradox” in a recent YouTube video.
In the basic case, the fact that the ball rolls is what keeps it on the turntable. As the turntable spins, the ball spins in the opposite direction, as per Newton’s first law of motion. As long as the ball is allowed to roll up to the same speed as the turntable, it will pretty much stay in place in the absence of any other perturbing forces. In the event the ball is nudged along the turntable, though, it quickly ends up in a more complicated circular motion, orbiting in a ratio to the speed of the turntable itself. [Steve] explains the mechanisms at play, and dives into the mathematics behind what’s going on.
Sometimes, demonstrations like these can seem like mere curiosities. However, understanding physical effects like these has been key to the development of all kinds of complicated and fantastical machinery. Video after the break.
Energy prices have been in the news more often than not lately, as has war. The two typically go together, as conflicts tend to impact on the supply and trade of fossil fuels.
Since their relatively recent appearance on the commercial scene, rare-earth magnets have made quite a splash in the public imagination. The amount of magnetic energy packed into these tiny, shiny objects has led to technological leaps that weren’t possible before they came along, like the vibration motors in cell phones, or the tiny speakers in earbuds and hearing aids. And that’s not to mention the motors in electric vehicles and the generators in wind turbines, along with countless medical, military, and scientific uses.
These advances come at a cost, though, as the rare earth elements needed to make them are getting harder to come by. It’s not that rare earth elements like neodymium are all that rare geologically; rather, deposits are unevenly distributed, making it easy for the metals to become pawns in a neverending geopolitical chess game. What’s more, extracting them from their ores is a tricky business in an era of increased sensitivity to environmental considerations.
Luckily, there’s more than one way to make a magnet, and it may soon be possible to build permanent magnets as strong as neodymium magnets, but without any rare earth metals. In fact, the only thing needed to make them is iron and nitrogen, plus an understanding of crystal structure and some engineering ingenuity.
Erasto Mpemba’s observations initiated decades of research into the Mpemba effect: whether a liquid (typically water) which is initially hot can freeze faster than the same liquid which begins cold.
There’s a name for the phenomenon of something hot freezing faster than something cold: the Mpemba effect, named for Erasto Mpemba (pictured above) who as a teenager in Tanzania witnessed something strange in high school in the 1960s. His class was making ice cream, and in a rush to secure the last available ice tray, Mpemba skipped waiting for his boiled milk-and-sugar mixture to cool to room temperature first, like everyone else had done. An hour and a half later, his mixture had frozen into ice cream whereas the other students’ samples remained a thick liquid slurry.
Puzzled by this result, Mpemba asked his physics teacher what was going on. He was told “You were confused. That cannot happen.” Mpemba wasn’t convinced by that answer, and his observations ultimately led to decades of research.
What makes this question so hard to nail down? Among many of the issues complicating exactly how to measure such a thing is that water frankly has some odd properties; it is less dense as a solid, and it is also possible for its solid and liquid phases to exist at the same temperature. Also, water in the process of freezing is not in equilibrium, and how exactly things act as they relax into equilibrium is a process for which — physics-wise — we lack a good theory. Practically speaking, it’s also a challenge how to even accurately and meaningfully measure the temperature of a system that is not in equilibrium.
But there is experimental evidence showing that the Mpemba effect can occur, at least in principle. How this can happen seems to come down to the idea that a hot system (having more energy) is able to occupy and explore more configurations, potentially triggering states that act as a kind of shortcut or bypass to a final equilibrium. In this way, something that starts further away from final equilibrium could overtake something starting from closer.
But does the Mpemba effect actually exist — for example, in water — in a meaningful way? Not everyone is convinced, but if nothing else, it has sure driven a lot of research into nonequilibrium systems.
Why not try your own hand at investigating the Mpemba effect? After all, working to prove someone wrong is a time-honored pastime of humanity, surpassed only in popularity by the tradition of dismissing others’ findings, observations, or results without lifting a finger of your own. Just remember to stick to the scientific method. After all, people have already put time and effort into seriously determining whether magnets clean clothes better than soap, so surely the Mpemba effect is worth some attention.
About a year ago, [Wyman’s Workshop] needed a fan. But not just a regular-old fan, no sir. A ducted fan. You know, those fancy fan designs where the stationary shroud is so close to the moving fan blades that there’s essentially no gap, and a huge gain in aerodynamic efficiency? At least in theory?
Well, in practice, you can watch how it turned out in this video. (Also embedded below.) If you’re more of a “how-to-build-it” type, you’ll want to check out his build video — there’s lots of gluing 3D prints and woodworking. But we’re just in it for the ducted fan data!
And that’s why we’re writing it up! [Wyman] made a nice thrust-testing rig that the fan can pull on to figure out how much force it put out. And the theory aimed at 652 g of thrust, which was roughly confirmed. And then you get to power: with a 500 watt motor, he ended up producing 47 watts. Spoiler: he’s overloading the motor, even though he used a fairly beefy bench grinder motor.
So he re-did the fan design, from scratch, to better match the motor. And it performed better than the theory said it would. A pleasant surprise, but it meant re-doing the theory, including the full volume of the fan blade, which finally brought theory and practice together. Which then lead him design a whole slew of fan blades and test them out against each other.
He ends the video with a teaser that he’ll show us the results from various inlet profiles and fan cones and such. But the video is a year old, so we’re not holding our breath. Still, if you’re at all interested in fan design, and aren’t afraid of high-school physics, it’s worth your time.
Don’t care about the advantages of ducted fans, but simply want to make your quad look totally awesome? Have we got the hack for you!
Of course, we all know that capacitors are conceptually two conductors separated by a dielectric of some sort. But outside of air-variable capacitors you normally don’t see them looking like that. For example, a film capacitor has its plates rolled up in a coil with an insulating film in between. You can’t really see that unless you take them apart. But [Electronoobs] makes some giant capacitors using large plates and does a few experiments to demonstrate their characteristics. You can see his work in the video below.
The arrangement reminded us of a Leyden jar except there’s no physical motion. He also had some entertaining footage of electrolytic capacitors exploding when connected backwards. The reason, by the way, is that electrolytic capacitors have conductive goo in them. By putting a controlled current through them during manufacturing, a very thin insulating layer forms on one electrode. The thinner the layer, the higher the potential capacitance is. The downside is that putting current in the opposite way of the formation current causes catastrophic results, as you can see.
The value of a capacitor depends on the area, the spacing, and the type of dielectric between the plates. The video covers how each of those alters the capacitor value. Real capacitors also have undesirable characteristics like leakage and parasitic resistance or inductance.
It used to be that capacitance meters were exotic gear, but these days many meters have that capability. This would be a great set of experiments for a classroom or as the basis for a kid’s science project. For example, measuring different dielectric materials to determine which is the best for different purposes.
Granted, capacitors are pretty basic physics, but it is easy to get wrapped up in using them and not think about what’s going on inside. This video is a good introduction or a refresher, if you need one. It is easy enough to make your own variable capacitors or even special capacitors for high voltages.
