Hilarious, inspirational and loaded with cosmic coincidences, this was one of the best evenings ever! Many of our currently favourite themes were mentioned in the show (such as the controversy of Francis Galton, the BED/ Banana Equivalent Dose, sound wave visualizations, laser, drawing and playing with ellipses, Euler’s formula). Plus Simon got to meet his teachers from several favourite educational YouTube channels, Numberphile, StandUpMaths and Steve Mould.
We have tried using an LED backwards: not get it to shine by letting an electric current pass through it but produce electricity by shining light on an LED (this is how solar panels work). It’s important to use a sensitive LED for this experiment, and as we have observed, it also seems to be important to use light photons of the same frequency as the colour of the LED (red laser didn’t work on a white LED, but it may have to do with the fact that red light is weaker than white light anyway, i.e. has a lower frequency). The picture below shows us measuring the voltage of the current produced by the LED.
We’ve have learned this and a a lot more from Steve Mould’s video on How diodes, LEDs and solar panels work: Photovoltaic cells and LEDs are both made of diodes. Diodes are designed to allow electricity to flow in one direction only but the way we make them (out of semiconductors) means that can absorb and emit light.
In the video, Steve shows how the semiconductor atoms share elctrons. Semiconductors are crystal structures of atoms are replaced by the atoms of neighboring elements, for example a structure where some silicon (Si) atoms are replaced by phosphorus (P) or boron (B) atoms, thus providing for free electrons inside the structure (N-type conductor) or for free “holes” unoccupied by electrons (P-type conductor). A diode is basically two semiconductors pushed together. With enough voltage, the electrones are able to jump from the N-type semiconductor across the depletion zone and into the P-type semiconductor, emitting light (photons) as they fill the holes and go from a high energy state into the low energy state.
If you shine a light at a diode, you can kick some electrons from their shells and thus create free electrons and holes that will move (because of the electric field in the depletion zone) and generate voltage.
This demo is inspired by a recent video on Steve Mould’s channel. It’s about creating a movable hole in soap film with a loop of cotton thread (the photo shows Simon sticking a pencil through such a hole). Once in the soap membrane, the cotton thread forms a perfect circle. It’s because the soap film tries to minimise its area and compress as much as it can. The only way that can happen is to maximise the area of the hole. And as we know, with a fixed perimeter, the biggest area that you can make with it is if you form that perimeter into a circle, Simon explains as I’m writing this.
Simon has been dreaming to try creating the Steve Mould effect, or the chain fountain phenomenon, also known as the self-siphoning beads. It’s a counterintuitive physical phenomenon, almost a magic trick, that occurs when you place a chain of beads inside a beaker and pull on one end of the chain, allowing it to fall to the floor beneath. This establishes a self-sustaining flow of the chain of beads which rises up from the jar into an arch ascending into the air over and above the edge of the jar with a noticeable gap (the higher the distance between the floor and the beaker, the higher the arch), as if being sucked out of the jar by an invisible siphon.
According to the Wikipedia page about the chain fountain phenomenon, a ball chain (or anything with rigid links) produces the best results. Indeed, we had beautiful results with a 50m long nickel ball chain, but a 1m long pearl necklace also worked, even though the links it had weren’t that rigid (just knots of cotton thread)! Anything for science, I’m a young scientist’s mom.
Simon was delighted to learn that this phenomenon has actually been officially named after one of his favourite science presenters on YouTube, Steve Mould. Mould’s YouTube video, in which he demonstrated the phenomenon of self-siphoning beads and proposed an explanation, brought the problem to the attention of academics John Biggins and Mark Warner at Cambridge University! They published their findings in Proceedings of the Royal Society A.
So what’s the scientific explanation? According to Wikipedia, the chain fountain effect is driven by upward forces which originate inside the jar. The origin of the upward force is related to the stiffness of the chain links, and the bending restrictions of each chain joint. When a link of chain is pulled upward from the jar, it rotates like a stiff rod being picked up from one end. This rotation produces a downward force on the opposite end of the link, which in turn generates an upward reactive force. It is this upward reactive force that has been shown to drive the chain fountain phenomenon. A similar effect is observed when pouring viscous fluids from a beaker, Steve Mould pointed out.
We should warn anyone who’s about to buy ball chain, however, that it’s not only the joy of watching the chain fountains flow, but the tears of spending hours of untangling the wretched thing!