Wikipedia:Reference desk/Archives/Science/2020 August 8

= August 8 =

Would a positively charged solar cell produce electricity in the absence of electrons?
From the article on the photoelectric effect, the implication seems to be that electrons are simply "knocked loose" and hence the flow of electricity. But what if the cell and all of its connected components were instead (mostly) devoid of electrons, say in deep space. Would the positively charged assemblage thereof be able to be powered up by a ray of sunlight, or would it simply remain "dead" until enough free electrons floating about in the vacuum of space had been "captured"? In other words, can electrons be created by the photoelectric effect? Earl of Arundel (talk) 06:42, 8 August 2020 (UTC)


 * Why do you think that objects in deep space are (mostly) devoid of electrons? Why do you think that the photoelectric effect involves capturing electrons from the environment? --Guy Macon (talk) 07:03, 8 August 2020 (UTC)
 * It was more of a spherical cow kind of question. Assuming those conditions were met, would the photoelectric effect in fact still occur? Earl of Arundel (talk) 07:30, 8 August 2020 (UTC)


 * Spherical cows are simplifications. You are essentially asking "if a cow was made entirely of bad science fiction stories posted on Facebook. would it still Moo?" You would no longer be talking about a cow, spherical or otherwise. Likewise, your question is no longer talking about the photoelectric effect. --Guy Macon (talk) 16:22, 8 August 2020 (UTC)


 * The direct answer to your question is that if a solar cell were "(mostly) devoid of electrons", it would instantly explode with unfathomable violence. It might be instructive to try to calculate the amount of energy required to bring that much positive charge all together in a small space. --Trovatore (talk) 07:08, 8 August 2020 (UTC)


 * Well it can't be totally impossible. There are things like alpha particles "out in the wild". Earl of Arundel (talk) 07:30, 8 August 2020 (UTC)


 * An alpha particle can be held together without electrons by the nuclear force, but that only works for things the size of an atomic nucleus. When dealing with something larger than a nucleus (or just a very large nucleus), it no longer works and the thing desintegrates. Without electrons, there would be no photoelectric effect, but neither would there be any structures larger than an atomic nucleus, so no solar panels. PiusImpavidus (talk) 09:56, 8 August 2020 (UTC)
 * Disintegrates is a rather mild word. Larry Niven created the notion of the "Slaver digging tool", which "suppressed the charge on the electron".  From his description of the effects, I can only conclude that either Niven never actually did the calculation, or he meant it to be a partial suppression. --Trovatore (talk) 17:44, 8 August 2020 (UTC)
 * Scientists at the Lawrence Livermore National Laboratory were in fact able to remove 100% of the electrons from at least a few uranium nuclei.[] So an atom stripped of its electrons isn't (necessarily) inherently unstable. Granted, given the number of free electrons available here on Earth, such a substance must be highly reactive. Earl of Arundel (talk) 00:25, 10 August 2020 (UTC)
 * A uranium atom stripped of all its electrons is a bare nucleus, and as I wrote above, this nucleus can be stable on its own. Except that uranium has such a large nucleus that it isn't. It's radioactive. On the other hand, when we take a molecule and strip it of all its electrons, it won't hold together. That's because molecules are held together by the electromagnetic force. PiusImpavidus (talk) 07:02, 10 August 2020 (UTC)
 * Ah, good point. It's settled then. Thanks. Earl of Arundel (talk) 15:24, 10 August 2020 (UTC)


 * I guess my real question here is whether or not electrons can be created from photons? Earl of Arundel (talk) 07:30, 8 August 2020 (UTC)


 * Two photons together can be converted into an electron and a positron. It's called pair production and has nothing to do with the photoelectric effect. PiusImpavidus (talk) 09:56, 8 August 2020 (UTC)


 * Right, and even if this were to occur in a solar cell the net charge would remain zero so... Earl of Arundel (talk) 00:25, 10 August 2020 (UTC)


 * A solar cell and connected components consisting of antimatter, say floating in deep space, could be mostly devoid of electrons. --Lambiam 12:11, 8 August 2020 (UTC)

Anyway, it was meant as a joke, which is why I put it in a smaller font. --Lambiam 07:44, 9 August 2020 (UTC)
 * Replacing all electrons with positrons, all protons with antiprotons etc. is not what most people think of when they say "take away all of the electrons". --Guy Macon (talk) 16:22, 8 August 2020 (UTC)
 * Who mentioned or suggested "taking away" the electrons? The hypothetical question implied their absence, but not the why or how of that absence. --Lambiam 22:22, 8 August 2020 (UTC)
 * Sigh... See Paraphrase. Replacing all electrons with positrons, all protons with antiprotons etc. is not what most people think of when they say "(mostly) devoid of electrons". Happy now? --Guy Macon (talk) 22:40, 8 August 2020 (UTC)
 * Very few people would say that, and if they do, I can only guess what they were thinking of.
 * To kind of sum up, photovolatic cells work because they're designed so that absorbed light produces a photo-induced charge separation via the photovoltaic effect, which can then be exploited to do work. You can do all kinds of violent things to a PV cell for experiments, but that tends to make it into something that is no longer what we think of as a "photovoltaic cell". Light will produce a charge separation as long as the material has a band gap with the appropriate energy. If the material is changed so it no longer has said band gap, then there's no more photovoltaic effect. --47.146.63.87 (talk) 00:14, 11 August 2020 (UTC)
 * Thank you. The article on band gap was particularly helpful. Earl of Arundel (talk) 05:18, 11 August 2020 (UTC)

Explanation of the continuum hypothesis
In fluid dynamics fluids are treated as continua, with assuming kinetic energy of a piece of a fluid stream as if it were a solid body, while in reality every molecule has its own velocity and, consequently, its own kinetic energy. For example, for a stream of propellant, being expelled from a rocket, we can calculate its kinetic energy given the flow rate and the thrust exerted. Would we assume the propellant stream to be a small set of uninteracting particles (molecules) we should come to the conclusion, that given momentum of the propellant may be consistent with multiple values of kinetic energy. As far as I remember, it is explained in statistical mechanics, that in a big enough set of interacting particles distribution of kinetic energies of the particles is such, that the sum of their components parallel to the direction of common motion tends to the kinetic energy of a continuous body. But I have difficulty with finding the exact theorem or equation in literature. Couldn't somebody give me a reference to an article (or articles) in Wikipedia, where this principle is explained and/or relevant sources are cited? Эйхер (talk) 14:54, 8 August 2020 (UTC)


 * Courtesy links: Our Continuum hypothesis article is for the mathematical hypothesis about the possible sizes of infinite sets, but it does contain the hatnote: "For the assumption in fluid mechanics, see Continuum assumption", that link being a redirect to the relevant portion of Fluid mechanics (which refers to it as both the "continuum assumption" and the "continuum hypothesis"). We also have Continuum mechanics and Aerodynamics.  Also relevant are Knudsen number and Compressible flow. -- ToE 15:54, 8 August 2020 (UTC) Edited to reflect hatnote cleanup; explicitly noted "Courtesy links". -- ToE 22:04, 8 August 2020 (UTC)
 * Thank You anyway! Here the concept is described, as well as boundaries of its applicability, but not the explanation. What I'm looking for is the theoretical explanation of validity of the "continuum assumption". Эйхер (talk) 17:18, 8 August 2020 (UTC)
 * Not a physicist but can't help noticing how similar your formulation is to the central limit theorem in mathematics. 93.136.85.102 (talk) 03:58, 9 August 2020 (UTC)


 * Suppose you have a black box with a 1 kg mass, moving at a speed of 1 m/s. What is its kinetic energy? Well, 0.5 J, of course. Now you open the box and inside you find a bunch of very active frogs jumping in all directions. Their kinetic energies, added up, are considerable, but do not contribute to the kinetic energy of the box. Now consider a volume of the fluid or gas enclosed in a thin membrane that moves along with the flow at the average velocity of the particles. This forms a virtual box that can be analyzed like the moving box with frogs. A difference is that the membrane is not impermeable, but on the grand scale of things the relatively few particles that enter the virtual box in one direction (passing through the membrane) are compensated by those that exit it in the same direction. So calculations treating this as a homogeneous mass will come out the same as those treating it as particulate. --Lambiam 07:36, 9 August 2020 (UTC)
 * Possibly out of scope, but the concept of stress-energy tensor deals nicely with boxes of frogs, and indeed other less interesting situations such as ideal fluids. HTH, Robinh (talk) 21:34, 9 August 2020 (UTC)