what causes a resistor to turn current into radiation? - Page 1
Forum section: electronics / beginners

author: iteratee
author_rank: Contributor
timestamp: August 01, 2020, 04:07:04 am
content: If I have a solid resistor in a vacuum with no nearby mutual inductor / antenna or adjacent particles to conduct heat, what makes some of the electron flow get tapped off by the lattice to make photon radiation? If an electron bumps into an ion or gets close enough for both to feel substantial mutual force, then some parasitic action must prevent 100% of the inertia from being conserved right? Otherwise the particles would just bounce around and resonate forever.There must be a probablistic mechanism due to the fact that one individual particle bounce can't give off an arbitrary fraction of its energy. Some bounces must conserve 100% of their inertia.Edit: crap, a multi-atom resistor is its own antenna. One-atom resistor then.

author: bob91343
author_rank: Super Contributor
timestamp: August 01, 2020, 05:38:29 am
content: Since neither conduction nor convection is possible, the temperature will keep increasing until the energy levels are high enough for radiation to occur.

author: Nerull
author_rank: Frequent Contributor
timestamp: August 01, 2020, 06:21:04 am
content: The current through the resistor produces heat, not radiation directly. All bodies with a temperature greater than absolute zero emit thermal radiation, no current is required. All atoms are composed of charge particles, in everything with a temperature these particles are constantly accelerating, accelerating charges emit radiation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 01, 2020, 06:58:09 am
content: There's no need to imagine a minimal case, indeed, the quantum effects will make that much more complicated and weird than the bulk case.Electrical power is dissipated as heat. This occurs through whatever mechanisms: free electrons (or in some materials, ions as well) get accelerated by the electric field, and bump into the lattice for various reasons, exchanging electron energy with phonon energy (lattice vibrations).Over long time scales (in this case, roughly femtoseconds I think?), the electron and phonon energies are equal, i.e. they are in thermal equilibrium. So we don't need to worry about the electron dynamics or the energy transfer mechanisms at all, just that it happens and energy is able to flow at a useful rate.So, then, phonons to radiation: we apply the same scheme again. This time, the phonon gas is coupled to EM fields near (and under) the surface of the material, and to free space. (Note that the penetration depth of EM fields is finite and nonzero -- the optical equivalent of skin effect is the Debye scattering depth.) There is a mechanism for this exchange, and again by the equipartition theorem, they have the same energies; so we get thermal radiation of the same temperature.That exchange goes something like, a given atom has some dipole moment, so when it's shaken (by phonon energy), EM waves are generated. And vice versa, when waves are absorbed. I don't actually know, offhand, the full quantum model of Planck radiation from condensed matter.By the way, the black body derivation doesn't make any reference to matter at all -- the classical setup is merely that there's a cavity which contains a photon gas (thermal radiation). Now, that cavity might have to be made of matter, and certainly in experiments it is, but that's not a necessity; if there were some way to create total internal reflection with space itself, it would have the same effect.(It should be no surprise, then, that black holes are predicted to have a temperature as well! The total bending of spacetime itself might be considered total internal reflection, and therefore a black hole seemingly forms a black body cavity. As containers of radiation go, they do happen to be particularly well sealed, so only radiate at exceptionally low temperatures, for any astronomically known black holes. A more convincing description of this does require more advanced physics, of course; it took a Stephen Hawking to discover!)The derivation, in turn, for black body radiation, uses the energy of resonant modes, up to a cutoff frequency; the solution is the Planck law. It may be interesting to note that the distributions (population and energy) of photons is different from that of phonons or electrons (or, wait, are phonons the same, I forget?), but that doesn't matter because there is no single one-for-one exchange where one becomes the other and vice versa; this is the magic of a thermalized system, energy is always coming and going, and so the populations and energy levels of the various media aren't supposed to equate directly.So, that is what determines the radiation: the statistics of the EM field. The temperature is related to the temperature of the body emitting it, which in turn is driven by the power dissipated.It doesn't need to be a simple process, indeed the whole process involves a truly unimaginable number of individual elements (atoms, or their electrons, and the modes of the EM fields they reside in). In a certain sense, it would actually be an easier question to ask about the emission of light from an LED -- a more direct quantum interaction. By great fortune, we can integrate over most of those interactions -- the particles act together in a nearly continuous way, making a holistic approach tractable.Statistical mechanics is a notoriously difficult topic in physics, but one well worth understanding, and intimidating though it may be, it is not at all beyond human understanding! (Or if that still leaves you feeling a bit hopeless, well...TimEd: I wanted that to be a hyperlink. I really did. This fucking forum Imgur plugin is too greedy. It will find anything and embed the whole god damned thing. UGH!

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2020, 01:26:53 pm
content: The radiation emitted by a black body is averystrong function of temperature, proportional toT4, whereTis the absolute temperature (Stefan-Boltzman law).

author: cdev
author_rank: Super Contributor
timestamp: August 01, 2020, 09:49:04 pm
content: "resistance" is what does it.Even the smallest amount of electricity will produce a proportionately smaller amount of heat.There is no magic threshold. Any resistance and any current creates heat according to the same basic rules. (with RF behaving a bit differently because of the skin effect and the phase relationships of the capacitivie and inductive reactance , which is also predictable.)Quote from: bob91343 on August 01, 2020, 05:38:29 am

author: bob91343
author_rank: Super Contributor
timestamp: August 01, 2020, 10:04:43 pm
content: Temperature is actually a measure of the energy of a mass. If it has energy, it will produce radiation.

author: iteratee
author_rank: Contributor
timestamp: August 01, 2020, 11:06:18 pm
content: Hm I should have asked about heat instead of radiation then. I knew about black bodies (where the radiation comes from) but not the "phonon" vibrations that are apparently responsible for the heat conversion in the first place.

author: iteratee
author_rank: Contributor
timestamp: August 01, 2020, 11:46:57 pm
content: Quote from: bob91343 on August 01, 2020, 10:04:43 pmOne electron flying through an empty universe "radiates" something other than itself? Or this implies the local thermal energy is sort of aggregated with the average velocity?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 02, 2020, 02:35:47 am
content: Quote from: cdev on August 01, 2020, 09:49:04 pmWhat's electricity? Superconductors may like to have a word with you.Quote from: bob91343 on August 01, 2020, 10:04:43 pmNot specific to mass -- massless particles have energy too! (If you meant in the sense of "an object" or "collection of points", then yes.)Quote from: iteratee on August 01, 2020, 11:46:57 pmParticles obey some statistics; a low density electron gas might obey Maxwell-Boltzmann statistics, whereas electrons in a solid obey Fermi-Dirac statistics. In any case, there is a distribution of velocities, energies whatever. The mean energy is related to temperature by Boltzmann's constant (and a small factor determined by the distribution).A single electron doesn't really have a distribution, but if you consider its own energy as the average, then yes, you can take that as its temperature.Temperature is relative, just as velocity and energy are. Say you have a gas of electrons at ~1MeV (e.g., solar wind), and you have a LINAC electron beam at 1MeV (e.g. used for sterilization, cancer treatment). Relative to a stationary reference frame, they have the same effective temperature as such. However, a particle in the gas will have about the same energy relative to any other particle in there, whereas the electron beam will have very little energy relative to other electrons in the beam: the beam is more coherent. So you do have to be more careful in these situations, and temperature may not be the most useful measure for the situation.Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 02, 2020, 03:01:50 am
content: Quote from: iteratee on August 01, 2020, 11:46:57 pmOne electron flying through empty space undergoing acceleration, for example going in a circular orbit due to an external magnetic field, will radiate electromagnetic radiation, which takes the form of photons.

author: Jwillis
author_rank: Super Contributor
timestamp: August 02, 2020, 07:50:43 am
content: Heat is radiation. Infrared radiation. Although at a much lower energy level than X-Ray or Gamma . non the less it is still radiation.

author: TimFox
author_rank: Super Contributor
timestamp: August 02, 2020, 01:33:35 pm
content: Radio waves, infrared light, visible light, ultraviolet light, x rays, gamma rays, etc. are all photons (electromagnetic radiation). The term refers to the wavelength range, but the only difference between x rays and gamma rays is the means by which they were produced (electronic transitions or nuclear transitions).

author: paulca
author_rank: Super Contributor
timestamp: August 03, 2020, 03:13:32 pm
content: Quote from: TimFox on August 02, 2020, 01:33:35 pmInteresting. I remember reading that dental x-ray machines often use arc light to produce the x-rays rather than a radioactive source and intermediary... (That's how they normally work in hospitals isn't it?)Does that mean you can't produce enough energy in an arc flash to get something hot enough to release gamma-rays?EDIT: Apparently lightening does produce both xray and gamma ray radiation.https://phys.org/news/2017-10-lightning-afterglow-gamma.htmlAlso EDIT: Calling what produces Xrays "arc-light" turns out to be an oversimplification.

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2020, 03:25:25 pm
content: Modern mammography, dental-imaging, chest-imaging, and CT-scanner x-ray sources are all vacuum tubes with a hot cathode and an anode serving as the electron target where the x-ray photons are generated. (Listed in order of increasing tube voltage, which defines the maximum photon energy of the source, from about 30 kV to typically 140 kV.)Roentgen's original sources, from the late 19th century, were Crookes Tubes, which were gas-discharge cold-cathode devices. Changing the vacuum pressure changed the voltage across the tube limited by the "glow discharge".Radioactive sources have some use in radiotherapy, and Co-60 radioactive sources are used for radiographic inspection of pipelines, but medical imaging uses x-ray tubes.

author: iteratee
author_rank: Contributor
timestamp: August 04, 2020, 04:35:24 am
content: I thought heat was just some local oscillation of the atom. I've been trying to build a CMOS voltage reference out of discrete transistors that includes a PTAT (that would normally be made from a "latteral" BJT). Apparently this concept called "thermal voltage" is important to the temperature independence. There are several issues though, maybe I'll make another post about that. It needs a "start-up" circuit that is ridiculously sensitive.

author: Berni
author_rank: Super Contributor
timestamp: August 04, 2020, 05:47:34 am
content: Quote from: paulca on August 03, 2020, 03:13:32 pmThe method of producing xrays using xray tubes is a different phenomenon than just thermal radiation. It is possible to radiate xrays due to being very hot, but the required temperature is ridiculously high so its not practical to do.Instead these tubes accelerate electrons to very high speed with a high voltage DC field and then slam them into a tough metal target. Since wiggling electrons around can cause electromagnetic radiation to be given off means and suddenly stopping such a fast moving electron is seen as a very fast wiggle, so it produces very high energy electromagnetic radiation... ie xrays. More voltage you give it, the faster the electrons go, the harder they smack in the target and the higher energy xrays they produce.This is the reason why running a CRT with too much voltage or running certain vacuum tubes at very high voltages can cause them to start generating xrays.

author: iteratee
author_rank: Contributor
timestamp: August 04, 2020, 06:11:10 am
content: Quote from: Berni on August 04, 2020, 05:47:34 amlike giant blue star fusion-invoking kind of hot

author: gcewing
author_rank: Regular Contributor
timestamp: August 04, 2020, 08:13:13 am
content: Quote from: iteratee on August 04, 2020, 06:11:10 amOr just ordinary fission bomb kind of hot. Apparently your typical nuclear fireball is initially created mostly by x-rays heating the surrounding air.

author: paulca
author_rank: Super Contributor
timestamp: August 04, 2020, 08:33:28 am
content: Quote from: gcewing on August 04, 2020, 08:13:13 amOr lightening strike kinda hot.As an off topic. If you look up on how fusion bombs work it's pretty interesting to see they have timing circuits which are working *while* the bomb is detonating. There are things that happen between the fission bomb going off and the fusion charge. The later being initiated by a focused Gamma Ray shock wave and the fissile tampers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2020, 05:42:08 am
content: Quote from: paulca on August 03, 2020, 03:13:32 pmMost electrical arcing is very limited in particle energy, because the density of ordinary matter in them (say for an arc in air) provides so many more charge carriers than even a very high current (say 10kA) can strip away. And the fact that it's so hot means heat and matter can leave the area very quickly indeed, so the temperature stays very modest over a wide range (only a few times hotter than the Sun's surface, say).This is the arc's CV regime, where higher current flow leads to, about the same voltage drop, in fact slightly lower for the most part.At some point I think the curve ticks back up again, as you generate so much heat in such a small space, that the atoms become fully ionized (there are no additional charge carriers to free up), and more particles never get a chance to exchange that energy with cooler surroundings. And with the higher particle energy, comes a higher voltage drop. Probably the voltage drop rises coincident with increasing ionization, and it rises ever faster as atoms become exhausted of bound electrons.It is in this regime where you'll find deeper UV emission, and so on.You need astronomical power inputs to get there -- the Z machine is an example where a spark discharge leads to deep x-ray generation and nuclear phenomena.QuoteWow, scary. So nuclear reactions really can happen (albeit to quite a small degree, I suppose). Further proof that early nuclear physicists' worry about igniting the atmosphere was unfounded -- if it could happen, it would've happened billions of years ago!I wonder, then; it must be somewhere in the 100k to 1MA range, and give or take spark/arc duration and size, where rather more aggressive and interesting (nuclear) reactions can occur, at increasing probability.- So below that, you have just ordinary, well, maybe not welding exactly, but ordinary phase change of matter, whether to liquid or vapor, and this in the 2-20kA range, say. Which very much fits with the explosions of brown smoke in arc-flash scenarios -- all the vaporized copper blown off.- And in the lower range, < 2kA say, you have a useful power output for arc welding and melting.- And it just so happens that lightning indeed teases the bottom end where it gets interesting (~100kA).- Meanwhile, the Z machine is somewhere up in the hot end (~18MA?), capable of very interesting reactions indeed.Also, the stripping of atoms is easiest with light gases, so it's more likely that lightning could cause nuclear reactions in air, than comparable currents (10-100kA fault currents aren't unheard-of) in arc flash conditions (where most of the plasma is copper and iron, much heavier atoms).Now I want to see someone set up a lightning rod through a cell of deuterium gas, or (high purity) heavy water. I'll bet helium and neutron flux are detectable.Tim

author: paulca
author_rank: Super Contributor
timestamp: August 05, 2020, 10:14:04 am
content: Quote from: T3sl4co1l on August 05, 2020, 05:42:08 amI understand that gamma photon are released to account for the energy to balance when nuclei change.However, I can't find anything that suggests if you get a black body hot enough that it cannot emit gamma rays. It's not easy to google though.

author: Berni
author_rank: Super Contributor
timestamp: August 05, 2020, 12:10:02 pm
content: Someone did some math about it and got a result of 5 million degrees kelvin to emit on average one xray photon per second:https://www.reddit.com/r/askscience/comments/3e89m8/how_hot_does_an_object_need_to_be_to_emit_gamma/So yeah this is going into fusion reactor kind of temperature territory.

author: paulca
author_rank: Super Contributor
timestamp: August 05, 2020, 01:48:36 pm
content: I was just checking I wasn't nuts and there still was a relationship between blackbody temperature and the wavelength of photons released and there weren't special cases for higher energies.I'm also wondering if the lightening example is caused by thermal energy alone or if it's the thermal energy high enough to cause nuclear interactions in the atmosphere resulting in xray/gamma ray emissions. Although I think the gamma emissions were noted in the upper ionosphere around sprites, so maybe more going on there too.EDIT: Nuclear interactions releasing photons in gamma range ... are they not going to require fusion or fission? In the air there are no heavy elements to split and anything heavier than ironrequiresenergy to fuse and does not realise it.... there is very little in the atmosphere as heavy as iron. That leaves fusion. Can the lightening bolt get hot enough to fuse atoms and get xray/gamma.EDIT2: Is that not where most of our nitrogen comes from? (or am I confusion something else)

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2020, 03:05:47 pm
content: Alpha, beta, and gamma radiation are common emissions from radioactive decay of nuclei. When discovered, they were named in order of penetrating power. Of these, gamma rays are high-energy photons. Alphas are helium-4 nuclei (charge +2) and betas are electrons (charge -1) or positrons (anti-electrons, charge +1), both of which have mass.For example, Cobalt-60 beta-decays first to a nickel isotope, which then emits gamma photons at well-defined energies of 1.17 and 1.33 MeV,No fission is involved.

author: paulca
author_rank: Super Contributor
timestamp: August 05, 2020, 03:24:09 pm
content: Yes. But for those you need radioactive elements.

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2020, 03:42:08 pm
content: Oops--I misread the context of your statement about requiring fission.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2020, 03:51:16 pm
content: Technically no, thermal is x-rays -- even at 1MeV+. X-rays are produced by electron interaction, gamma by nuclear reaction.So the ~10keV emission from tritium is gamma, even though it's eminently within the realm of electrically produced x-rays.Well, I suppose it might be worth arguing that x-rays from a hot proton, neutron, and other nucleons, plasma (or all the way up to quark-gluon plasma even) might be gammas, but anyway, such cases are clearly where the distinction lacks value.As for black body, it may get complicated at extreme temperatures, where pair production (>1MeV) enables additional degrees of freedom. You still have the same old Planck distribution among photons -- there are just have additional forms of radiation as well.I think the consequence of this is, not so much the spectrum changing, as the heat capacity of space itself going up -- in addition to EM modes, electron and etc. modes become available. Again using the equipartition theorem, when all these modes are in thermal equilibrium, each has a distribution given by its statistics (an electron-positron gas will have a Maxwell-Boltzmann distribution, I think?).Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2020, 03:55:48 pm
content: Because of the definition (by source, not energy), there is a large overlap between x-ray energies and gamma energies.

author: paulca
author_rank: Super Contributor
timestamp: August 05, 2020, 04:03:25 pm
content: Quote from: TimFox on August 05, 2020, 03:55:48 pmYeah. Sounds like two slightly out of sync models interacting. Not uncommon I suppose. As long as the electro-magnetic effects remain the same I suppose and ... gammas are electromagnetic.

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2020, 04:58:57 pm
content: No, just a question of definition. The other common forms of radiation are defined (roughly) by wavelength (some overlap between short microwave RF and long infrared wavelengths) but X rays and gammas were named before it was determined that they were EM radiation as well. We now understand that they are all of the same nature. Luckily, we no longer need to consider “N rays” (q.v.).

author: Berni
author_rank: Super Contributor
timestamp: August 05, 2020, 05:28:06 pm
content: Yeah the exact borders of what we call different wavelengths of electromagnetic radiation are pretty fuzzy since the effects they are usualy named after morph into one another gradually rather than at one single sharp wavelength.You could just as easily split up X-rays into various bands of higher or lower energy ones, Like for example we have UV-A, UV-B and UV-C, tho with that we had a good reason to give it separate names since it has different effects. UV-A is considered fairly safe, higher energy UV-B not anymore since it gives you burns, even higher energy UV-C instead is so energetic that it rips oxygen apart just by shining trough air. But once you go even higher and get to x-rays they get powerful enough to rip apart just about anything. But they still have similarity as in high energy UV-C has a lot of the chemical bond wrecking ability of X-Rays, while low energy X-Rays have the property of easily being stopped by almost anything (like UV light).For example Extreme UV (EUV) straddles this line as its actually considered low energy X-rays rather than UV and the latest and greatest chip making lithography process uses 13nm (or about 100eV) EUV light to get around the wavelength limitations. Tho making such EUV light and focusing it gets really really tricky.

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2020, 05:34:44 pm
content: Again, definitions. Usually, people say “light” if you can focus it. However, radio waves can be focused with antenna structures (Hertz made lenses out of pitch) and X-ray focusing mirrors have been made using glancing incidence. The distinction is more engineering than physics.

author: iteratee
author_rank: Contributor
timestamp: August 06, 2020, 12:33:15 am
content: Many of my teachers at one point referred to the "visible" spectrum as "narrow". They've all been looking at the same logrithmic chart zeroed at D.C.

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2020, 01:06:11 am
content: The visible spectrum is less than one octave wide. The radio classifications (mf, hf, vhf, uhf, etc.) are each a full decade wide. This is a normal way to express spectrum width. Full audio bandwidth (20 Hz to 20kHz) is 10 octaves.

author: cdev
author_rank: Super Contributor
timestamp: August 06, 2020, 02:45:24 am
content: Heat motion of electrons is a givenat any temperature above absolute zero? Is that right?

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2020, 03:01:34 am
content: At absolute zero, due to quantum mechanics, there is still zero-point energy. This is a complicated subject: I refer you to the textbooks. Since electrons have spin-1/2, they obey Fermi statistics, another complicated subject.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2020, 03:14:43 am
content: There can be motion (for whatever counts as motion in the quantum world) at zero temperature, just not non-degenerate state transitions, because there's no energy to move between states.Tim