Author Topic: what causes a resistor to turn current into radiation?  (Read 1164 times)

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Offline iteratee

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what causes a resistor to turn current into radiation?
« on: August 01, 2020, 04:07:04 am »
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.
« Last Edit: August 01, 2020, 05:06:49 am by iteratee »
 

Offline bob91343

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Re: what causes a resistor to turn current into radiation?
« Reply #1 on: August 01, 2020, 05:38:29 am »
Since neither conduction nor convection is possible, the temperature will keep increasing until the energy levels are high enough for radiation to occur.
 

Offline Nerull

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Re: what causes a resistor to turn current into radiation?
« Reply #2 on: August 01, 2020, 06:21:04 am »
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.
 

Offline T3sl4co1l

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Re: what causes a resistor to turn current into radiation?
« Reply #3 on: August 01, 2020, 06:58:09 am »
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...


Tim

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Offline TimFox

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Re: what causes a resistor to turn current into radiation?
« Reply #4 on: August 01, 2020, 01:26:53 pm »
The radiation emitted by a black body is a very strong function of temperature, proportional to T4, where T is the absolute temperature (Stefan-Boltzman law).
 
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Offline cdev

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Re: what causes a resistor to turn current into radiation?
« Reply #5 on: August 01, 2020, 09:49:04 pm »
"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.)


Since neither conduction nor convection is possible, the temperature will keep increasing until the energy levels are high enough for radiation to occur.
« Last Edit: August 02, 2020, 11:19:44 pm by cdev »
"What the large print giveth, the small print taketh away."
 
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Offline bob91343

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Re: what causes a resistor to turn current into radiation?
« Reply #6 on: August 01, 2020, 10:04:43 pm »
Temperature is actually a measure of the energy of a mass.  If it has energy, it will produce radiation.
 
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Offline iteratee

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Re: what causes a resistor to turn current into radiation?
« Reply #7 on: August 01, 2020, 11:06:18 pm »
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.
 

Offline iteratee

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Re: what causes a resistor to turn current into radiation?
« Reply #8 on: August 01, 2020, 11:46:57 pm »
Temperature is actually a measure of the energy of a mass.  If it has energy, it will produce radiation.
One 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?
 

Offline T3sl4co1l

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Re: what causes a resistor to turn current into radiation?
« Reply #9 on: August 02, 2020, 02:35:47 am »
Even the smallest amount of electricity will produce a proportionately smaller amount of heat.

There is no magic threshold. Also your use of the word "radiation" seems intended to make people think that resistance and heat are something more than they are.

What's electricity?  Superconductors may like to have a word with you. :)


Temperature is actually a measure of the energy of a mass.  If it has energy, it will produce radiation.

Not specific to mass -- massless particles have energy too!  (If you meant in the sense of "an object" or "collection of points", then yes.)


One 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?

Particles 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
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Offline TimFox

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Re: what causes a resistor to turn current into radiation?
« Reply #10 on: August 02, 2020, 03:01:50 am »
Temperature is actually a measure of the energy of a mass.  If it has energy, it will produce radiation.
One 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?
One 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.
 
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Offline Jwillis

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Re: what causes a resistor to turn current into radiation?
« Reply #11 on: August 02, 2020, 07:50:43 am »
Heat is radiation. Infrared radiation. Although at a much lower energy level than X-Ray or Gamma . non the less it is still radiation.
 
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Offline TimFox

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Re: what causes a resistor to turn current into radiation?
« Reply #12 on: August 02, 2020, 01:33:35 pm »
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).
 

Offline paulca

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Re: what causes a resistor to turn current into radiation?
« Reply #13 on: August 03, 2020, 03:13:32 pm »
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).

Interesting.  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.html

Also EDIT: Calling what produces Xrays "arc-light" turns out to be an oversimplification. 
« Last Edit: August 03, 2020, 03:24:18 pm by paulca »
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Offline TimFox

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Re: what causes a resistor to turn current into radiation?
« Reply #14 on: August 03, 2020, 03:25:25 pm »
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.
 

Offline iteratee

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Re: what causes a resistor to turn current into radiation?
« Reply #15 on: August 04, 2020, 04:35:24 am »
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.
 

Online Berni

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Re: what causes a resistor to turn current into radiation?
« Reply #16 on: August 04, 2020, 05:47:34 am »
Interesting.  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.html

Also EDIT: Calling what produces Xrays "arc-light" turns out to be an oversimplification.

The 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.
 

Offline iteratee

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Re: what causes a resistor to turn current into radiation?
« Reply #17 on: August 04, 2020, 06:11:10 am »
The 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.
like giant blue star fusion-invoking kind of hot :D
 

Offline gcewing

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Re: what causes a resistor to turn current into radiation?
« Reply #18 on: August 04, 2020, 08:13:13 am »
The 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.
like giant blue star fusion-invoking kind of hot :D
Or just ordinary fission bomb kind of hot. Apparently your typical nuclear fireball is initially created mostly by x-rays heating the surrounding air.
 

Offline paulca

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Re: what causes a resistor to turn current into radiation?
« Reply #19 on: August 04, 2020, 08:33:28 am »
The 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.
like giant blue star fusion-invoking kind of hot :D
Or just ordinary fission bomb kind of hot. Apparently your typical nuclear fireball is initially created mostly by x-rays heating the surrounding air.

Or 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.
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Offline T3sl4co1l

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Re: what causes a resistor to turn current into radiation?
« Reply #20 on: August 05, 2020, 05:42:08 am »
Interesting.  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?)

Most 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.


Quote
EDIT:  Apparently lightening does produce both xray and gamma ray radiation.
https://phys.org/news/2017-10-lightning-afterglow-gamma.html

Wow, 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! ;D

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. :o

Tim
« Last Edit: August 05, 2020, 05:48:54 am by T3sl4co1l »
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Offline paulca

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Re: what causes a resistor to turn current into radiation?
« Reply #21 on: August 05, 2020, 10:14:04 am »
Wow, scary.  So nuclear reactions really can happen (albeit to quite a small degree, I suppose). 

I 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.
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Online Berni

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Re: what causes a resistor to turn current into radiation?
« Reply #22 on: August 05, 2020, 12:10:02 pm »
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.
 

Offline paulca

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Re: what causes a resistor to turn current into radiation?
« Reply #23 on: August 05, 2020, 01:48:36 pm »
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 iron requires energy 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)
« Last Edit: August 05, 2020, 01:52:45 pm by paulca »
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Offline TimFox

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Re: what causes a resistor to turn current into radiation?
« Reply #24 on: August 05, 2020, 03:05:47 pm »
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.
 


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