# Hypothesis about temperature (split from Physical mechanism how matter absorbs radiation.)

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Posted (edited)
On 3/10/2024 at 11:10 AM, avicenna said:

Maybe no easy answer because we know too little yet about light.

The wiki says it is the bound electrons that absorbs radiation; then how the electrons energy get transferred  to the nucleus kinetic energy. I think temperature not dependent on the KE of electrons, but only in the KE of the nucleus or center of mass.

I think the wiki is right, the temperature of an atom is directly related to the energy of the bounding of the electrons in the atom. When an atom absorbs EM radiation it gets "excited". You can think in terms of the equation E =mc2. According to it, and as Einstein said, a "body" of mass m can absorb some quantity ΔE of EM radiation augmenting its mass in a quantity Δm = ΔE/c2. Mass is something directly related to the internal energy of a "body" and in your case of an atom.

Seems to me you are getting confused with the gases behavior in which "heat" is absorbed verifying the gases' law PV = nRT. In this case part of the energy of a radiation is transformed to some kinetic energy on the atoms/molecules and then the gases can exert pressure in a closed volume but the temperature of the gases is the temperature of its atoms/molecules as described above.

Edited by martillo

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Posted (edited)
1 hour ago, martillo said:

I think the wiki is right, the temperature of an atom is directly related to the energy of the bounding of the electrons in the atom. When an atom absorbs EM radiation it gets "excited". You can think in terms of the equation E =mc2. According to it, and as Einstein said, a "body" of mass m can absorb some quantity ΔE of EM radiation augmenting its mass in a quantity Δm = ΔE/c2. Mass is something directly related to the internal energy of a "body" and in your case of an atom.

Seems to me you are getting confused with the gases behavior in which "heat" is absorbed verifying the gases' law PV = nRT. In this case part of the energy of a radiation is transformed to some kinetic energy on the atoms/molecules and then the gases can exert pressure in a closed volume but the temperature of the gases is the temperature of its atoms/molecules as described above.

This seems awfully garbled not to have much to do with the topic of the thread.

1) Temperature is a bulk property. An individual atom does not have a temperature. If you think you have read that it does on Wikipedia, I feel sure you have misunderstood what you have read. If you can provide a link perhaps I can explain what it is actually saying.

2) E=mc² has got nothing to do with the topic under discussion.

3) The Perfect Gas Equation, or Ideal Gas Law, has nothing to do with the topic either. Firstly the question is about metals,  and secondly, the effect of temperature  on pressure and volume has got nothing to do with how radiation is absorbed and causes a rise in temperature.

Edited by exchemist
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3 hours ago, martillo said:

I think the wiki is right, the temperature of an atom is directly related to the energy of the bounding of the electrons in the atom.

No

3 hours ago, martillo said:

When an atom absorbs EM radiation it gets "excited".

Which isn’t necessarily what’s going on here, with a solid metal, where we have a band structure for electron energy. You don’t have to promote an electron to a higher band. Since the OP specified IR, this means the energy of the photon is probably less than 1 eV

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Posted (edited)

The main question of the OP is:

On 3/9/2024 at 12:33 PM, avicenna said:

I'll like to know the actual physical mechanism how matter absorbs EM radiation.

I will try to explain my reasoning in a more comprehensible way.

I have found very appropriated to mention what Einstein said about the subject because just applying classical electrodynamics he reached to the conclusion that If a body absorbs an EM energy ΔE its inertial mass augments in Δm = ΔE/c2. This gives us the notion that if a massive object absorbs EM radiation it stores it internally in its basic structure constituted by the nucleus and the surrounding electrons of the atoms of the object. This can be applied to any kind of object, solid, liquid, gas, whatever. We can think they are all constituted by nucleus of atoms and electrons everything linked by some bonds and those bonds made by electromagnetic forces. We can think then that the energy is stored as an electromagnetic energy in an entire electromagnetic structure. I think this is totally compatible with the notion of "lattice vibrations" in crystalline metallic structures as @exchemist mentioned although I don't think they need to vibrate. I think the same is accomplished with static structures where the lattice distances can vary accordingly. Furthermore I think the same reasoning would apply to any material object, metallic or not and even for isolated molecules and I think the same could in principle be extended to isolated atoms but I must agree this is just a thought of mine and not having any credited reference to mention.

Hope my reasoning could have sense now...

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34 minutes ago, martillo said:

We can think then that the energy is stored as an electromagnetic energy in an entire electromagnetic structure. I think this is totally compatible with the notion of "lattice vibrations" in crystalline metallic structures as @exchemist mentioned although I don't think they need to vibrate

If there is no vibration, how is it compatible with the notion of lattice vibrations?

The mass increases because there’s more energy. The two are equivalent statements; mass is a form of energy. But an energy increase does not necessarily mean a temperature increase - a spinning object has more energy than a non-spinning one, but the spinning has no effect on the temperature. In classical terms, temperature is dependent on the KE of the constituent atoms, but not the translational KE of the center-of-mass. QM recognizes that the atoms will undergo collisions, and that can cause excitations, so the distribution of states also indicates the temperature.

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11 minutes ago, swansont said:

The mass increases because there’s more energy. The two are equivalent statements; mass is a form of energy. But an energy increase does not necessarily mean a temperature increase - a spinning object has more energy than a non-spinning one, but the spinning has no effect on the temperature.

Einstein's conclusion is related to the inertial mass, this means the mass at "rest" and the total energy which includes the kinetic energy. If a massive object absorbs EM energy it augments its inertial mass, the "rest mass". This means the object stores the EM energy in its internal structure which in principle could involve internal vibration, I agree, but also could be stored in the non vibrating static electromagnetic field of the structure. In principle both cases are possible. Why not to consider the second one?

Temperature is a magnitude related to the intensity of the EM radiation emitted by an object. An object that increases its internal energy emits more EM radiation and so its temperature is increased. For instance the electrons in the atoms can absorb extra EM energy reaching configurations of higher levels of energy in the atom and being capable to jump to lower levels emitting correspondent EM energy. This implies more energy radiated by the object and so a higher temperature. This is the way EM energy can be absorbed by an object without involving a kinetic vibration of the atoms.

Seems to me temperature can be explained in both ways, with or without vibrating atoms. I would prefer the second one with no vibrations.

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22 minutes ago, martillo said:

Einstein's conclusion is related to the inertial mass, this means the mass at "rest" and the total energy which includes the kinetic energy. If a massive object absorbs EM energy it augments its inertial mass, the "rest mass". This means the object stores the EM energy in its internal structure which in principle could involve internal vibration, I agree, but also could be stored in the non vibrating static electromagnetic field of the structure. In principle both cases are possible. Why not to consider the second one?

Temperature is a magnitude related to the intensity of the EM radiation emitted by an object. An object that increases its internal energy emits more EM radiation and so its temperature is increased. For instance the electrons in the atoms can absorb extra EM energy reaching configurations of higher levels of energy in the atom and being capable to jump to lower levels emitting correspondent EM energy. This implies more energy radiated by the object and so a higher temperature. This is the way EM energy can be absorbed by an object without involving a kinetic vibration of the atoms.

Seems to me temperature can be explained in both ways, with or without vibrating atoms. I would prefer the second one with no vibrations.

That’s ridiculous. Temperature is proportional to the mean thermal kinetic energy of the molecules or atoms of the substance. 1/2kT per degree of freedom. This is basic kinetic theory.

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17 minutes ago, exchemist said:

That’s ridiculous. Temperature is proportional to the mean thermal kinetic energy of the molecules or atoms of the substance. 1/2kT per degree of freedom. This is basic kinetic theory.

Not ridiculous if you consider that EM radiation has momentum and carries energy (in both models: electromagnetic wave or photon). This way EM radiation can produce movement and kinetic energy when colliding with atoms or molecules. KMT theory is not altered by this, by the way, it offers a good explanation of some things.

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10 minutes ago, martillo said:

Not ridiculous if you consider that EM radiation has momentum and carries energy (in both models: electromagnetic wave or photon). This way EM radiation can produce movement and kinetic energy when colliding with atoms or molecules. KMT theory is not altered by this, by the way, it offers a good explanation of some things.

What is ridiculous is to think you can explain temperature without the concept of motion of atoms and molecules. It is fundamental.

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Posted (edited)
1 hour ago, martillo said:

Seems to me temperature can be explained in both ways, with or without vibrating atoms. I would prefer the second one with no vibrations.

Preference?

We know that metals expand appreciably with increasing temperature, so some of the thermal energy is absorbed in the 'static' phenomenon of increased interatomic bond lengths. But this isn't thermal energy anymore and doesn't contribute to the temperature.

You can't discard lattice vibrations just because you don't like them.

And I think you'll struggle to explain the typically high thermal conductivity of most metals if you ignore the free electrons.

Edited by sethoflagos
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Posted (edited)
58 minutes ago, exchemist said:

What is ridiculous is to think you can explain temperature without the concept of motion of atoms and molecules. It is fundamental.

What do you think cannot be explained this way? Think in EM radiation as an intermediary in the transferring of kinetic energy between particles when no direct collisions are present.

54 minutes ago, sethoflagos said:

And I think you'll struggle to explain the typically high thermal conductivity of most metals if you ignore the free electrons.

It is well known metals are excellent in absorbing and emitting EM radiation. No free electrons involved.

54 minutes ago, sethoflagos said:

We know that metals expand appreciably with increasing temperature, so some of the thermal energy is absorbed in the 'static' phenomenon of increased interatomic bond lengths. But this isn't thermal energy anymore and doesn't contribute to the temperature.

Right, thanks for the comment. I would just correct that the same way thermal energy can be absorbed in the static structure the inverse can also happen, the energy gained in the structure can be transformed again into thermal energy while emitting EM radiation contributing to the temperature.

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Posted (edited)

Just something may be wrong. Thinking about...

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3 hours ago, martillo said:

Einstein's conclusion is related to the inertial mass, this means the mass at "rest" and the total energy which includes the kinetic energy. If a massive object absorbs EM energy it augments its inertial mass, the "rest mass". This means the object stores the EM energy in its internal structure which in principle could involve internal vibration, I agree, but also could be stored in the non vibrating static electromagnetic field of the structure. In principle both cases are possible. Why not to consider the second one?

Because it’s not how temperature is defined. It’s insignificant in most situations anyway, because E/c^2 tends to be small.

3 hours ago, martillo said:

Temperature is a magnitude related to the intensity of the EM radiation emitted by an object. An object that increases its internal energy emits more EM radiation and so its temperature is increased.

That’s backwards; the radiation is increased because the temperature is higher.

3 hours ago, martillo said:

For instance the electrons in the atoms can absorb extra EM energy reaching configurations of higher levels of energy in the atom and being capable to jump to lower levels emitting correspondent EM energy. This implies more energy radiated by the object and so a higher temperature. This is the way EM energy can be absorbed by an object without involving a kinetic vibration of the atoms.

The radiated energy from a blackbody is a continuum; it’s not directly related to electrons jumping between states. IOW any quantum jumps are typically an insignificant part of the spectrum in terms of radiated energy. (in fact it might have no net effect, since the excitations come from the collisional energy)

3 hours ago, martillo said:

Seems to me temperature can be explained in both ways, with or without vibrating atoms. I would prefer the second one with no vibrations.

Then I would suggest your understanding is incomplete.

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2 hours ago, martillo said:

What do you think cannot be explained this way? Think in EM radiation as an intermediary in the transferring of kinetic energy between particles when no direct collisions are present.

Yes but now you are acknowledging there is kinetic energy in motion of the particles. So thinking in terms of radiation is NOT any kind of alternative to the kinetic theory of temperature, which as I say is that it is proportional to their mean kinetic energy.

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Posted (edited)
1 hour ago, swansont said:

The radiated energy from a blackbody is a continuum; it’s not directly related to electrons jumping between states. IOW any quantum jumps are typically an insignificant part of the spectrum in terms of radiated energy. (in fact it might have no net effect, since the excitations come from the collisional energy)

I don't understand how "collisional energy" can produce EM radiation. Can you explain that? On the other side I think I understand how it is produced by "electrons jumping between states". I also stay thinking now that may be at the end the radiated energy from a black body could actually be not a continuum but discrete in minimal increments as originally considered by Planck. As said at Wikipedia (https://en.wikipedia.org/wiki/Planck's_law) :

At the end of the 19th century, physicists were unable to explain why the observed spectrum of black-body radiation, which by then had been accurately measured, diverged significantly at higher frequencies from that predicted by existing theories. In 1900, German physicist Max Planck heuristically derived a formula for the observed spectrum by assuming that a hypothetical electrically charged oscillator in a cavity that contained black-body radiation could only change its energy in a minimal increment, E, that was proportional to the frequency of its associated electromagnetic wave. While Planck originally regarded the hypothesis of dividing energy into increments as a mathematical artifice, introduced merely to get the correct answer, other physicists including Albert Einstein built on his work, and Planck's insight is now recognized to be of fundamental importance to quantum theory.

1 hour ago, exchemist said:

Yes but now you are acknowledging there is kinetic energy in motion of the particles. So thinking in terms of radiation is NOT any kind of alternative to the kinetic theory of temperature, which as I say is that it is proportional to their mean kinetic energy.

I'm not thinking in EM radiation as an alternative to to the kinetic theory of temperature. I'm saying that the EM radiation is the cause for the particles to acquire their kinetic energy and that temperature is a magnitude related to its intensity. The Kinetic Theory of Gases with its main law PV = NKT, for instance, remains exactly the same. I do not modify anything in it. I'm just saying that temperature actually measures the EM radiation present in the volume which is related to the kinetic energy of the particles by that equation.

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1 hour ago, martillo said:

I don't understand how "collisional energy" can produce EM radiation. Can you explain that? On the other side I think I understand how it is produced by "electrons jumping between states".

Accelerating charges radiate. During a collision, atoms are deformed, temporarily giving them a dipole moment, while they are accelerated.

1 hour ago, martillo said:

I also stay thinking now that may be at the end the radiated energy from a black body could actually be not a continuum but discrete in minimal increments as originally considered by Planck. As said at Wikipedia (https://en.wikipedia.org/wiki/Planck's_law) :

The spectrum from blackbody sources I’ve seen look like a continuum.

1 hour ago, martillo said:

I'm not thinking in EM radiation as an alternative to to the kinetic theory of temperature. I'm saying that the EM radiation is the cause for the particles to acquire their kinetic energy

How can emitting EM radiation cause particles to acquire KE? That violates conservation of energy.

1 hour ago, martillo said:

and that temperature is a magnitude related to its intensity.

Again, you need evidence of this. The evidence we have is that the emission spectrum depends on temperature. The radiated power depends on the surface area, but a hot source can radiate less power than a cooler source, e.g a 100L pot of boiling water will radiate more power than a 1 cm^3 chunk of solid at 150 C.

1 hour ago, martillo said:

The Kinetic Theory of Gases with its main law PV = NKT, for instance, remains exactly the same. I do not modify anything in it. I'm just saying that temperature actually measures the EM radiation present in the volume which is related to the kinetic energy of the particles by that equation.

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Posted (edited)
2 hours ago, swansont said:

The spectrum from blackbody sources I’ve seen look like a continuum.

I said I was thinking about the subject. I don't have evidence, at least yet. May be the spectrum could be continuous after all. Good point, I must think about.

2 hours ago, swansont said:

How can emitting EM radiation cause particles to acquire KE? That violates conservation of energy.

Energy is conserved. I didn't explain it properly. I'm considering the EM radiation that enters in a volume of gas' particles increasing its temperature. Energy must be supplied to the system to augment its temperature. An EM radiation has momentum and so the supplied one enters in the volume colliding with the existent particles altering their movement (I mean linear movement not "vibration"). In average extra energy is delivered to the particles augmenting their kinetic energy. Once a new thermal equilibrium is reached the moving particles also absorb and emit radiation and a new equilibrium is reached between the moving particles and the EM radiation present in the volume of the system.

2 hours ago, swansont said:

Again, you need evidence of this. The evidence we have is that the emission spectrum depends on temperature. The radiated power depends on the surface area, but a hot source can radiate less power than a cooler source, e.g a 100L pot of boiling water will radiate more power than a 1 cm^3 chunk of solid at 150 C.

The total intensity of the radiation depends on the quantity of the emitting particles. A 100L of water contains more emitting particles than a 1 cm3 chunk of solid and so the intensity of the radiation (quantity of energy radiated) by the 100L of water would be higher.

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Posted (edited)
6 hours ago, swansont said:

The spectrum from blackbody sources I’ve seen look like a continuum.

I have been thinking in this your point against my approach of the absorbed EM radiation stored in the internal electromagnetic structure of atoms/molecules. Your objection is related to the fact that in this case the emission spectrum would be discrete according to the discrete levels of energy of the atoms (discrete quantum levels of energy of the electrons in the atoms). I'm considering now that the original discrete spectrum is lost to a continuous one due to the "Compton scattering" interactions of the originally emitted photons with the electrons (free or bounded) they find while travelling from inside the body you are analyzing to the outside. Also other electrons could be found outside in the environment traveled before reaching the final energy measuring place. They would also contribute to more Compton scattering on the original photons. The original photons' energy could be altered in infinite possible ways and so a continuous spectrum is produced at the end.

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9 hours ago, martillo said:

I said I was thinking about the subject. I don't have evidence, at least yet. May be the spectrum could be continuous after all. Good point, I must think about.

Energy is conserved. I didn't explain it properly. I'm considering the EM radiation that enters in a volume of gas' particles increasing its temperature. Energy must be supplied to the system to augment its temperature. An EM radiation has momentum and so the supplied one enters in the volume colliding with the existent particles altering their movement (I mean linear movement not "vibration"). In average extra energy is delivered to the particles augmenting their kinetic energy. Once a new thermal equilibrium is reached the moving particles also absorb and emit radiation and a new equilibrium is reached between the moving particles and the EM radiation present in the volume of the system.

You can alter the linear motion, but it’s a very small effect, since the momentum of EM radiation is p=E/c. Negligible for bulk material. Small even for an atom. Plus, if this is an excitation, the photon gets re-emitted, reversing the effect. And if the radiation is isotropic the net momentum is zero.

And above all, temperature is not affected by linear motion.

9 hours ago, martillo said:

The total intensity of the radiation depends on the quantity of the emitting particles. A 100L of water contains more emitting particles than a 1 cm3 chunk of solid and so the intensity of the radiation (quantity of energy radiated) by the 100L of water would be higher.

Yes. So it’s not higher just because the temperature is higher. The power per unit area of the material is the relevant quantity, as given by the Stefan-Boltzmann law.

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52 minutes ago, swansont said:

You can alter the linear motion, but it’s a very small effect, since the momentum of EM radiation is p=E/c. Negligible for bulk material. Small even for an atom. Plus, if this is an excitation, the photon gets re-emitted, reversing the effect. And if the radiation is isotropic the net momentum is zero.

And above all, temperature is not affected by linear motion.

Is just me failing to explain the phenomenon properly. I completely agree with the Kinetic Theory at all times.

We have a system of particles occupying a volume V and having some temperature T verifying PV = NKT. The temperature T is related to the intensity of the thermal energy present in the interior (not a direct relation: Stefan-Boltzmann Law). The kinetic energies of the particles produce the pressure P because of their momentum. The system is heated with incoming external EM radiation. Enough EM energy to augment its temperature in such a way that a new set of values verify these equations for a new thermal equilibrium reached.

1 hour ago, swansont said:

Yes. So it’s not higher just because the temperature is higher. The power per unit area of the material is the relevant quantity, as given by the Stefan-Boltzmann law.

Right. The temperature is related to the thermal energy of the system but is not a direct relation, they verify Stefan-Boltzmann Law.

What I'm trying to say is just that the temperature T (measured by a thermometer) is more related to the energy of the internal EM radiation present in the interior while the pressure P is more related to the momentum of the particles and so their kinetic energy even though they are all interrelated by those two equations.

I must remind here that the only disagreement I have in all this is that I think in an EM energy stored in the internal electromagnetic structure of the particles and not as a vibration in their atoms. As for now I don't have an evidence in favor of this although I'm thinking about. Meanwhile all I can do is to try to show that everything could be also well explained under this assumption.

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1 hour ago, martillo said:

Is just me failing to explain the phenomenon properly. I completely agree with the Kinetic Theory at all times.

We have a system of particles occupying a volume V and having some temperature T verifying PV = NKT. The temperature T is related to the intensity of the thermal energy present in the interior (not a direct relation: Stefan-Boltzmann Law). The kinetic energies of the particles produce the pressure P because of their momentum. The system is heated with incoming external EM radiation. Enough EM energy to augment its temperature in such a way that a new set of values verify these equations for a new thermal equilibrium reached.

The original topic was about a solid, so the ideal gas law and kinetic theory is moot. However, the kinetic theory shows that it’s the KE of the atoms that matters; these atoms collide elastically with other atoms - these are the analogue of the vibrations in a solid.

1 hour ago, martillo said:

Right. The temperature is related to the thermal energy of the system but is not a direct relation, they verify Stefan-Boltzmann Law.

What I'm trying to say is just that the temperature T (measured by a thermometer) is more related to the energy of the internal EM radiation present in the interior while the pressure P is more related to the momentum of the particles and so their kinetic energy even though they are all interrelated by those two equations.

There is no “internal EM energy”

1 hour ago, martillo said:

I must remind here that the only disagreement I have in all this is that I think in an EM energy stored in the internal electromagnetic structure of the particles and not as a vibration in their atoms. As for now I don't have an evidence in favor of this although I'm thinking about. Meanwhile all I can do is to try to show that everything could be also well explained under this assumption.

You can’t have motion of atoms, having KE, and have a bunch of EM radiation interior to the system, and have the theory work.

kT is directly related to mv^2 (with a constant related to degrees of freedom)

If there’s energy stored as EM radiation, there’s less KE, but that means a lower temperature.

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1 hour ago, martillo said:

Right. The temperature is related to the thermal energy of the system but is not a direct relation, they verify Stefan-Boltzmann Law.

And yet when we measure temperature gradients between systems at different temperatures, in non-extreme conditions they are generally linear in agreement with the dominant mechanism for transfer of heat being by momentum exchange.

If the dominant mechanism were EMR as you suggest, then the measured gradients would be highly non-linear (cubic in delta T I think).

1 hour ago, KJW said:

One thing should be mentioned: Only the translational modes of molecular motion contribute to the temperature.

I understand your POV but I think it's a misleading one. Particle collisions in gases that support rotational and vibrational modes only follow conservation of linear momentum on average. In the general case, some momentum is transferred via the other modes.

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Posted (edited)
1 hour ago, swansont said:

There is no “internal EM energy”

There must be EM energy in the interior of the system. In a totally empty space, without EM radiation, the particles (atoms/molecules) would lose all their internal energy becoming totally cold (0º K). All the electrons of them jumping to their minimum level of energy possible radiating all the possible internally accumulated energy. Some temperature higher than the 0º K can be reached only in a dynamical equilibrium in which the particles continuously absorb and emit some quantity of EM radiation. You can think in photons in the interior of the system travelling in all directions.

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1 hour ago, martillo said:

There must be EM energy in the interior of the system. In a totally empty space, without EM radiation, the particles (atoms/molecules) would lose all their internal energy becoming totally cold (0º K). All the electrons of them jumping to their minimum level of energy possible radiating all the possible internally accumulated energy. Some temperature higher than the 0º K can be reached only in a dynamical equilibrium in which the particles continuously absorb and emit some quantity of EM radiation. You can think in photons in the interior of the system travelling in all directions.

That's the electrostatic interaction, which involves virtual photons. That's typically not referred to as EM radiation, which consists of real photons. The vibrational modes of a solid can be described in terms of phonons (not photons);  there are non-radiative ways of changing those states.

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On 3/14/2024 at 8:55 AM, martillo said:

the temperature of an atom is [...]

There is no such thing.

Thermodynamics defines temperature based on thermal equilibrium. Statistical mechanics relates it to average kinetic energy per degree of freedom. For statistical mechanics to make the connection between both concepts through the partition function and the Maxwell distribution, we need approximations on really big numbers of molecules.

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