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A thermodynamic question: is electromagnetic radiation released because of a difference in temperature or because instability of electrons inside matter (atoms and molecules)?

The thermodynamic law states that energy goes from higher temperature to lower temperature in my study book, but I think that it's not the case for radiation. Could someone elaborate?

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Temperature is just a measure of how fast the atoms/molecules of an object "jiggle". The concept of temperature makes sense only for large number of atoms/molecules. Because atoms are electrically charged, and moving charges emit electromagnetic radiation, it means that all objects with $T>0$ ($T$ is absolute temperature) will emit thermal radiation. The higher the temperature is, the higher the average kinetic energy of the atoms will be - thus the faster they will "jiggle". This is called thermal radiation.

Electromagnetic radiation can also be released when electrons make a transition from one higher energy state to a lower one. But this release of radiation happens for different reasons. The energy levels in atoms are quantized, meaning that the atom can occupy only particular energy levels: $E_{1}, E_{2}, E_{3},\dots E_{n}$. For example, when an electron from an atom makes a transition from state $2$ to state $1$, there will be a photon emitted in the transition with an energy of: $$\hbar\omega=E_{2}-E_{1}$$

Now going back to thermal radiation and thermodynamics laws. Heat is just the energy transfer/exchange between thermodynamic systems. Heat transfer goes on average from hot object to the cold object because the atoms of the hot object have higher kinetic energy than the atoms of the cold one.

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Just to make a simplistic analogy: you can view this process just like (billiard) balls colliding. When two balls collide, the one with higher kinetic energy will lose energy by transfering to the one with lower kinetic energy. We can further imagine a frictionless billiard table with $N$ balls of different kinetic energies that are colliding with each other and with the table margins. In the end, given enough time, what will happen is that the energy will be equally distributed on the whole table.

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  • $\begingroup$ It should be noted that not all forms of heat transfer are due to collisions $\endgroup$ – Aaron Stevens Jul 2 at 14:35
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Is Electromagnetic radiation released, because of difference in temperature? or because instability of electrons inside matter (atoms and molecules) ?

Both.

It is because of temperature difference because temperature difference is a difference in the magnitude of the motions (kinetic energies) of the atoms and molecules of objects, and those motions reflect a difference in the magnitudes of charge- acceleration or dipole acceleration which produces electromagnetic radiation.

The main differences between heat transfer by radiation vs conduction and convection, are the mechanisms by which the kinetic energy of the higher temperature object is transferred to lower temperature object and the medium necessary for the transfer.

Heat is energy transfer between two objects (in solid, liquid or gaseous state) due solely to a temperature difference between those objects.

Although it is not a formal thermodynamic definition of temperature, an operational definition of temperature is that it is a measure of the average kinetic energy of the atoms and molecules of an object (the so called kinetic temperature). The object with higher average kinetic energy has a higher temperature than an object with lower average kinetic energy.

Heat transfers kinetic energy from the higher temperature object to the lower temperature object. This results in a decrease in the average kinetic energy of higher temperature object (a decrease in its temperature) and an increase in the average kinetic energy of the lower temperature object (an increase in its temperature).

The mechanisms of heat transfer (conduction, convection, and radiation) determine how this transfer of kinetic energy occurs.

Heat transfer by conduction involves direct contact between the objects so that the atoms and molecules of the higher temperature object collide with those of the lower temperature object.

Heat transfer by convection combines transfer by conduction with the transfer of energy by the movement of fluids in contact with solids.

Heat transfer by radiation requires no contact between the objects and can occur through a vacuum. Without getting into details, objects emit energy by electromagnetic radiation due to the vibrational and rotational motions of their molecules and atoms. These particle motions result in charge- acceleration or dipole acceleration which is what produces electromagnetic radiation. The higher the temperature of the object, the greater the radiant heat flux emitted. Energy of the electromagnetic waves from the higher temperature object is transferred to the lower temperature object.

Hope this helps.

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Yes EM radiation is released due to heat, this is called infrared radiation (IR). Molecules and matter contain heat which is kinetic energy of the atoms within including many vibrational and rotational components about bonds. This vibrational energy is the motion of the nuclei as well as motion of the electrons. Electrons in molecular bonds are able to jump (up and down) in small energy steps within matter and this is why we have IR radiation. Visible photons are more energetic, we usually don't refer to them as heat but they can they can be, ex they can burn objects , ex lasers. Yes we have light because of nuclear energy in stars for example, that energy excites electrons and excited electrons are able to radiate their energy across space due to the magical EM field.

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    $\begingroup$ Molecules and matter contain heat... I believe this is imprecise language. I think the sentence would be better by just saying Molecules and matter have kinetic energy... and so on $\endgroup$ – Aaron Stevens Jul 2 at 14:28
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At any temperature higher than 0K, any material emits e&m radiation. This is caused by moving charged particles inside, not by temperature difference.

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    $\begingroup$ It also absorbs radiation. Only in case of a temperature difference there is a net emission or absorption. $\endgroup$ – my2cts Jul 2 at 18:24
  • $\begingroup$ Yes, absorption and radiation proceed simultaneously $\endgroup$ – Cang Ye Jul 3 at 1:05
  • $\begingroup$ So any net emission or absorption is caused by a temperature difference. $\endgroup$ – my2cts Jul 3 at 19:41

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