Re: How does kinetic energy become radiant Energy?

Hi Ken, thanks for your question: I should say questions!  

You asked how the kinetic energy of atoms and molecules, for example 
atoms in a plate of iron is translated into radiant energy when the iron 
is heated to a sufficiently high temperature to emit not just heat but 
visible light.  We call the radiation for example “thermal radiation” if 
it is not sufficiently energetic to contain visible light 
or “incandescence” if visible light is emitted.  

You also asked how much energy do you need to emit a photon.  This second 
question is a little easier to answer.  Whatever the process responsible 
for producing the photon, the energy needed is at least the amount of 
energy possessed by the emitted photon.  This energy is calculable from 
the wavelength of the photon of radiation emitted.  The energy of a 
photon (E) is linearly related to the frequency of the light (f) and the 
constant of proportionality is known as Planck's constant (h) such that 
E=hf. Planck's constant is around 6.626*10-34 Js.  To produce a visible 
photon the energy needed is very high compared with the energy of normal 
thermal vibrations at an atomic level.

The more difficult issue is how the transformation of energy occurs.  I 
like this type of question because it gets down to basic physical 
behaviour. This shows that you are not satisfied with just knowing the 
laws which describe phenomena but want to be able to explain them.  This 
is a good attitude. 

Heat energy in any body is the energy which is associated with the 
vibrational motion (and rotational if rotation is possible) of the body’s 
atoms and molecules.  The more heat the body contains the more energetic 
these vibrations and the buffeting that the atoms and molecules 
experience become.  Teir motion gets more violent and faster.  Atoms and 
molecules contain the positively charged nuclei and the 
surrounding “clouds” of negatively changed electrons.  As a result of the 
turbulent oscillating motions these charges are continuously being 
accelerated one way then another.  Scotsman, James Clerk Maxwell 
developed the theory and predictive equations linking the acceleration of 
charge with the generation of a wave of electrical disturbance we know as 
electro-magnetic radiation.  Accelerated charge causes the emission of 
this type of radiation and conversely absorption of radiation causes 
acceleration of charge.  This is how radio aerials and receivers work and 
explains the processes of light absorption and emission in atoms and 
molecules.

I had some difficulty finding a reference to confirm this explanation for 
thermal radiation.  The best I found was Michael Fowler’s  http://galileo.phys.virginia.edu/classes/252/black_body_radiation.html 

You asked whether this kinetic energy is a fixed amount as with photons.  
This is also an interesting question too especially because of the link 
between the phenomenon of thermal radiation and the beginnings of quantum 
theory.  It is true that thermal motion is quantised, that is not all 
values of the energy of the motion are allowed.  In practice, in bulk 
materials heated to temperatures sufficient for them to visibly glow, 
there are so many different levels of energy available that the light 
emitted continuously varies across the range of wavelengths emitted.  
However when people tried to explain the distribution of light energy 
emitted by black bodies (ideal emitters which absorb all wavelengths 
completely) the predictions were not always good.  Max Planck rightly put 
more faith in observation than theory and his attempts to improve the 
predictions involved the assumption that the vibrational energies were 
quantised.  The predictions were then substantially accurate and this 
improvement was seen as strong evidence for the quantisation of energy.  
Physics never looked back !