MadSci Network: Physics

Re: specifically, how do you obtain heat from the electric current?

Date: Tue Oct 26 18:10:17 2004
Posted By: Kenneth Beck, Senior Research Scientist, Chemistry and Physics of Complex Systems, Pacific Northwest National Laboratory
Area of science: Physics
ID: 1098387411.Ph

Dear Pavel,

You know my fellow scientists will sometimes remark in a friendly, joking 
way (and only to one another) that a little science can be a dangerous 
thing.  I see in your question alot of thought and a desire to understand 
the physical process known as Ohmic or Joule heating. But it’s also 
riddled with a lot of labels and buzz-words (“color charge”?!) which will 
only confuse rather than clarify.
Joule discovered the process of resistive heating in the 1840’s after Ohm 
published his famous equation. Joule’s law is easily stated in terms of 
Ohm’s law as,

Q = Pt = I^2Rt

where Q is the heat generated in time, t, I is the current, and R is the 
resistance (P, as you probably guessed, is the power).

I’ve read over the earlier response from November, 2000.  The correct 
URL, by the way, is…

with an “r.html”, not a “q.html”.

I think a little research about Joule heating on your part may help you 
understand the process much better. But, I will answer you questions as 
stated in “Facts/premises” 1-4:

(1) If we are only considering Joule heating, electron collisions 
determines the amount of atomic vibrations within a material.   
Collisions of electrons set in motion by the applied electric field with 
(still) bound electrons in the material. This motion is rather fast 
compared to the motion of nuclei within the atoms.  They will be slower 
to respond.  But when they respond to the momentum of multiple electron 
collisions, they will excite the structure of the material 
vibrationally.  This can be measured as a rise in temperature.

(2) Well, no.  This is not a fact or a valid premise.  Free electrons do 
concern us.  They are set in motion by an applied field and are in motion 
until they collide/  They transfer their momentum and energy to bound 
electrons of the material.

(3)  Let’s be specific.  We’re talking about Joule heating, not 
ionization or photoluminescence (“releasing a photon”).  I know you’re 
trying to understand all this in terms of the extremely ancient 20th 
Century notion - the Bohr atomic model.   The reason electrons occupy 
different states in an atom is predominantly because of their electronic 
energy, in addition to their, generally, smaller angular ( orbital and 
spin) energy.

(4) Again, we’re talking about Joule heating, not thermonuclear 
transformation of mass and energy (e=mc2). Electrons in a material are 
set in motion by an applied electric field.  They collide with (still) 
bound electrons, setting atoms in motion.  This motion we measure as a 
temperature rise and generated heat.  Here’s a reference for you…

and another with worked example and further references to heating an 

Good luck in your future investigations!

---* D.r Ken Beck

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