The nature of heat

Discussion in 'Physics & Math' started by Magical Realist, Oct 31, 2015.

1. Q-reeusValued Senior Member

Messages:
3,700
True. So I decided to do a search just using heat as term, and lo and behold Wkipedia article starts off with essentially verbatim of paddoboy's cut&paste piece. So yes I stand corrected in respect of standard terminology. One wonders how to understand then the logic of 'heat transfer' as not inherently redundant, as also 'heat exchanger' or more tellingly 'heat content'. Still, I now realize it always pays to do a web search rather than relying on a dimly recalled school education when as here definitions can be all-important.

3. Russ_WattersNot a Trump supporter...Valued Senior Member

Messages:
5,051
I think the reason is probably due to the fact that unlike a compressed spring or gallon of gas, things don't have thermal energy by themselves, only when measured against something else. Like potential energy.

Q-reeus likes this.

5. Q-reeusValued Senior Member

Messages:
3,700
Yes that makes sense. Still doesn't remove some of those redundancy issues imo, but there is a distinction there.

7. The GodValued Senior Member

Messages:
3,546
The definition of heat in the literature is not very clear. Some associate it with temperature (quality of being hot) and there are some definitions which talk of heat as transfer of thermal energy (so called 'in transit' thermal energy). The second definition is actually the cause of confusion.

a. The terms 'heat flow' and 'heat transfer' sufficeintly define transfer of energy, so defining the term 'heat' as the transfer of energy is not appropriate.

b. There are certain terms as 'Heat of Hydration', Heat of fusion, Latent Heat, Heat of combustion, Heat of Reaction etc.....these are absolute quantities under given conditions, and they do not refer to 'in transit' state, thus definition of heat as transfer of energy is not appropriate.

c. First Law of ThermoD also has a distinct term as Q for heat, which is not a state function, but has certain value at each state in a given experiment, suggesting heat without being in transit.

So, even though, the term transfer is used in the definition of heat by many sources, it appears sloppy. I feel that the simple dictionery definition is quite ok....

"A form of energy associated with the motion of atoms or molecules and capable of being transmitted through solid and fluid media by conduction, through fluid media by convection, and through empty space by radiation."

Q-reeus likes this.

Messages:
26,404
Except it wasn't a cut n paste. Just a logical common sense definition of what was asked for. Keep trying though.
Of course you do!

9. The GodValued Senior Member

Messages:
3,546
I think you want to say that 'things don't have heat by themselves'.
Heat is not a system property, but thermal energy certainly is. Absolute temperature, the measure of thermal energy of a system, is not frame specific.

10. The GodValued Senior Member

Messages:
3,546

You have offered two definitions of heat, one says heat is energy and another says heat is transfer of energy.....

This statement would be more meaningful if type of energy is specified. Assuming this to be thermal energy,

This definition contradicts the second definition offererd by you. Take for example a system at 1000 deg K and another also at 1000 Deg K, then will the transfer of energy be more in this case as compared to a system at 900 deg k and 900 deg k.

No doubt this definition is also there in the literature, but as I have already explained and Q-reeus also stated, it confuses with the term Heat Flow, Heat Exchange, heat Transfer and some others like heat of fusion, heat of combustion, heat of fusion, heat of reaction etc.

Moreover please understand conduction can be a single body (single system) issue (a metal bar heated from one side will see heat conducted to other side),

and of course radiation does not require two systems and its independent of temperature difference. Any object at > 0 deg K will emit (absorb/reflect) radiation, so if there are two systems irrespective of temperature difference they are emitting radiation, absorbing radiation and even reflecting the same. This is complex for you and in fact second part of MR question wherein he talks of photon / wavelength etc, you need to google first for Planck's law etc, then we can pursue.

11. Q-reeusValued Senior Member

Messages:
3,700
Here's one occasion where imo you are showing quite some acuity. Do try and maintain that as a general rule.

Messages:
26,404

13. Q-reeusValued Senior Member

Messages:
3,700
My instincts say otherwise - the correspondence in wording betrays you. That you may have manually reproduced rather than direct cut&paste is technical cop-out, though of course I cannot prove it.

Messages:
26,404
Your instincts can say what they like.

But keep trying.

15. arfa branecall me arfValued Senior Member

Messages:
7,013
The argument can be settled by understanding what enthalpy and thermodynamic potential are.

And we can note that a hot object, say a hot plate, has a temperature field over it which is not a vector field, nor are changes in temperature because temperature has no direction. Which is to say temperature is a scalar quantity and so is thermodynamic potential.

16. tashjaRegistered Senior Member

Messages:
715
Hi Magical,
I hope the answers provided by Prof. Beale help. If you have any more questions, just ask:

Last edited: Nov 1, 2015

Messages:
26,404
18. Magical RealistValued Senior Member

Messages:
12,960
Yes..very helpful. Thank you and tks to all who have given answers so far..

19. exchemistValued Senior Member

Messages:
9,876
Bit late to the party but let me have a pop at this.

Photons are a wavelike disturbance in the electromagnetic field that carries energy, as waves always do. When they encounter electrically charged particles that resonate with the same frequency as they do, they may be absorbed, in the process giving up their energy to the charged particles by making them move in some way. This is because the oscillating electric field of the photon can stimulate a sympathetic motion in the particles, thereby transferring energy to them. (This explanation is not rigorous, as I am not going to bore you with the QM - though you can go google "transition dipole" if you are interested - but it conveys roughly what goes on.)

With visible light the frequency is right for exciting the outer electrons in atoms and molecules, with X-rays it is the inner electrons in atoms, with IR it is vibration and with microwaves it is the rotation of molecules, in both cases of those with an uneven charge distribution (dipole) , and so on. All these forms of motion either directly or indirectly lead to an increase in the kinetic energy of molecules.

The case of electronic excitation is quite interesting (to a physical chemist

). This initially produces an electronically excited state in the molecule and so does not itself increase thermal energy. However, there is a cascade of subsequent processes (which are far from trivial and would take too long to explain properly here) by which electronically excited molecules lose energy and drop back to their ground state. These involve a conversion of electronic energy to molecular vibration - which is a mode of thermal kinetic energy.

We can talk more if you have further questions.

20. Magical RealistValued Senior Member

Messages:
12,960
Very detailed explanation. Thanks. One other question: most waves we know of in nature transfer energy thru their momentum or movement thru space. This momentum seems to assume the transfer of some force due to the mass of the waveform or medium it is propagated thru. But in the case of electromagnetic waves, there is no mass present to apply a force to the atoms. In what sense does a wave propagate energy at all and resonate with particles if there is no mass or force being carried by the wave? Is this where we invoke the rest mass of photons to make up for the limits of the waveform metaphor of light?

Bonus question: How does a photon from a bioluminescent lightsource differ say from a photon from a candle flame? Is this an example of light NOT carrying heat energy?

Last edited: Nov 4, 2015
21. Russ_WattersNot a Trump supporter...Valued Senior Member

Messages:
5,051
That's pretty much all correct.
Kind of, yes, you get the idea: photons (EM radiation) create and ride on their own electro-magnetic field, which does not require a medium. But yes, by the equivalence of mass and energy, you can calculate the rest mass of a photon.
No fundamental difference, except perhaps in wavelength. Now, a photon that is generated through a chemical process like bioluminescence may be generated differently from black-body radiation, but two photons of the same wavelength are identical/indistinguishable and therefore carry the same heat energy.

22. Q-reeusValued Senior Member

Messages:
3,700
NOTE: Posts #39 to #44 below are in-correct-sequence reproductions, following recent crash. Text all there, but minor formatting issues unavoidable. Q-reeus.
exchemist, Nov 4, 9:39 PM
Just to add to the reply from Russ, in the case of bioluminescence light is generated by a chemical reaction that creates a product of reaction in an electronically excited state. This is photochemistry. Because electronic excitation in itself does not involve motion of the molecule (vibration, rotation or translation) it does not contribute to its thermal kinetic energy.

So it is perfectly possible for light to be produced without something becoming hot. It can seem odd, because we are more familiar with processes that create light through heating something up to incandescence (the sun, a filament light bulb, etc). But a fluorescent tube light or "neon" strip light use a similar idea - generate specific electronic excitations of an atom or molecule, that emit light of a particular frequency (=colour) when they drop back to the ground state.

Last edited: Nov 8, 2015
23. Q-reeusValued Senior Member

Messages:
3,700
John Connellan, Nov 5, 10:38 AM
The only force which derives directly from mass is gravity. You have to realize that the other forces at the fundamental level have nothing to do with mass. In our everyday world, most non-gravitational forces are the result of the electromagnetic interaction. Think about the huge amount of potential energy (and corresponding electric force fields) generated by a few million isolated electrons even though the total mass of the bunch of electrons is almost negligible.

Now you can see why photons, which have no mass but still hold electromagnetic energy can easily interact with matter and get electrons moving around or vibrating. This atomic/molecular movement is the definition of heat energy.

Last edited: Nov 8, 2015