# Constant velocity of heat?

1. Apr 11, 2010

### dirtyd33

Since heat is a form of electromagnetic radiation, does it travel at the constant velocity of light?

2. Apr 11, 2010

### starthaus

Heat radiation in vacuum travels at c.

3. Apr 12, 2010

Staff Emeritus
That's not what heat is.

4. Apr 12, 2010

### PaulS1950

there is a difference between heat radiation and heat in an object. The object will radiate heat (in a vacuum) at c but it will disperse into its surroundings at varying speeds dependent on the thermal qualities of the substances involved.

5. Apr 12, 2010

### dirtyd33

first of all vanadium you're wrong; heat is infrared, a type of electromagnetic radiation.. so emr travels constantly relative to any inertial reference body, according to special relativity. as pauls1950 stated heat travels at varying speeds depending on the bodies involved; but heat is emr, so is this not emr traveling not at c relative to reference bodies?

6. Apr 12, 2010

### Staff: Mentor

No. Radiation is one of the ways heat can be transferred, but it is not the heat itself. The sooner you realize that's correct, the better for this thread.

I have a cup of hot tea on my desk. It doesn't 'contain' infrared radiation, even if it emits it.

7. Apr 13, 2010

### dirtyd33

I am operating on the premise that since emr travels constantly in vacuum and relative to any reference body, then vacuum's are present about and within every reference body. so on this ground any heat transfer is through vacuum, whether via conduction, convection, or radiation, and if it is indeed true that "it will disperse into its surroundings at varying speeds" (pauls1950), then that is emr traveling not at c relative to reference bodies. i can understand heat transferring at varied rates based on the qualities of the bodies involved, but for heat to move about bodies it must be moving through vacuum.

8. Apr 13, 2010

### Staff: Mentor

No, that's not correct. Conduction and convection really are conduction and convection. Conduction happens when molecules touch/hit each other. Convection happens when molecules move from one place to another. They are not radiation. Only radiation is infrared, so only that travels at C (as infrared radiation is a form of "light").

9. Apr 13, 2010

### ZapperZ

Staff Emeritus
Your "operating premise" isn't universal, i.e. it is not the most general situation. You are focusing on ONE particular form of heat, which you should have clarified in the very beginning. Other forms of heat include the average kinetic energy of particles, which you would have come across in any lesson in thermodynamics. This isn't mediated as IR radiation but rather as the transfer of kinetic energy of particles.

If you dispute this, which is a standard definition in physics, then the impetus is on you to make exact references to support your claim.

Zz.

10. Apr 13, 2010

### starthaus

Yes, I already told him this at post #2 (sigh)

11. Apr 13, 2010

### sophiecentaur

To avoid this sort of confusion, Big Boys don't even use the word "Heat".
The Internal Energy of a substance is a better description - refering to the random KE of the particles in it. Hence, the Infra Red thing is not included in the oft-used term Heat. IR is just one form of energy transfer from one place to another.
Temperature is the average Energy of the particles.

12. Apr 13, 2010

### dirtyd33

So you are saying that the temperature, or internal energy of a particle, is a different form of heat than infrared radiation right? You are certain that heat in one form is not equal to heat in another? You're absolutely sure it's not the very same energy of quantum particles exchanging through space between bodies? Ok.

13. Apr 13, 2010

### mate0

heat is specifically the energy contained in matter because its molecules are moving internally. radiation is not a form of heat, though it is a form of energy.

14. Apr 13, 2010

### jrosen13

This is one of the issues with thermodynamics specifically, where the language has to be carefully defined in terms of real things one can measure, such as the temperature of an object.

Heat that we can feel due to IR is a subjective definition, and it is not useful either. If I shine gamma rays on the object, it will definitely get hot, so for one lets debunk the IR = heat myth.

IR feels hot because it is readily absorbed by molecules and turned into vibrational energy that can be measured as an increase in temperature. But it is just light, so we can basically extend that to all other wavelengths, from radio waves to gamma rays, as long as we dont worry too much about the quantum processes that are involved to transfer energy from the electromagnetic field to the vibration of molecules. IR may be the most efficient at warming an object, but thats as far as it goes.

At the end of the day, heat takes a lot of different forms because it just describes transfer of energy from one body to another in a way that agrees with statistical probability.

15. Apr 13, 2010

### dirtyd33

My knowledge grew throughout this discussion, thank you all. I still hold my reservations about certain things, but I am clearer on the difference of radiated energy and inertial energy.

"its all heat in this day and age, i rage your grave, anything it takes to save the day" - del the funkee

Last edited: Apr 13, 2010
16. Apr 13, 2010

### Staff: Mentor

That's not true. From the dictionary:

17. Apr 14, 2010

### sophiecentaur

Dictionary definitions of technical terms are not necessarily to be relied on technically. They may reflect, accurately, the general usage of a term but that's another matter.

18. Apr 14, 2010

### Staff: Mentor

While that may well be true in some cases, in this case, they got it right.

19. Apr 14, 2010

### SpectraCat

As others in this thread have pointed out, it is largely a question of carefully defining one's terms. However, overall I think I agree with Russ, in that one must account for radiative mechanisms when considering the processes that bring a system to thermal equilibrium. Consider a block of room temperature material ejected suddenly into space. There aren't enough molecules around for convective mechanisms in the surroundings to have any effect to cool it, yet we know it will cool. Assuming it is not a volatile material, the only mechanism for removing heat from the body that I can think of is radiation. In fact, in a cosmological sense, I guess virtually all of the "heating" and "cooling" that goes on in the universe happens radiatively.

20. Apr 14, 2010

### dirtyd33

Thank you spectracat for acknowledging/ reaffirming the universal concept of energy, in the presence of vacuums, that i was getting at.