Q & A: what is heat?

What is heat. I know what heat energy is, but what is the actual heat. Atoms causing friction seems more of a cause than an explanation. Am I digging to deep? But why does heat exist? I hope my question makes sense, but I know why water feels wet, so why can’t I know why heat is hot.
- James (age 26)
BC,canada

Just to make sure other readers are starting at the same point, here’s what heat energy is. There are all sorts of random jigglings and wobbles etc of the atoms, electrons, spins, etc in any material. These all require some energy above what a material would have in its lowest energy state. The higher the temperature, the more likely the system is to be found in some high-energy state, and the lower the temperature the more likely to be found in a low energy state. I don’t know what to say that ’heat’ is beyond this heat energy. What more definition would you be seeking? In formal scientific usage, ’heat’ refers not to the total amount of that thermal energy but only to the transfer of thermal energy caused by a temperature difference between objects. However, I think that sort of formality is not what you were asking about.

So why does heat exist? I guess that amounts to asking why energy gets distributed around among all the little random microscopic vibrational modes rather than concentrated in a few large-scale modes, things like bouncing balls. The reason is that there are many more microscopic states of the world with the energy distributed into the tiny modes than there are with the energy concentrated in a few big modes. Nature seems to become progressively indifferent to what quantum state it is in (that’s called the 2nd law of thermodynamics), so the best bet on what you’ll see is based on giving equal probabilities to different quantum states which have the known amount of energy, etc. The vast majority of those states take almost all the energy from the big visible modes and instead have it distributed in the little modes.

Why does heat feel hot? Some of the heat energy from a hot object flows into your hand, if you touch it. The reasons why your nerves are designed to respond to that are pretty obvious from an evolutionary point of view. As to why anything has any particular subjective feel, that’s way too deep a mystery for us.

I’m not sure if that answers your question, so feel free to elaborate on it if needed.

Mike W.

(published on 10/22/2007)

every state of energy should belong in a certain region of the electro-magnetic spectrum. so.. my quarry is whether HEAT belongs in the IR region? if so to produce an EM wave(of any frequency)there should be an osillating charge. now how the hell do we produce such a charge while say, rubbing our hands? i have an explanation for that too. now if my theory is true that means we are continously emmiting light(of lower frequency)in the form of "HEAT" the very form of energy what people take for granted... this is mainly because of the fact that in lower school we all studied about light and heat as two distinct chaptes. what people dont recognise is that "HEAT" and "LIGHT" are THE SAME... p.s. please correct me if i am wrong
- ADARSH K GEORGE (age 18)
CHENNAI,TAMILNADU,INDIA

Sure, every hot object emits electromagnetic radiation. This has been known for a long time, with the exact description discovered by Max Planck in 1900, the first step in the invention of quantum mechanics. I think that on the average a person will emit several hundred watts of thermal radiation, while absorbing nearly as much back from the environment.  Most of this radiation is in the infrared range.
"Heat", more precisely described as "thermal energy" can take lots of forms, and electromagnetic radiation is just one of them. Likewise, electromagnetic radiation can be either of the thermal (highly random) kind emitted from your body or of non-thermal (much more organized) forms, such as that emitted by a laser or a radio transmitter.
So "heat" and "light" are only the same in certain circumstances.

Mike W.

(published on 12/22/07)

How is thermal(heat) radiation emitted? Is it an atomic phenomenon? Does it involve electrons emitting photons? How is it that any body emits thermal radiation over such a wide range of wavelengths? How does this differ from line spectra(that involves atoms)? Thank you, Amoghavarsha
- Amoghavarsha
Bangalore, Karnataka, India

You're right that usually thermal radiation comes from electrons emitting photons. In principle it could come from any charged particles, or even (at very high temperatures) from the annihilation of uncharged particle-antiparticle pairs.

Usually, the electrons in materials are in a variety of different states, with a broad range of energies. That allows emission and absorption over a broad spectrum of frequencies. In the special case of a gas of isolated atoms, the allowed energy levels are just the discrete atomic levels, with just a bit of broadening from Doppler effects and occasional collisions. That means that it's hard for the gas to emit or absorb light except at a narrow set of frequencies, the line spectra. On a long time scale, even such a gas would set up an equilibrium with a Planck distribution of thermal radiation, thanks to the occasional processes that allow energy exchange at other frequencies.  More typical environments (e.g. metallic boxes) have electrons running around in continuous energy bands, allowing rapid thermalization over a broad range of frequencies.

Mike W.

(published on 11/20/10)