What exactly is heat defined as?
My first guess was the kinetic energy of an atom, but this cannot be right. If you have a metal, each atom may be hot, but they are held in a rigid lattice.
Which leads to another question - how may heat be transferred?
In a metal, heat is transferred by free electrons, and in a gas I think heat is transferred by one particle colliding with another, although I am unsure whether the nucleus or electrons are interacting.
So how is heat transferred in more exotic states of matter, such as the neutron soup found inside neutrons stars, or the Quark-Gluon plasma that apparently exists during supernovae.
Thoughts?
-Oli
Heat is the kinetic motion of atoms, transferred through photons. If you heat a metal hot enough, the lattice will break down and you’ll get a liquid. Same with a lot of things, unless they go through a chemical reaction first.
Heat == Energy == Motion == Entropy.
Heat is the same no matter what scale you’re at, AFAIK.
Minor nitpick:
I doubt heat is the kinetic energy of atoms specifically. If that was so, then the core of a neutron star could not be hot, and neither could quark matter.
Out of interest, does anyone know the relationship between the rise in temperature of 1K for one atom and an increase in kinetic energy of x joules?
-Oli
I think heat is kind of amorphous. Heat is usually defined in terms of its effects. It is the form of energy that makes the temperature of objects rise or causes the material in the object to change state, for example from a solid to a liquid. Sometimes heat is involved in physical changes not involving a change in termperature, such as the dissolving of one substance in another or a change in crystylline structure. In this case it is called “latent heat.”
Great Scott!!! “crystylline”??? Try “crystalline.”
Stealing a definition from physlink.com, heat is “a measure of the amount of energy transferred from one body to another because of the temperature difference between those bodies. Heat is not energy possessed by a body.”
The amount of energy exchanged per unit rise in temperature is the heat capacity, and it varies from substance to substance.
Temperature reflects average kinetic energy of the atoms in a substance, but it can be defined without any atoms involved at all. From about.com, under temperature, definition:“The property of a body or region of space that determines whether or not there will be a net flow of heat into it or out of it from a neighbouring body or region and in which direction (if any) the heat will flow.”
To answer Starman I think heat really is a measure of the kinetic energy in atoms/molecules.
To raise the temp. of one atom 1k you would need to know the specific heat of that atom. i.e. it would take more energy to heat iron than it would to heat hydrogen.
Contrary to your intuition, the heat of a body is in fact motion at the particle scale.
Atoms in a struction do “jiggle” in place. This jiggling also easily explains the different phases of normal matter (solid, liquid, gas).
As atoms in a solid begin to jiggle harder, eventually they will jiggle hard enough to disrupt the bonds holding them in fixed position to their neighbors and a liquid is born. Keep increasing the jiggling and atoms will begin being expelled entirely from the body (evaporation) and you get a gas.
An excellent account of this process is covered in “The Feynman Lectures on Physics Volume 1” in the very first chapter.
BTW, this is an overall excellent book for those curious about the way the world works. It brings many concepts in to clear focus for the reader.
Ack… what the hell is a “struction”? I think I wanted to type “structure”
While “heat” is commonly used to mean “thermal energy”, strictly speaking, in thermodynamics, heat refers only to the transfer of thermal energy. And temperature can be roughly considered to be proportional to the kinetic energy per particle (whatever the relevant sort of particle is), with the proportionality constant being Boltzman’s constant. More precisely, however, temperature is defined in terms of heat flow: If heat flows from body A to Body B, then A is hotter than B.
Is heat also transferred at a subatomic level? That is, is there any variation in the temperature in the nucleus of an atom?
Given that heat can be transferred by subatomic particles (i.e. neutrons in a neutron star) I would have thought the temerature could vary within the nucleus. BTW, thanks for the informative responses.
-Oli
Disclaimer, this is only a guess and really needs to be confirmed with (or discredited) by someone with more phyics than I have.
My guess is that even within a neutron star core, there must still be some “wiggle room” left for neutron packed together. Or, the whole ball wiggles are a unit (I find this idea pretty unlikely).
Note that “heat” has two components: acoustic and electromagnetic; phonons and photons; high frequency sound and low frequency light. In other words, a hot atom can heat its neighbors by wiggling them across atomic bonds (thus transferring a phonon) or by emitting blackbody EM which the neighbors absorb (thus transferring a photon.) The thermal energy doesn’t only travel through a substance via conduction, it also travels internally as electromagnetic radiation.
One portion of heat is actually hypersonic sound. Phonons inside a warm object are incoherent, with high broadband frequency spectrum, so think of this mechanical-vibratory part of heat as being like pink-noise which is REALLY LOUD and really really high pitched. When sound waves have a very short wavelength, one which approaches the size of atomic spacing, the speed of sound goes to zero. Heat moves slowly through an object because the speed of hypersonic sound is very low.
And if “stored heat” is allowed as a concept[1], then atoms can store it in several ways: by having electrons jump to higher-energy orbitals, by vibrating in many ways (linear or torsional,) or by altering chemical bonds (melting, endo/exo chemical reactions, etc.) If you whack an atom, it might vibrate as a whole, but instead it’s electrons might vibrate (in other words change shape of electron cloud.)
Also note that “heat” means something different for metals, as opposed to heat in any non-metal substances. When you heat a metal, you mostly affect its sea of electrons and not its actual atoms.
[1]Many instructors won’t allow the “stored heat” concept in their classrooms or in their textbooks. Their rallying cry is HEAT IS A VERB. By their philosophy you can “heat” an object (transfer thermal energy), but the object does not store an energy called “heat.” If heating an object causes chemical reactions, where did the “heat” go? Are heat and chemistry the same thing? Not if “heat” is never a noun.
Heat, like all thermodynamic concepts, is only well-defined when you have a large number of particles. What qualifies as “large” is debateable, but the number of nucleons in a nucleus would probably not qualify (so heat would be meaningless, in a single atom). The number of neutrons in a neutron star, however, is easily large enough to treat thermodynamically.