Question

what is heat? i know heat can be radiation and also energy but can it be a wave? and 2nd question is what causes heat? we know heat can be obtained from the change of various energy. 3rd question people say infared ray is heat so actually heat generate light or light generate heat or they both generate at the same time by something else?

Answer 1

Heat is a form of energy. All forms of energy are equivalent. A given amount of energy in one form cannot disappear without an equal amount of energy appearing in some of the other forms.
Infrared rays are electromagnetic radiation. The energy in the IR radiation is converted to heat energy when the radiation is absorbed by material in the path of the radiation.

Answer 2

There is unfortunately more than one answer to your question, depending on the context. "Heat" is a word that is widely used, and used to mean different things.

The purest possible definition is perhaps that from thermodynamics, which says that "heat" is the change in energy of a system that (1) appears at the boundary of the system, and (2) does NOT involve a change in the value of any macroscopically observable degree of freedom, except for the temperature. That is, the system is NOT observed to expand or shrink, change its electric or magnetic dipole moment, change its chemical composition, have any part of itself move against a directly measureable force (like gravity or a spring), and so forth. It's essentially energy that is transferred directly to or from the microscopic, invisible degrees of freedom that are such things as the uncoordinated rapid movements of individual atoms and molecules.

Note that from this point of view, heat is not something a substance has, but something that happens to it. It's like the distinction between electric charge and electric current (moving or changing charge). Heat is the MOVEMENT of energy, a particular kind of movement -- one that is effectively not directly observable macroscopically.

Now, that said, with our understanding of atomic-scale physics, we can go deeper and ask what, exactly, is going on when heat is transferred to or from a substance. What we find is that for the most part, what happens is that the uncoordinated vibrations of the chemical bonds in molecules, or the bonds between atoms in a metal or extended network solid (e.g. diamond or ice), or the lattice vibrations of an ionic solid, are becoming faster and stronger. This isn't all that happens, certainly. When water is microwaved, for example, it is primiarly the rotation of the water molecules that accelerates. When gases are heated, a lot of the energy goes into acceleration of their translational motion (bouncing around the box), and in the case of monatomic gases, like helium, all of it goes there.

Infrared radiation is often (carelessly) referred to as "heat" because when you apply heat o a substance, you can do so with infrared radiation, and the substance will also freely radiation infrared radiation. The reason is that infrared radiation doesn't have enough energy to break chemical bonds, usually, so that applying IR radiation to a substance will NOT cause a chemical change. (Remember energy changes that result in changes in chemical composition are not heat, because changes in chemical composition are macroscopically observable.) Radiation at higher frequencies, e.g. visible or UV light, does. That doesn't mean you can't heat a substance with visible or UV light -- in fact, the Sun heats the Earth that way every morning -- but it does mean that not all of the energy you add will correspond to heat. Some of the energy will probably do chemistry and hence count as "work" (the thermodynamic complement of heat).

Heat is fundamentally a macroscopic thermodynamic concept, in the end. If you want to get down to the atomic detail and look at precisely where energy is going, then heat tends to lose its usefulness as a concept.

Answer 3

ya fully agree with what you said because now i am reading the phonon model by Einstein very interesting its talking about the vibrational wave cause by wave ( well i think heat is still an abstract concept and not fully develop yet) cause you see even in thermodynamics the enthropy idea is still the measurement of ignorane ( meaning we do not deeply understand the system yet - like the quantum mechanics just using probability) well thanks any way....