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Q & A: What is heat?

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Most recent answer: 04/13/2020
May I have an authoritative definition of heat?There is a lot of confusing information out there on this subject. I think I understand that heat is atomic motion (Is it vibration?)If I�m correct:. Heat is the condition or quality of being hot or otherwise possessing atomic motion or vibration.. Heat flow is the movement of heat through a material or from one material to another. It flows from a hot to a cold object. Is this definition of heat and heat flow correct?If my definition is correct then heat cannot travel through space because there is no medium to transfer the energy. I constantly hear people referring to �the heat from the sun�. These are seemingly educated people. I think that they may mistakenly believe that Infrared light is heat. I feel sure that IR is part of the electromagnetic spectrum and therefore light, not heat. Am I wrong?I can understand that an uneducated person could easily describe feeling the heat from the sun on their face but is it not the interaction between the light and the atoms in their skin that generate heat? This question comes to me often from young people and I want very much not to provide a confusing answer.
- John Randel (age 52)

When we're being careful, by "heat" we mean the transfer of energy in random thermal forms. That can include via vibrations of atoms and molecules and also via the thermal radiation given off by hot objects. 

When we speak loosely, sometimes "heat" means just the thermal energy itself, not its tranfer from one object to another.

Either way, the key featire of heat as opposed to mechanical energy (or its transfer, "work") is the random, disorganized form taken by the energy. It's energy thay is accompanied by "entropy", meaning that for any large-scale description of what's going on, there are a huge number of microscopic states fitting that description. It's this entropy that characterizes heat, not the particular fundamental interactions (electromagnetic, strong nuclear, etc.) that are involved.

For radiation, absorbing light given off by a hot filament or the sun is an ordinary form of heating, since the light already has a lot of entropy. Absorbing light from a laser, for which the initial form has little entropy, fits your description of a process where the heating occurs only as the lght is absorbed.

The frequency range of the light (visible, infrared,...) has nothing to do with the distinction. The characteristic frequency rages of random thermal radiation epend on the temperature involved. For cold obejects (say some liquid helium) the thermal radiation can be mostly in the microwave. For hot objects (say inside the sun) it can be ultraviolet or even gamma rays. For the surface of the sun, it's visible light. For or bodies, it's infrared.

Mike W.

(published on 04/13/2020)

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