How does heat travel? I have two competing thoughts here!!

Firstly some form of atomic/molecular process liberates a photon in the infrared region of the spectrum which is detected as heat by a sensor.

Secondly thermal excitations and the what not propagating through matter causing an increase in the average kinetic energy and hence a temperature rise.

Heat is just the change in entropy with respect to temperature, so how do I unify these ideas? (sorry if its basic)

  • $\begingroup$ When molecules bounce off each other, the slower one usually gets faster and vice versa. So the average kinetic energy of the colder/slower molecules increases while the kinetic energy of the hotter/faster molecules decreases. Your "firstly" occurs during a chemical reaction. Additionally, radiation is how heat travels through empty space. $\endgroup$ – user854 Nov 4 '14 at 21:00
  • $\begingroup$ It seems to me that you are specifically interested on how does heat travel inside solids. Maybe you can be more specific about that. $\endgroup$ – Asaf Nov 4 '14 at 21:34
  • $\begingroup$ I think what I am confused over is the difference between saying heat is thermally induced vibrations $in\ solids$ and then comparing that to electromagnet radiation of infra red photons. How say could a heat sensor tell me that a metal object was hot from across the room. The answer to that question would probably answer most of my queries. :) $\endgroup$ – user58536 Nov 4 '14 at 21:41
  • $\begingroup$ Most objects near room temperature have a simple relationship between their temperature and their IR spectrum. The motion of the atoms causes photon emission and the motion and temperature are linked. $\endgroup$ – BowlOfRed Nov 4 '14 at 22:09
  • $\begingroup$ @JanetthePhysicist, there are 2 concepts, thermal radiation or infrared radiation (aka heat radiates as EM waves) and temperature related to kinetic energy of particles of a solid $\endgroup$ – Nikos M. Nov 5 '14 at 0:02

Heat travels in three ways: Conduction, convection, and radiation. Conduction is when objects of different temp come into contact, and the vibrating molecules from the hotter object increase the vibration of the molecules in the cooler object, thereby cooling the hotter object and heating the cooler one. Convection is similar, but occurs when one of the materials is a gas or liquid. Radiation refers to the black body electromagnetic waves emitted by all objects that are above absolute zero temperature. It looks like you already understand all that, so this may not answer your question.

  • $\begingroup$ Thank you very kindly for your answer. I have posted a comment above which maybe you could expand a little on to help me understand ? $\endgroup$ – user58536 Nov 4 '14 at 21:42
  • $\begingroup$ I think I understand your question now: How does kinetic energy from vibrating molecules of a hot solid get converted into electromagnetic waves? The simple explanation is that the mechanical vibration causes charged particles to wiggle back and forth. Moving charges produce time-varying magnetic fields, which launches the EM wave. It's similar to an antenna on a radio transmitter: you move charge up and down the antenna, and that generates the radio wave. $\endgroup$ – David Rose Nov 5 '14 at 0:44

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