# How does heat travel?

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)

• 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. – user854 Nov 4 '14 at 21:00
• It seems to me that you are specifically interested on how does heat travel inside solids. Maybe you can be more specific about that. – Asaf Nov 4 '14 at 21:34
• 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. :) – user58536 Nov 4 '14 at 21:41
• 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. – BowlOfRed Nov 4 '14 at 22:09
• @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 – Nikos M. Nov 5 '14 at 0:02