What causes hot things to glow, and at what temperature? I have an electric stove, and when I turn

Blaine Stein 2022-05-18 Answered
What causes hot things to glow, and at what temperature?
I have an electric stove, and when I turn it on and turn off the lights, I notice the stove glowing.
However, as I turn down the temperature, it eventually goes away completely. Is there a cut-off point for glowing?
What actually is giving off the light? Does the heat itself give off the light, or the metal?
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Answers (2)

Julius Johnston
Answered 2022-05-19 Author has 17 answers
It is due to thermal radiation. Bodies with temperature above absolute zero emits radiation. If frequency of the radiation is in the visual range the body "glows".
"When the electrons in the atom are excited, for example by being heated, the additional energy pushes the electrons to higher energy orbits. When the electrons fall back down and leave the excited state, energy is re-emitted in the form of a photon. -wikipedia(emission spectrum)"
This explains that heat itself is not giving off the light.
Spectral energy density as function of of wavelength and temperature
u ( λ , T ) = 8 π h c λ 5 1 e h c λ k T 1
If you integrate u ( λ , T ) wrt λ from λ = 380 n m to λ = 750 n m you will find that there is always some radiation in visual range. But it is very very small at room temperature(T = 300K) and hence undetectable to naked eye.
To get cut off temperature( T c ) for glow. You need to solve
0 T c λ = 380 n m λ = 780 n m u ( λ , T ) d λ d T = minimum required to be detected by human eye.
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Annabel Sullivan
Answered 2022-05-20 Author has 4 answers
I would like to emphasize a point mentioned in the first answer.
The classical black body radiation has to do with a gas of photons in thermal equilibrium. The photons interact with each other and that interaction gives the usual Planck spectrum for their energies.
Inside stars, there are photons produced which interact with the stellar matter and they both come to a thermal equilibrium. When the photons escape the star, they have a black body spectrum.
When you take some amount of matter (an electric stove) and heat it, there is the question of where do the photons come from. In low temperatures, you don't have enough thermal energy to excite electrons in atoms. So, where do the photons come from? The answer is that they must come from the excitation of the background electromagnetic field. Even if you tried to isolate the heated matter from every electromagnetic field, you could never block out the vacuum fluctuations.
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