I What happens when an object starts to glow?

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I understand it as heat is simply the motions of atoms/molecules (kinetic energy). The warmer an object gets (a solid), the more the molecules are vibrating. But what is happening when an object starts to glow? I guess that the electrons of the vibrating atoms get excited and then falls back and hence emitting photons (visible light). So why are the electrons excited? Why do they care about the vibrations of their molecules?
I understand it as heat is simply the motions of atoms/molecules (kinetic energy). The warmer an object gets (a solid), the more the molecules are vibrating. Or possibly the other way around; the more vibrations, the more heat we say that the object has (right?). But what is happening when an object gets so hot that it starts to glow? I guess that the electrons of the vibrating molecules get excited and then falls back and hence emitting photons (visible light). So why are the electrons excited? Why do they care about the large vibrations of their molecules?

Thanks!
 

Drakkith

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There are many different ways to partition thermal energy into a material. If we model our material as a monoatomic gas, then the only way to do so is in their kinetic energy and any electronic transitions of the electrons. The more complexity we add to our model, the more ways we can add energy. Changing to a diatomic gas adds things like the vibration of each atom along their shared bond. For molecules that aren't diatomic, the vibrations take on upwards of six different forms: symmetric and asymmetric stretching, scissoring, rocking, wagging and twisting. See this video for an example of each one.

In addition, molecules can also rotate around certain bonds, and the more complicated the molecules get, the more ways all these different types of vibrations and rotations add together. The end result is a chaotic jumble of moving charged particles that are constantly bumping into and off of one another, getting pulled and twisted at random, and undergoing different transitions all the time. And don't forget that solid objects also generally share electrons, which are in motion throughout the bulk material and can collide with ions and other electrons, transferring energy to/from them. This is especially true for metals, which is why they are very good conductors of both heat and electric current.

The key idea here is that these particles undergo acceleration when they are subjected to pushing or pulling forces. What happens when charged particles undergo an acceleration? They radiate EM radiation!! That's why hot objects glow. It's not solely because the electrons are undergoing electronic transitions. That's not even the dominant effect except for sparse gasses (like what you find in a neon light). The bulk of the light is the result of the acceleration of many charged particles in many different ways.

This also explains why hotter objects glow brighter and with a higher average frequency. The hotter the object is, the more energy each of these collisions have and the higher the acceleration, which results in a higher frequency of emitted light and more of it.
 

Mentz114

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Summary: I understand it as heat is simply the motions of atoms/molecules (kinetic energy). The warmer an object gets (a solid), the more the molecules are vibrating. But what is happening when an object starts to glow? I guess that the electrons of the vibrating atoms get excited and then falls back and hence emitting photons (visible light). So why are the electrons excited? Why do they care about the vibrations of their molecules?

I understand it as heat is simply the motions of atoms/molecules (kinetic energy). The warmer an object gets (a solid), the more the molecules are vibrating. Or possibly the other way around; the more vibrations, the more heat we say that the object has (right?). But what is happening when an object gets so hot that it starts to glow? I guess that the electrons of the vibrating molecules get excited and then falls back and hence emitting photons (visible light). So why are the electrons excited? Why do they care about the large vibrations of their molecules?

Thanks!
A suffciently large cloud of matter can collapse under its self-gravity and begin nuclear fusion reactions - i.e. become a star.
 

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