I Why doesn't the atom absorb heat energy when it is low?

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(In my last thread)
Mentor Dale said:
"
An atom in the ground state can absorb energy from the environment including thermal radiation.
Once it has done so it will be excited and will no longer be in the ground state. An excited atom
can radiate and go to a lower energy state, but an atom in the ground state cannot radiate since
there is no lower energy state.
"
I then asked:
"
But how if the energy it absorbed is lower than that of the gap between the first excited state and ground state?
"
Dale said:
"
Then it is transparent to that radiation and cannot absorb it.
"
Now I have my question: Why cannot it absorb it ?
 
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Just to confirm, I did recommend asking this question in the public forums
 
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Sorry I am new to this forum. Did you mean public forum as another sub-forum different from General Physics? If so, please move it.
 

Nugatory

Mentor
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Now I have my question: Why cannot it absorb it ?
Because if it absorbed it, then its total energy would be neither the ground state energy nor the energy of the lowest excited state but somewhere in between, and that's physically impossible.
If you are trying to ask why the atom can only have those energy levels and not something in between, we'll need some quantum mechanics and even the most minimum answer is going to go beyond a B-level thread: google for "Schrodinger hydrogen atom" to get started.

But note that we're talking about radiation here. In your thread title you asked about heat energy, and it never makes sense to talk about a single atom absorbing heat energy. Heat energy is (loosely speaking) the kinetic energy of a large number of molecules moving at random.
 
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Sorry I am new to this forum. Did you mean public forum as another sub-forum different from General Physics? If so, please move it.
No worries, you did correctly!
 
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Because if it absorbed it, then its total energy would be neither the ground state energy nor the energy of the lowest excited state but somewhere in between, and that's physically impossible.
If you are trying to ask why the atom can only have those energy levels and not something in between, we'll need some quantum mechanics and even the most minimum answer is going to go beyond a B-level thread: google for "Schrodinger hydrogen atom" to get started.

But note that we're talking about radiation here. In your thread title you asked about heat energy, and it never makes sense to talk about a single atom absorbing heat energy. Heat energy is (loosely speaking) the kinetic energy of a large number of molecules moving at random.
Thanks. You are more correct about the title should be "thermal radiation energy". By my question, I mean why I can't regard that the atom absorbs the energy and at the same period radiates out the same amount?
 
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Why cannot it absorb it ?
When you solve the Schrodinger equation for an atom, one of the things that naturally falls out of the solution is that only discrete energy levels are possible. If an atom is exposed to radiation for which it has a corresponding energy level then it can absorb it and transfer to an excited state. Conversely, if no corresponding state exists then it cannot transfer and therefore cannot absorb the radiation. This is the cause of the absorption lines in gas spectra.
 
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When you solve the Schrodinger equation for an atom, one of the things that naturally falls out of the solution is that only discrete energy levels are possible. If an atom is exposed to radiation for which it has a corresponding energy level then it can absorb it and transfer to an excited state. Conversely, if no corresponding state exists then it cannot transfer and therefore cannot absorb the radiation. This is the cause of the absorption lines in gas spectra.
Can I regard that the atom absorbs the energy and at the same period radiates out the same amount?
 
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Are you trying to ask about scattering of light?
Here I didn't relate it to the scattering of light. Why do you think there is connection between them?
 
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Can I regard that the atom absorbs the energy and at the same period radiates out the same amount?
No. The atom cannot absorb the energy. There is nowhere in the atom for it to go. Such a scheme would therefore not conserve energy.
 
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Do you mean you regard that if it absorbs and radiates in the same period, the energy it absorbs and radiates out cannot balance ?
 
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Do you mean you regard that if it absorbs and radiates in the same period, the energy it absorbs and radiates out cannot balance ?
If it absorbs energy then that means that the energy enters into the atom. There is nowhere for the energy to go, so it cannot be absorbed.

It sounds like you are trying to redefine the word “absorb” such that it is possible for an atom to “absorb” energy without that energy entering the atom. This is not what “absorb” means. When an atom absorbs energy its energy level increases.
 
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I guess it absorbs under the thermal agitation in its environment, and radiates out as classical electrodynamics said when it is accelerating. The net result is zero. You could say the net absorption is zero.
 
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I guess it absorbs under the thermal agitation in its environment
Only if the thermal agitation has an energy corresponding to an available transition. If not, then it cannot absorb the energy.

At this point it seems like you have an agenda you wish to push and are not here to learn. Please review the forum rules which prohibit personal speculation.
 
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So you regard that contrary to classical electrodynamics, even it is accelerating, it doesn't radiate electromagnetic wave ?
 
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So you regard that contrary to classical electrodynamics, even it is accelerating, it doesn't radiate electromagnetic wave ?
The atom is neutral. According to classical electrodynamics it won’t radiate because it accelerates.
 

sophiecentaur

Science Advisor
Gold Member
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When you solve the Schrodinger equation for an atom, one of the things that naturally falls out of the solution is that only discrete energy levels are possible.
It's worth pointing out that the familiar Hydrogen Atom equation (one proton and one electron) shows the first Energy Level above ground state to be around 10eV (iirc) which is an UV transition. Thermal frequencies are associated with molecular transitions so the H atom model is not the best to hold in your mind when discussing this. The states associated with molecular vibration are more what you want but the model is not so familiar. The same principle applies, of course.
 
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Well, I meant it radiates through the electron outside of the nucleus.
 
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It's worth pointing out that the familiar Hydrogen Atom equation (one proton and one electron) shows the first Energy Level above ground state to be around 10eV (iirc) which is an UV transition. Thermal frequencies are associated with molecular transitions so the H atom model is not the best to hold in your mind when discussing this
Agreed, which is also why many gasses are transparent to IR or visible light.
 
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The atom is neutral. According to classical electrodynamics it won’t radiate because it accelerates.
Well, I meant it radiates through the electron outside of the nucleus.
 
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