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Can accelerated charges explain the light from a fire?

  1. Jul 21, 2014 #1
    I have heard that when charged particles are accelerated, they emit radiation. Combining this with the knowledge that atoms within a fire, are colliding at more then roughly the order of 1021 collisions per second. Is this why fire emits light? Is it the constant high acceleration from the collisions with other particles which gives us radiation in the form of visible light?
     
  2. jcsd
  3. Jul 21, 2014 #2
    I believe the reddish-orangish glows you see represent the EM radiation of accelerating soot particles at the temperatures of the fire. In this way, the radiating soot is similar to hot lava that glows red. The frequency of the emitted EM radiation can be predicted from the temperature of the soot particles. The general phenomenon of objects emitting EM radiation when above 0 K due to colliding/accelerating particles is known as blackbody radiation.

    On the other hand, I think the blues that you see represent emissions of photons as the electrons of the particles transition down from higher energy levels. I believe this second mechanism is distinct from the first and is similar to how spectral lines are produced (absorption/emission lines).
     
    Last edited: Jul 21, 2014
  4. Jul 21, 2014 #3
    Cant this change in energy level be a change in the electrons velocity around the nucleus?
     
  5. Jul 22, 2014 #4
    I don't believe so. Quantum mechanics throws out the idea that the electron is moving around the nucleus. It throws out the notion that, prior to measuring the electron, it has a defined location at any given moment in time. One way to see this: if the electron were moving circularly, it would be constantly accelerating (centripetally). Hence, the electron would radiate away energy since this is what accelerated charges do. But the bound electron is typically found somewhere in an incredibly tiny radius around the nucleus. Given this tiny radius, the electron would have to be moving so fast and would require such a great acceleration that it would radiate away all of its energy is around 10^–14 seconds (I think). This means that, after 10^–14 seconds, the electron would lose all of its energy and spiral into the nucleus. But that's not what happens if you watch an atom--atoms are stable, and their electrons don't spiral into their nuclei. Hence, the electrons can't be circling the nucleus or even moving in a classical sense.
     
  6. Jul 22, 2014 #5

    Jano L.

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    This would be true if there were no EM fields acting on the atom and the only fields were due to the atom and were retarded. Retardation is good but there are noise EM fields present due to other particles and possibly background radiation, which can prevent such collapse.

    http://www.sciencedirect.com/science/article/pii/S0375960103012696
    http://www.calphysics.org/articles/ColeHydrogen.pdf
     
  7. Jul 22, 2014 #6

    sophiecentaur

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    This is not a concept that works in QM. It is normal to talk in terms of Energy Levels or States within the atom. Some atoms can absorb optical frequencies, when the steps between permitted energy states correspond to the energies of optical photons. The electron, in its bound state cannot be discussed as if it were a little satellite in orbit. It doesn't even have a location that could be identified like an object in Earth orbit.
    The energy associated with the optical glow you see in a fire only corresponds to a few eV. The inner electrons are at much deeper levels and would not be touched until the 'fire' temperature was much higher to include significant amounts of X rays. Electrical Arcs (welding) tend to be at high temperatures and will produce significant UV radiation.
     
  8. Jul 22, 2014 #7
    So... its partially beacouse of the constant high acceleration from the collisions with other particles, but altso a change in the electrons energy level? Is that whats beeing said here?
     
  9. Jul 22, 2014 #8
  10. Jul 22, 2014 #9
    But i still believe it makes sense to talk about the electrons acceleration even though QM teaches us that they dont have a definite position or speed. Why? Becouse we use the electrons high speed to explain why it doesnt get pulled into the nucleus, and i have learned that importent and valid equations within QM such as bhors equation for the hydrogen atom is based on the electrons speed. I believe his equation was found using the idea that it had a given path, combined with λ=h/p. But i could be wrong...
     
  11. Jul 22, 2014 #10
    Yes, you are exactly right about everything you've said here. Teachers often present an older and incomplete model of the atom whereby electrons orbit the nucleus along a circumference that "fits" an integer number of de Broglie wavelengths. I believe this derivation you're thinking of can be found here. This model persists in physics education, despite the fact that we know it to be wrong and inaccurate in predicting spectral lines (particularly compared to quantum mechanics/QM).

    So, why does this cruder model persist? Because of pedagogy: this model helps students to begin thinking about and understanding quantum-mechanical features of the atom, like quantized energy levels and energy level transitions. It also provides an application of the de Broglie wavelength. However, as one continues to learn more about QM, the somewhat antiquated model of "orbiting electrons" eventually should be supplanted by the QM understanding of the electron bound to a nucleus (particularly wherever the two theories disagree).

    This is not entirely correct, in my understanding. In physics education, it is common to be told that an electron "must" be orbiting when the teachers are attempting to justify Bhor's model. In fact, Bhor offered a similar rationale in support of his model. This rationale that an electron must be orbiting the nucleus is tempting to believe in because of the strong (but somewhat incorrect) analogy to planetary orbit. The typical defense of the position that the electron must be orbiting is correct mostly just in one regard: this position correctly explains why it's impossible that the electron be stationary in a single location (because it would fall into the nucleus). But ultimately, the notion of a moving electron has its own equal problems which makes it no better than the notion of a stationary electron. The issue that is conventionally raised about a moving electron is this: because (a) charges emit EM radiation and emit energy when they accelerate, (b) an electron moving (even at uniform speed) in a circle around the nucleus constitutes an accelerating charge, and (c) the required orbital radii are so small, an orbiting electron would rapidly emit all of its energy (potential and kinetic), causing it to fall into the nucleus. It seems from the post above that this spiraling situation wouldn't happen in every single case, but rather only in some cases, depending on what the external environment is. Even accounting for this, the notion of orbiting electrons suggests that atoms would be much less stable (and much more short-lived) than what we see when we look out at the universe.
     
  12. Jul 22, 2014 #11
    I will take this to consideration and think about it. Thank you!
     
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