Why is classical physics = ultraviolet catastrophe (1 Viewer)

[jamesfirst]

In the section of black body radiation, the classical theory people determined that the intensity of the black body radiation would increase infinitely. But I don't get why this violates the conservation of energy???


The heat is been applied to the black body to convert thermal energy into EM radiations. So if the heat was constantly inputted into the black body, wouldn't it make sense that it would increase infinitely. I understand that Planck discovered that there was a definite peak.


I just don't understand why it violates the conservation of energy...


And what did Planck exactly do to explain this ???? I get that he introduced photon but... what??? lol

 

[SpreadTheWord]

Isn't it as the heat increases, the frequency changes due to induction of different electromagnetic radiations?..but anyway to answer question, the principle of conservation of energy is violated because energy cannot be created nor destroyed, and in this case energy increases and increases reaching an infinite amount as you have said. However planck explained this through naming them as 'small packets of energy'. He assumed that the radiant energy, although exchanged between the particles of the black body and the radiant energy field in continuous amounts, may be treated statistically as if it was exchanged in multiples of a small 'lump'

I might be wrong in this part, so forgive me. If you have covered the photoelectric effect, you will understand that there needs to be a certain amount of uv light energy to overcome the workfunction in order to release the electrons (other em's might not be able to supply sufficent amounts of energy) So in relation to the previous , as the heat increases it produces different electromagnectic radiation all of which have different properties (energy levels), possibly the reason for the energy increasing and thus violating the principle of conservation of energy.

Hope this helps mate.

 

[jamesfirst]

I know about the threshold frenquence from the photoelectric effect.


Anyway that explanation doesn't really help because it's copied from Jacaranda's Physics textbook, on the Planck and black body radiation section.

 

[Fizzy_Cyst]

In the section of black body radiation, the classical theory people determined that the intensity of the black body radiation would increase infinitely. But I don't get why this violates the conservation of energy???

The heat is been applied to the black body to convert thermal energy into EM radiations. So if the heat was constantly inputted into the black body, wouldn't it make sense that it would increase infinitely. I understand that Planck discovered that there was a definite peak.

I just don't understand why it violates the conservation of energy...

And what did Planck exactly do to explain this ???? I get that he introduced photon but... what??? lol

Wave theory suggests that as wavelength decreases, intensity increases infinitely. Intensity is essentially power, or rate of use of energy, so basically infinite energy being used up infinitely quickly. Obvious violation of LOCOE.

Planck was a mathematician. He looked at the graph wave theory was suggesting, he looked at the graph from the actual results and he derived an equation from the actual results. His equation suggested that the energy transitions of the 'atomic oscillators' within the blackbody were quantised, there were only discrete energy transitions which the oscillators could undergo, rather than the continuous spectrum of energy transitions as previously thought by the wave theorists.

 

[romesh]

The heat is been applied to the black body to convert thermal energy into EM radiations. So if the heat was constantly inputted into the black body, wouldn't it make sense that it would increase infinitely. I understand that Planck discovered that there was a definite peak.

Also, you have to remember that the emission of radiation constitutes energy loss from the heated object. That is, if you heat an object so that it is glowing *and* at a constant temperature, then the amount of energy you are putting in to keep it hot must be same as the amount of energy it is losing to the environment (from both convection and radiation). The examples here are not time dependent (like you heat an object, and it keeps getting hotter), it is just about a constant situation where the temperature is fixed in some way

 

[khorne]

ALl of these explanations suck. It's pretty simple.

Classical theory stated that as wavelength decreased, intensity of radiant energy increased. This was because at large wavelengths, the particles (electrons etc) could not oscilate nor physical create a wave of such great wavelength. If you were to consider the oven analogy of a blackbody, the wave could not fit in the cavity, so it could not be created nor radiated.

However, as the wavelength decreased, more and more waves would be formed. However, this violated conservation of energy, as it should theoretically increase without bound. Plank, by assuming energy was quantised, made a statistical model to show that the curve would drop off after a certain point. Assuming the energy = hf and that a whole photon must be emitted, not partial, at higher oscilating (again, this is in terms of thermal energy) electron is needed. In such a black body, the energy distribution of such is that the higher the freq. the less electrons possess that amount of energy. Thus as wavelength decreases, less electrons exist with such energy, and as they can't emit partial photons, then they don't emit, reducing the intensity. You then get to a point were hardly any electron has the energy needed for high freq wave emissions, so none occur.

So, at low frequency, the long wavelength limits the number of possible photons. At high frequency, the high energy makes emission unlikely. So the distribution goes to zero at both f = 0 and f = infinity, and has a maximum in between.