Confused about the quantum theory (1 Viewer)

[allstarr69]

hi.. Ill tell you what I know and would be greatful if anyone tells me where ive gone wrong...
In a black body, energy that is absorbed and re-emmited by the walls causes the walls to increase in energy. Classical physics thought the energy would increase indefintely as the frequency of the energy increased in the black body. This was called the ultraviolet catastophe.

Planck however proposed the quantum theory which means that energy is only discreted in small amounts called quanta, and that a quantum is the smalllest amount of energy possible for a particular wavelength. He gives a formula, E = hf.

Ok so I understand this, but i dont understand how this solved the ultraviolet catastophe. Just coz he shoed that energy existed in small amounts doesnt show how the energy strats to decrease as frequency increases. An the formula E = hf doesnt describe this either as it only gives a linear relationship between Energy and frequency.

Would someone smart and nice pls tell me what im missing? Thanks in advance

 

[Zarathustra]

I think the actual equation is this. By E=h.f - it can be seen that light is in multiples of 'h' - but if the wavelength is zero then E is infinite - just like the classical theory - I never really got that part either but Planck's other equation is obviously the one used to plot the blackbody graphs (if it were E=h.f it would be linear - which is used elsewhere in the course), and there is no singularity in the actual equation (F(w)=C1/[w^5(exp(C2/wT)-1]) - I think that this means that wavelength is never zero... I highly doubt that this course aims to make us understand quantum theory.

------> This is the blackbody curve, methinks. If wavelength is zero in this equation Energy is still infinite but the other equation proves that it never will be, maybe....

 

[BlackJack]

http://www.phys.unsw.edu.au/~jw/FAQ.html#black

This may help you much, and there's another from there. The point of emphasis is that an electron only exists in multiples of some fundamental frequency (think deBroglie), and when light is emitted from said electron it therefore can only have quantised energy. You should read those.

To fully understand this well, you can also lead in the fact that in the material, fewer elecrons are in a high state of energy. To understand this, think of a heated material. It has (say, for argument) atoms in excited states. You can put into each a quantised amount of energy, and the total energy you have to distribute is determined by temerature.
On a statistical basis, you are likely to end up with some atoms at the base and at low excitation levels, while high levels are much less likely to be found. It is these highly excited atoms that emit the high frequency photons. Since they get exponentially rarer as you look for even more excited atoms, you never get the uv catastrophe.

 

[allstarr69]

thanks ppl for those links.. things are a bit clearer now.
So basically as frequency increases it still releases MORE energy, but fewer of that more energy because there is less chance that it will release it due to the quatisation thingy.