Really silly syllabus (1 Viewer)

[robertdapice]

Yes, it's true, some of the physics syllabus is effectively retarded.

I am speaking through the blinding frustration of summarising the unit 'From ideas to Implementation' for my 3rd assessment block starting on monday.

Anywho, this:

"Indentify Plancks hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised."

If anyone understands this, and I mean REALLY understands it, please, fill me in. I understand what a black body is, how as temperature increases, wavelength of emitted waves becomes shorter and intensity becomes higher.

But does this 'max out'? Does the intensity only get so high (as a certain temperature), and then suddenly, the intensity begins to lower again?

My text book is fuzzy, and I've read several sources on the net. Plus, my teacher is useless.

Further, the text book doesn't explain why quantum theory satisfies this pattern, but classical physics doesn't.

The description of a 'black box cavity' is also relatively vague.

How frustrating.

Any help would be appreciated!

Rob

BTW: I do know that the keyword is 'identify', but I really feel like knowing this - that's the problem with this ridiculous course.

 

[kini mini]

Originally posted by robertdapice
Yes, it's true, some of the physics syllabus is effectively retarded.

This you call news?

BTW: I do know that the keyword is 'identify', but I really feel like knowing this - that's the problem with this ridiculous course.

It's hard to tell how much you have to know for this. Here are my notes from last year:

Identify Plancks hypothesis that radiation emitted and absorbed by the walls of a black-body cavity is quantised

 A model black-body: a small hole in the wall of an induction furnace (temperature can be set accurately, very efficient)
 At 1000 deg C, the walls of the black-body will emit all types of radiation, but this radiation can only escape through the small hole
 The radiation will bounce around the furnace cavity until absorbed by the walls, causing the walls to emit radiation of a different wavelength  eventually equilibrium established
 So radiation from hole is characteristic of the equilibrium temperature  known as black-body radiation
 Classical wave-theory of light predicted that the radiation intensity should increase without limit as the wavelength of emitted radiation decreased
 This increase in energy violated the principle of conservation of energy
 Planck proposed that energy was exchanged between particles of the black-body and the equilibrium radiation field in a manner analogous to the transmission of radio waves
 That is the relatively high frequency of light emitted by a black-body required an atomic-sized aerial for its production
 Assuming that radiant energy could be treated as if it consisted of multiples of a discrete amount (quanta), Planck said that each quanta was characteristic of each frequency of radiation emitted

This is a summary of Jacaranda and Marsden

 

[NSBHSchoolie]

Dap, I had exactly the same problem, as almost all texts skimp over that explanation. After much soul-searching and rereading of numerous sources, I think I have the answer.

The problem with quantum physics is that everything is based on probability. As the atoms in the black body cavity are heated up, their electrons are pushed up into the higher quantised energy levels. When the electrons emit their photons, the wavelength/frequency given out depends on the difference between the two energy levels. However, the probability of a long enough jump, say for UV to be produced is extremely small, while a red one is much more common. This is why there is a peak in the middle of the graph.

 

[Constip8edSkunk]

NSBHSchoolie, could you expand on this please?

However, the probability of a long enough jump, say for UV to be produced is extremely small, while a red one is much more common. This is why there is a peak in the middle of the graph.

i thought the peak is the point of maximum photons, where an equilibrium is reached between the theorized increase in intensity and the idea that increase in frequency means more energy per quantum of photon(quantised energy --> less photons --> less intensity)

 

[NSBHSchoolie]

Well I'm not quite sure what you mean Skunk, but here's a guess.

Each of the curves on the graph corresponds to a specific temperature. As far as I know, the peak of the graph represents the dominant frequency being radiated (ie. the most photons of that frequency), but all the quantised frequencies are being emitted at once.

As temperatures increase, the atoms in the cavity are more excited, which creates more frequent movements between energy levels in the atom. And as I said, because of probablity, less UV will be emitted than say, red.

I'm not sure this is the answer but its the best damn thing I've been able to come up with. A bigger question is why no textbook explains this to you straight.

 

[Constip8edSkunk]

From what i've read, there will always be less UV light emitted than red light, unless the temperature is increased extremely high, far beyond 6000K(which is in the diagram of most textbooks)... generally there are less radiance measured for UV light than say Red light because a photon of UV light has greater amount of energy than a photon of red light, by E=hf. In the smaller frequencies(longer wavelengths) the increases in distribution of radiant energies as the wavelengths shortens is greater than the increase in quantised energy in each quantum of photon, therefore the number of photin increases as do the radiance. But at a certain point, the increase in energy per packet would be greater than the increase in radiance energy associated with the wavelength of teh emitted radiation as it shortens. at this point, as one studies decreasing wavelengths of EMR emitted, there would be less photons emitted as the wavelengths decreases(or frequency increases). This means there would be less radiance and teh turning point of the graph of this data would be the peak represented.

As one increases the overall inputted energy/temperature, the distribution of the radiance energy tend to shift slightly towards the left, by probability as you say, as oscillations are at a higher quantised frequency or between higher energy levels. I think this can also be shown by thermodynamics. By the conservation of energy, there would also be an overall increase in radiance as the graph shifts upwards to accomadate the increase in energy.

Have i got it right?



and [rant] yeah i think the textbooks doesnot explain it clearly because ithe syllabus does not require much of an indepth understanding, just know that this happens, and know the buzz word quantised. like in the motors topic, all you have to do is memorise all the phenomenons like induction, motor effects and what transformers do without actually understanding the theory behind it and how they come together. all the syllabus wants is what (cram) not how (think) [/rant]

sorry bout the large post

 

[NSBHSchoolie]

Whoa, you're good. Seems to have cleared it up for me!!

As for the rant, I know what you mean. But I think its better to at least understand the workings of the babble you are required to reproduce in the HSC.