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Post by carnyx on Jan 13, 2011 23:48:24 GMT 1
STA, here is my understanding of the photoelectric effect;
Light of a certain frequency and amplitude will shake off electrons from a metal surface.
A specific frequency for a given metal is required get any electrons, and then increasing the frequency above this starting value, will progressively yield more electrons.
But, increasing the amplitude at any frequency will also produce more electrons, but only in discrete steps.
I suspect that you could produce a mechanical analog of such an effect via a system of springs and weights ..
I also suspect that a mathematical analog model can exist, and subsequent manipulations would alos be easy, if you invented the concept of a 'photon' ... i.e. discrete 'lumps' of energy that was defined in terms of frequency plus amplitude.
(PS; If you constrain yourself to comment on the above post within the parameters of sense set by these sentences .. I think you woudl be doing a service to all board readers. )
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Post by speakertoanimals on Jan 14, 2011 14:08:10 GMT 1
Wrong. Only light ABOVE a certain frequency (which depends on the metal) will eject electrons. There is NO dependance on amplitude, apart from the fact that the RATE at which electrons are ejected (number per second) increases as you increase the amplitude.
First part right, but second part totally wrong. Increasing the frequency, once above the threshold value, doesn't yield more electrons.
In fact, if the power of the beam remains the same, but frequency increases, what happens is you get FEWER electrons, but eacn has greater energy.
The greater the frequency, the greater the kinetic energy of the liberated electrons.
Not really.
For a given frequency, the energy (well, actually maximum energy, but that's a bit of a complication!) of the liberated electrons is constant. As you increase the amplitude, what increases (and increases continuously), is the RATE at which electrons are ejected (number per second).
Well, then you'd be a greater physicist than Einstein or Planck, the whole point about the effects abover being that it is fuindamentally inconsistent with classical physics -- the same classical physics that could be modelled by a system of springs and weights.
As I suspected, you couldn't even get the basics right, so really had no idea WHY photons HAD to be introduced, what problem they were introduced to solve, or what exact properties they have which did solve the problem.........................
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Post by speakertoanimals on Jan 14, 2011 14:32:03 GMT 1
What are the essential aspects of the photoelectric effect that were unexplainable using classical ideas (which includes classical ropes and pulleys analogies as well BTW)?
Let's take a classical wave, a nice smooth, continuous wave. If we have larger amplitude, we have more energy.
Okay, now imagine electrons in a metal, who require a certain energy in order to make their way out of the metal. We just suppose they can absorb energy from the incoming waves.
Now the obvious thing is to say that if you have a lower energy wave, the electron just has to wait a bit longer to soak up enough energy to get out. Frequency here doesn't matter much, all energy is good energy. (note that unlike electrons in atoms, electrons being liberated from a metal is NOT like a classical resonance process, where we have maximum effect over some narrow range of frequencies near the resonant one, and very little happening outside this frequency).
Hence we have the view that BRIGHT light should give lots of electrons, and dim light very few, but frequency doesn't matter much.
totally wrong. Take light of too low a frequency, and no matter hoW bright (the surface is being bombarded with energy!), no electrons at all come out! Not one.
Whereas take the dimmest light source of a high-enough frequency, and out they come!
Doesn't make sense if electrons just sit there soaking up the energy that is being continuously delivered.
And that's the key -- it just doesn't fit with any continuous picture of energy delivery.
It only fits with energy being delivered in chunks, where the size of a chunk only depends on the frequency AND NOTHING ELSE. And that each electron absorbs only one chunk, if any.
And got einstein his nobel prize, which tells you what a hard problem it had been!
(There are further technicalities to do with the exact shape of the blackbody spectrum, but since that is even more technical, and may people don't seem to believe in cavity radiation anyway..............)
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Post by Progenitor A on Jan 14, 2011 15:32:31 GMT 1
STA, here is my understanding of the photoelectric effect; Light of a certain frequency and amplitude will shake off electrons from a metal surface. A specific frequency for a given metal is required get any electrons, and then increasing the frequency above this starting value, will progressively yield more electrons. But, increasing the amplitude at any frequency will also produce more electrons, but only in discrete steps. I suspect that you could produce a mechanical analog of such an effect via a system of springs and weights .. I also suspect that a mathematical analog model can exist, and subsequent manipulations would alos be easy, if you invented the concept of a 'photon' ... i.e. discrete 'lumps' of energy that was defined in terms of frequency plus amplitude. (PS; If you constrain yourself to comment on the above post within the parameters of sense set by these sentences .. I think you woudl be doing a service to all board readers. ) Well Carnyx, do you like the responses of our 'physicist? Are they what you expected? Wrong. Only light ABOVE a certain frequency (which depends on the metal) will eject electrons. There is NO dependance on amplitude, apart from the fact that the RATE at which electrons are ejected (number per second) increases as you increase the amplitude. Note that you are WRONG in your statement which she then repeats in different words! Can you believe the 'physicality' of it? 'There is NO dependance on amplitude, apart from the fact that the RATE at which electrons are ejected (number per second) increases as you increase the amplitude’ She does not see the contradiction within this one sentence.! Anyone with the slightest intelligence will see that she does not have a clue what she is talking about! First part right, but second part totally wrong. Increasing the frequency, once above the threshold value, doesn't yield more electrons. In fact, if the power of the beam remains the same, but frequency increases, what happens is you get FEWER electrons, but each has greater energy. She could be right here but from what she has written above her word is not to be trusted. Note the dogmatic WRONG! Not really. For a given frequency, the energy (well, actually maximum energy, but that's a bit of a complication!) of the liberated electrons is constant. As you increase the amplitude, what increases (and increases continuously), is the RATE at which electrons are ejected (number per second). Note how this response evades your implicit question, then burbles on about about how the RATE of electrons is increased with the amplitude of the light. In fact she (and you) has already stated above that the kinetic energy is increased with increasing frequency. She gets so confused! As I suspected, you couldn't even get the basics right, so really had no idea WHY photons HAD to be introduced, what problem they were introduced to solve, or what exact properties they have which did solve the problem......................... Ah the expected autistic response! Its a defensive mechanism you know!
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Post by carnyx on Jan 14, 2011 15:41:40 GMT 1
@sta
Thank you for your responses. Here is a set of axioms which represent my understanding, so far;
1. Light above a certain frequency will shake off electrons from a given metal surface.
2. Increasing the amplitude of the light will cause an increased flow of electrons.
3. At this stage, increasing the frequency of the light reduces the flow of electrons, but increases the energy of these electrons.
Now, from my own experience of things, it seems that this set of statements could apply to systems that have multiple and cross-coupled resonance modes. (e.g. analogic apparatus such as springs and weights, pendulums, organ pipes, parallel tuned circuits, and so on )
The simulation model could be visualised as a matrix of multiple second-order differential equations simulating Newton's laws of motion, with the 'electron behaviour' being simulated via reduction of energy caused by peak stress detection and consequent ejection of momentum, as pulses of varying amplitude and rate.
As this is obviously not a Nobel-winning insight, I clearly need more axioms to constrain our understanding.
One clue is in your statement;
I wonder if you could expand on this a little more ?
(and at the risk of starting another hare, what is interesting is that the observed 'lumps' of energy appear in the output (... electrons), and not in the input (....light). In other words, the quanta do not seem to exist prior to the interaction. It follows that whilst the input could be defined as consisting of 'potential quanta' ..aka 'photons' .. they do not actually exist in the EM domain)
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Post by speakertoanimals on Jan 14, 2011 16:06:21 GMT 1
Nope. I think you're still missing the point that the kinetic energy of the ejected electrons is FIXED with regard to frequency (and just gives a smooth curve when plotted against frequency), and is totally independant of amplitude.
Many people did try to build semi-classical models of the photoelectric effect.
The use of analogies here -- what is the point of that, really? What we are really interested in is the actual physics of the interaction of radiation with matter, NOT a model of it.
I think you need to go back to the books -- when Planck first introduced the photon for blackbody radiation, he viewed it as some sort of calculational device, not a fundamental statement about the nature of radiation. However, given the photoelectric effect (quantum nature of absorption), ideas changed, but still only gradually.
You have to remember that physicists themselves were very reluctant to accept the full quantum hypothesis, they in effect had to be pushed into it, kicking and screaming, because NOTHING ELSE that they tried could explain the actual results of various types of experiments. And they tried damn hard!
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Post by speakertoanimals on Jan 14, 2011 16:13:51 GMT 1
Here is another point - you might come up with some complicated model that 'explains' why in the photoelectric effect, energy seems to eb absorbed in chunks.
BUT, I bet you couldn';t come up with a reason as to why the number that determines the relation between frequency and the energy of a chunk (Plancks constant), is the SAME for very different metals (even though they have very different key frequencies needed to get ANY electrons), or why the SAME number comes up when we consider the problem of blackbody radiation, which is to do with emission of radiation within a cavity, and nothing to do with ejecting electrons or metals.
Why is Plancks constant a constant if the chunkiness of radiation is some sort of second-order effect caused by something else classical going on? Why does the SAME number occur for all metals, and for the very different physics of the photoelectric effect and cavity radiation?
The point being, the only thing that was common was em radiation, hence the very constancy of Plancks constant seems to be telling us its a fundamental property of the radiation itself.
And when the SAME number turns out to relate the momentum and wavelength of an electron (wavelength as measured by the diffraction of electrons), then we have to admit that Plancks constant is even more fundamental than that!
Plancks constant relates the properties of a particle (mass/energy/momentum) to the wavelike properties of the SAME particle (wavelength). Not just em radiation and photons, but the wave-behaviour of massive particles (and not just electrons). It hence explains the photoelectric effect, the exact spectrum of cavity radiation, as well as the diffraction of particles such as electrons, and what happens when photons (X-rays, gamma rays) bounce off charged particles such as electrons (Compton scattering).
That's the key thing really -- a single hypothesis (waves AND particles), a simple relation between the particle-like properties and the wave-like properties, which then expains a whole range of seemingly disparate phenomena.
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Post by carnyx on Jan 14, 2011 16:57:15 GMT 1
And yet ...
Thanks for pointing out that Planck himself introduced the notion of the 'photon' as a computational convenience .......
But can you answer the question of whether EM radiation itself is quantised, or is it quantised only by reference to the outputs of subsequent conversion processes? If so, then Bohr's atom model of valency shells will provide a good reason as to why the quantisation occurs ... and why it is explainable via newtonian mechanical behaviour of the electrons.
And you asked an interesting question as to use of analogies. Had it not struck you that the very act of thinking is via analogues, abstracts, models .. representations... and not the blunt physical actualité?
And that such thought processes themselves induce other parallel processes of linkage and association ... in other words creativity, metaphysics, and is basically what makes homo so bloody sapiens?
The problem of QM as I point out, is that as it deals with fundamental units, it also encompasses the law of the excluded middle, and so is not a fruitful source of analogy; or metaphor if you like. A lego-world of quantum bits that are 'there' or 'not-there' ... is sterile, in reality.
And so Physics, sadly, has become less and less culturally relevant. A glance at the appalling state of Climate Science .. a branch of Physics after all .. shows how much moral authority has been lost, apart from considerations of 'truth'.
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Post by speakertoanimals on Jan 14, 2011 17:14:23 GMT 1
That isn't at all significant! Given the problems, that might have been what anyone else would have done, especially since at the time there would have been no actual REASON for thinking it was the case. So, if ALL techniques you had tried treating radiation as continuous had failed, then obvious to ask -- okay, just for the sake of argument, lets pretend its not, and see what answer we get then.............
when that worked, the next question is than -- is this somehow just a lucky accident, or is it the real physics?
The fact that the SAMe hypothesis works for many different scenarios is the answer to that one...........
The first sentence makes no sense!
Bohrs model doesn't explain why quantisation occurs, it ASSUMES quantisation occurs, in that the difference between energy levels is emitted as ONE photon, and Planck gives you the wavelength of that photon.
And what is used in Bohrs model is NOT the newtonian behaviour of electrons, but the behaviour of atroms according to quantum theory (which goes beyond the initial quantum hypothesis of particle/wave and instead explains exactly what 'standing-wave' states of the electron exist within an atom.).
O, let's not get into this same daft analogies game yet again!
Plus if you claim thinking is via analogs, what our are basic thoughts ananlogs OF, there has to be some basic thought-structure there below analog, else what do we build the first analog on?
Fruitful source of analogy? WHY should anyone bloody care? It EXPLAINS a wide-range of known phenomena, what more do you want?
Which is nothing to do with whether or not quantum theory is a good scientific theory....................
Sterile? It explains (almost all) physical phenomena. not bad. Good enough for any scientist.
I think you still have almost no bloody idea as to what quantum physics is, and what is says despite all my efforts, and seem to wantt o criticise quantum theory on some weird philsophical/political basis, rather than as physics.
I have no idea where you are coming from, to be frank (apart from somewhere with very little knowledge of actual physics), seem to have a very weird idea of what science is FOR (as opposed to what misguided use others may make of science), and what science is.
And I don't think I care either. I just try to keep pointing out where you have got your physics plain wrong, and will try not to worry about what misguided musings you choose to indulge in along with that............................
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Post by carnyx on Jan 15, 2011 23:23:38 GMT 1
@sta,
Oh dear, another loss of temper ....
I'll try again:
With regard to the photoelectric effect, does this show that the EM radiation itself is quantised, or is it an assumption based on the observation that the flow of emitted electrons show the characteristics of quantisation?
In other words, is the assumption of the quantisation of light itself based on a kind of post-hoc argument?
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Post by speakertoanimals on Jan 17, 2011 17:46:33 GMT 1
The FACT that electrons are quantised in that they are particles is no surprise.
But the ENERGY of the electrons ISN'T quantised, they can have various kinetic energies once they have escaped, from zero to a maximum determined by the wavelength of the light.
The essential point is as I detailed it earlier -- and the observations can only be explained under the assumption that the electrons (in fact, the whole metal system of which the electrons are a part) can only ABSORB the em energy in discrete chunks (the quanta).
This on its own would just say that em absorption is quantised, but black-body radiation shows that photon emission is also quantised. Then the Compton scattering shows that a photon bouncing off an electron is also quantised. Hence we end up with em field just being quantised.
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Post by speakertoanimals on Jan 17, 2011 18:30:29 GMT 1
I should just add that no single experiment could show directly that em radiation is quantised.
But what is already clear is that the behaviour in the photoelectric effect is certainly at variance with the idea of continuous em waves.
another thing I failed to mention is that IF radiation was continuous, then you would expect a slight time-delay for emission, as the electrons gradually soak-up the em energy. This was not seen.
The use of photons in the blackbody spectrum COULD be seen as a mathematical tricl, required to get the correct answer. But when the photoelectric effect is considered as well, now we have a different situation where the SAME quantisation hypothesis is needed to get the correct answer.
We don't need to PROVE quantisation (physics doesn't deal with proof, that is for mathematicians!), but the fact remains that the wave hypothesis DOESN'T explain blackbody and photoelectric effect, whereas the photom hypothesis DOES. Hence in physics terms, the photon hypothesis is a better explanation than the wave hypothesis.
Of course, what became clear was in the full quantum theory, em radiation was in some sense neither particles nor waves, but either (or neither) depending on the circumstances of the experiment. Just as electrons came to be seen as particle-like in many cases, but also as wave-like in other circumstances (like electron diffraction).
Hence the particle or wave question (which was initially answered as being particle for electrons, waves for em radiation), was instead replaced by -- you were asking the wrong question, and as far as we can judge, everything has both a particle-like AND a wave-like nature. how exactly those fit together is the essence of a full quantum theory, which is a more complicated thing that just the simple quantum hypothesis (em radiation emitted/absorbed as photons of a given energy and momentum, which depend on the frequency).
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