Talk:Photoelectric effect
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The point is whether we should define the photoelectric as "flow of electric current in a material when it is exposed to light" or a "emission of electrons from a material when it is exposed to light". I think the latter is preferable for the following reasons:
- The historical argument. I think that the first demonstration of the photoelectric effect was the observation of discharges (due to emission of electrons) between electrodes induced by ultra-violet light by Heinrich Hertz.
- A current will flow in a metal when exposed to ultra-violet light, but that is a secondary effect due to the holes left behind in the photoemission process. One can also knock electrons out of free atoms by photons, and in this case it does not make much sence to speak of current flowing.
- Photoemission spectroscopy, the detection of electrons from a solid (or liquid) with respect to kinetic energy and emission angle, is an important technique in solid state physics.
What do you think?
- I agree with the latter. A current is caused by energetic electrons, and is not always present in a demonstration of the effect. -Twinxor
As a reader of an encyclopedia, I would like to see reference to application of the scientific discoveries. Did this discovery brought us technologies that we used today or in the past? How did it change our life? Are solar power, digital cameras etc. based on this effect?
Please consider adding an "Applications" section to all entries that are related to science and technology.
My impression was that the current-flow effect was discovered first; if I am mistaken about that, and the first observations were simple discharges, then that might be a better starting place. I just wanted to make clear the distinction between Einstein and others' explanation of what causes the effect from the simple observation of the effect itself. I agree that mentioning applications would be good. Probably the earliest and best known was talking pictures: the soundtrack on a piece of movie film works by shining light through that portion film as it moves, stimulating an electrical current attached to sound amplifiers. --Lee Daniel Crocker
That's not the best example. The first optical soundtrack system, Movietone, used a photocell, which is a light-dependent resistor and has nothing to do with the photoelectric effect, and I believe that modern analogue optical soundtracks still use photocells. I think this is because photocells are more linear than photodiodes. Digital optical soundtracks, such as the Dolby digital system, are a different matter: they probably use photodiodes, because linearity is not necessary in a digital system. -- Heron 10:34, 10 Jul 2004 (UTC)
- Oops. I have just checked my facts, and found that my comment above was wrong. It seems that photocells and photodiodes both use the photoelectric effect. -- Heron 10:39, 10 Jul 2004 (UTC)
The emission of electrons is a more precise statement and should therefore replace "current flow." In fact, "current flow" brings to mind electrons (or charge) flowing within the material, but in the photoelectric effect the electrons are ejected and completely escape the material. --Carlos M.
"Emission of electrons" is clearly wrong, precisely because it is more accurate! Again, let me be clear: physicists (probably Hertz, but others might have noticed it earlier) observed some effect long before they knew that it was in fact the emission of electrons. It was later discovered what caused the effect, but the effect itself should be described as whatever the actual measurement or observation was that led us to figure it out; not what we currently understand as its cause--that's circular definition, and bad science. If what Hertz first measured was the discharge of a plate or a Leyden jar (as I now suspect), then define it that way. If what he first measured was a current flow (which I first assumed, perhaps erroneously), then say that. I'm not sure which it was, but I do know for sure that Hertz did not observe the emission of electrons. --LDC
- I see no a priori reason why the definition should be based on what led scientists to discover the effect.
- Such an approach is clearly anthrocentric, which some might not mind.
- Such a definition would mean that many phenomena would no longer be consider manifestations of the photoelectric effect. For example, there is no current caused in the levitation of moon dust.
- Such a convention would require rather severe changes in other terms. Do we want to define electric current as "the thing that happens in a frog's leg when placed on a moist tin dish and prodded with a lead knight"?
Phantym 19:27, 22 May 2005 (UTC)
Tesla / radiant energy
Tesla specifies that the exposed plate is insulated. It seems that the insulation should prevent light from hitting the metal plate directly. Or if transparent, the insulator should prevent electrons from being ejected. If so, how is this consistent with the photo-electric effect?
I have always been under the impression that static electricity actually drove Tesla's invention (triboelectrification between dust and the insulator). Alternatively, the device may be driven by photon induced ionization of the air. In either case, the capacitor may then be inductively charged.
Can anybody resolve this objection, or link to a careful discussion of Tesla's "radiant energy" device and the photoelectric effect?
- (William M. Connolley 19:30, 31 Aug 2004 (UTC)) I can't answer your point, but I have deleted the "Radiant energy" section, which appears to be dubious at best. If Tesla described/explained photoelectricity in 1901, as the section claims, he has great priority: Lenard's obs were not until 1902 [1] (http://www.phys.virginia.edu/classes/252/photoelectric_effect.html). So I don't believe this claim. At best, Tesla obtained a patent on a device using PE. He did not describe PE (important points being dep n freq not intensity) - or if he did, there is no evidence presented for this.
Connolley, read the damn patents! Specifically ...
- US685957 (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=685957.WKU.&OS=PN/685957&RS=PN/685957) - Apparatus for the Utilization of Radiant Energy
- US685958 (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=685958.WKU.&OS=PN/685958&RS=PN/685958) - Method of Utilizing of Radiant Energy
The tube (marked as S) is cited in the patents as a Lenard tube or Roentgen tube. As to describing the PE, a basic description is in the patent ... if you read it!
- (William M. Connolley 18:08, 20 Apr 2005 (UTC)) Tesla isn't in the std history of the photoelectirc effect. If you really think he belongs in the history, then he needs to be put in explicitly, not sneaked in via a "patents" section. And to do that, you need a proper source, because the patents don't seem explicit enough to me. Note that you have ignored my points above, viz: He did not describe PE (important points being dep n freq not intensity) - or if he did, there is no evidence presented for this.. I don't see anything in the patents addressing this - do you? I hope someone mopre competent will comment.
- It's IRRELEVANT if Tesla isn't in the std history of the photoelectirc effect. He has been left out of a great many things in which he should be credited for. But the main source of information is from his patents ... so this probably would be better place for him. It's not "sneaked in", just a statement of facts.
- The patents are pretty explicit, if you read them carefully (not just brush over them). Remember ... this (1901) is before much of the terminology is coin that we use today. The evidence is the 2 patents! NOTICE what the examiner wrote on the patent [it's in terms similar of which Thomson used].
- As to describing the PE (I think you are menting depletion and frequency [dep n freq )... this is in both ... let's take US685957, in the 1st page of the description on lines 11-34 he mentions "vibrations" of "small wavelengths" ... he then goes on to note that HIS observations call for a new theory that is being advance in the patent about "minute particles" "electricified" [eg. electrons] and being thrown off.
- If you can't comprehend this, you are not ignorant ... but an idiot
- (William M. Connolley 19:39, 20 Apr 2005 (UTC)) It is highly relevant that T is not in the std history, since wiki is a summary of the general state of knowledge, not of your personal research. The patents do not seem at all explicit to me, and I note that you have not actually quoted the text from them: please do so if you think it is relevant. Your third point remains off the point. For your fourth point, you are reminded of the "no personal attacks" rule.
- Wikipedia is a secondary source (smetimes a tertiary source) ... not just a summary of general knowledge ... eg., material that has generalization, analysis, synthesis, interpretation, or evaluation. This isn't my "primary" research ... but public knowledge from primary sources.
- Patents are explicit and I have cited lines in the patent! (eg., 1st page of the description on lines 11-34)
- I applogize for any "attack" ... but there are only so many conclusions to come to when facts are stated and then are ignored.
User:204.56.7.1 has put nonsense about Tesla discovering superconductivity on List of Tesla patents and Superconductivity. He seems impervious to facts. pstudier 20:45, 2005 Apr 20 (UTC)
- He added stuff to solar cell too. - Omegatron 20:51, Apr 20, 2005 (UTC)
Ok, I didn't really have an opinion before (except skepticism of hero worship), but I've been reading a lot and this does look like the photoelectric effect to me. Yes, it's original research, but I'm finding some cool things to include in various articles while I do it. :-)
"It is well known that certain radiations — such as those of ultra-violet light, cathodic, Roentgen rays [X-rays], or the like — possess the property of charging and discharging conductors of electricity, the discharge being particularly noticeable when the conductor upon which the rays impinge is negatively electrified." As long as the charging is positive and the discharging is of a negative object (as he seems to say in the second phrase), this is pretty much a statement of the photoelectric effect. However, his explanation for it ("that sources of such radiant energy throw off with great velocity minute particles of matter which are strongly electrified, and therefore capable of charging an electrical conductor") is wrong. He also says it is "well-known", though, so he isn't claiming to have discovered it; just demonstrating a way of capturing the energy.
A metal like aluminium has a work function of 4 eV. This means that to eject electrons from the surface through the photoelectric effect it needs to be hit by EM with a frequency above about 1 petahertz: math (http://www.google.com/search?&q=4+eV+%2F+planck%27s+constant+in+petahertz) This is equivalent to a wavelength less than 310 nanometers math (http://www.google.com/search?&q=speed+of+light+%2F+0.967195907+petahertz). This is in the range considered "near UV". So, through the photoelectric effect, UV and X-rays hitting a piece of metal will knock off electrons and give it a positive charge, as he said ("As the rays or supposed streams of matter generally convey a positive charge to the first condenser-terminal"). (But not visible light, so this has nothing to do with solar cells.)
As the charge gets bigger, though, the work function changes, so this only goes on until the charge is high enough that the electrons can't leave anymore. By connecting a large-value capacitor to the plate and the other side connected to a large charge reservoir (the earth), you can knock more electrons out of the plate than normal, because the capacitor "condenses" charge into a smaller space than the charge would normally fill by attracting it to opposite charges on the other plate. That's why they used to call them "condensors". So far, so good.
(Yes he does say something about the "receiver" being an "insulated plate or conducting-body", which made me think "coated in plastic" at first glance, but he also says "the surface should be clean and preferably highly polished or amalgamated", so I think he just means supported in a way that it is insulated from other objects, like you would say an insulated (http://www.thebakken.org/artifacts/database/artifact.asp?type=category&category=A6.2&id=3504) sphere (http://www.physics.mun.ca/~jjerrett/induction/induction.html) of metal has a charge on it.)
- Here is a sort of similar experiment: (fig. 3) (http://www.cas.muohio.edu/~marcumsd/p293/lab1/lab1.htm)
The one thing I'm not completely sure of yet is whether the photoelectric effect would create a net charge on a piece of metal in air.
- This (http://resources.yesican.yorku.ca/trek/radiation/final/assignment_em1.html) seems to indicate that yes, you can emit the electrons into the air (only if the metal is charged negatively in the first place?)
- This (http://acept.la.asu.edu/PiN/rdg/pixe/pixe.shtml) says "Absorption in air by X-rays is minimal and the arriving X-rays have their full energy. Electrons, on the other hand, lose energy and are scattered by air". Since their testing depends on catching those electrons, they do their tests in a vacuum. In this case, though, the scattering of the electrons doesn't matter.
- Tesla's (clever) metal foil switchy-thing discharger is specifically "inclosed in a receptacle, from which the air may be exhausted", which implies to me that the rest of the device is not. Hmmm...
But where do the electrons go? They just attach to the air and ionize it? Wouldn't they then drift back and stick to the metal and neutralize? Maybe it takes a while for them to drift back and neutralize, and you can discharge it into the earth before that happens (and then they will have to drift down to the earth to neutralize instead). Does a charged sphere eventually discharge by contact with the air? I'm used to thinking of circuits; not isolated charged objects. "The typical atmospheric molecule has an energy of about 0.03 eV." Aha! The electrons are tiny and also have >100 times as much energy. So maybe that is what happens. This paragraph should be considered much more speculative than the others, though. :-) I'm really out of my element with the air charge stuff. - Omegatron 02:37, May 21, 2005 (UTC)
Counter Intuitive?
The dual nature of the photoelectric effect, wave or particle, made it very hard to accept. Does this count as Counter-intuitive?
Tabletop 10:56, 31 Mar 2005 (UTC)
Moon dust
Are we sure that the moon dust gains its charge from the photoelectric effect and not from the charged particles streaming from the Sun? Enochlau 10:29, 21 Jun 2005 (UTC)