Qualities of electrons

[FREEFALL1984]

So, I'm a bit of a noob to physics, I enjoyed it in school but never really followed it up in my adult life and particle physics was never on my schools curriculum. I'm starting to get interested in it once again, and was wondering about the properties of electrons.

Firstly if I fire an electron at a surface and it is absorbed by the surface, does it impart a force in that surface. Secondly does the act of firing the electron impart a force on the electron gun, I would assume the answers to be yes since electrons have mass like any other particle but would like confirmation.

Also is there a material which can reflect electrons fired from an electron gun, and what happens if I fire electrons at a superconductor.

Also what puzzles me is the phenomenon known as the photoelectric effect. Meaning photons hitting a surface cause the surface to release electrons, once the surface has lost too many electrons does it simple stop releasing them or do the properties of the material change as the material would have to shed protons to balance itself. also is there a way to force he electrons and protons back into the surface to reverse the process. Also a remedial question I know, but could an electrical charge help to prevent the material shedding protons and maintain its composition, if indeed it would shed them in the first place.

If you guys could help me I would much appreciate it, I'm eager to become learned in the witchcraft that is particle physics....

[Z-Man]

Firstly if I fire an electron at a surface and it is absorbed by the surface, does it impart a force in that surface. Secondly does the act of firing the electron impart a force on the electron gun, I would assume the answers to be yes since electrons have mass like any other particle but would like confirmation.

Yes and yes. Your assumption is correct: Electrons have mass and therefore momentum, and to conserve that, the gun and surface have to provide/absorb it.

Also is there a material which can reflect electrons fired from an electron gun

Like visible light on a mirror? A contained magnetic field perpendicular to the boundary, such as the one inside a very long coil, does that. The Lorentz force makes the electron turn around and for symmetry reasons the angle it leaves the field at is the same as the angle it enters at. You have to make the field stronger to reflect higher energy electrons, of course.

what happens if I fire electrons at a superconductor.

Nothing much different from when you fire it at a regular conductor: it gets absorbed, the object gains a small net charge and, if grounded, will get rid of another electron somehow to compensate. Superconductivity will not break down because of a single electron, or even a stream of electrons, and it won't do anything funny to the new electron either.

Also what puzzles me is the phenomenon known as the photoelectric effect. Meaning photons hitting a surface cause the surface to release electrons, once the surface has lost too many electrons does it simple stop releasing them or do the properties of the material change as the material would have to shed protons to balance itself. also is there a way to force he electrons and protons back into the surface to reverse the process. Also a remedial question I know, but could an electrical charge help to prevent the material shedding protons and maintain its composition, if indeed it would shed them in the first place.

As you suspect, the material charges up (positively) as it sheds electrons. If the electrons are not replaced (they usually are), after a long while, the field induced by that charge would stop further electrons from escaping the material. They would still be released, but quickly fall back.

Photons of the energies usually used to trigger the photoelectric effect are not strong enough to knock away protons.

[Idobox]

A way to reflect electrons is to have to parallel planes, one having holes, and a voltage between them. Most electrons will go through one hole, and if the voltage is the right polarity, slow down. If the voltage is high enough, the electron will not reach the second plane, but stop and accelerate backwars, leaving with roughly the same velocity it had (a little bit is lost to radiation).

Of course, some electrons will collide with the grid, leading to inefficiency and heating.

The photoelectric effect is quite interesting. Electrons have a binding energy, the energy you need to get tear them out from their atom. If your photons have more than this energy, they will realease electrons, and the remainder of the energy will be converted to kinetic energy of the electron.

If you shine a UV light on a perfectly insulated piece of metal, it will start shooting electrons in all directions, charging itself positively. As the charge increase, the electrostatic forces pulling on electrons will increase. Like With gravity, you have an escape velocity depending on charge. At first, the charge is tiny, so electrons can escape, but as time goes on, this limit velocity increase, until it exceeds initial velocity of electrons, which means they go away, slow down, and turn back. At this point, you have a gas of electrons around the piece of metal that is pretty much contained.

[FREEFALL1984]

A way to reflect electrons is to have to parallel planes, one having holes, and a voltage between them. Most electrons will go through one hole, and if the voltage is the right polarity, slow down. If the voltage is high enough, the electron will not reach the second plane, but stop and accelerate backwars, leaving with roughly the same velocity it had (a little bit is lost to radiation).

Of course, some electrons will collide with the grid, leading to inefficiency and heating.

If some of the kinetic energy is lost as radiation then would that not violate conservation of momentum?

[Z-Man]

No, radiation has momentum, too.

[FREEFALL1984]

Ah ok I just thought radiation was just considered energy and is therefor without mass or momentum.

[Seret]

Radiation is made up of particles like photons, so does have momentum.

[Kryten]

Ah ok I just thought radiation was just considered energy and is therefor without mass or momentum.

It's without mass, but momentum is dependent on energy content, not mass.

[FREEFALL1984]

Sorry I'm really confused. So let me say what I'm thinking and you guys can correct me where I'm wrong and let me know if and where I'm right.

Radiation is the transference of energy into subatomic particles with no mass. But the energy stored in these particles can only be measured by assessing the waveform, ie a high frequency radiation contains more energy, My layman's opinion is telling me due to the fact that a particle with no mass cannot contain "momentum" in the conventional kinetic energy sense the actual energy is not contained within the particle itself but in the waveform. So the momentum is in fact the momentum of this waveform, which then dissipates due to entropy. Also operating on my layman's logic this means that the universe must be always flooded with photons otherwise there would be no way for these waves to propagate. I know I'm probably miles away from the point here but does that not mean photons don't actually move? instead they just oscillate and propagate waves like water molecules in a pond.

[Ralathon]

Also operating on my layman's logic this means that the universe must be always flooded with photons otherwise there would be no way for these waves to propagate. I know I'm probably miles away from the point here but does that not mean photons don't actually move? instead they just oscillate and propagate waves like water molecules in a pond.

You're getting into particle-wave duality here, this is difficult stuff to grasp without a good idea of how either particles or waves work.

The way I suggest you visualize it is to imagine a photon as a wave packet:

The peaks and valleys are related the probability that you'll encounter a photon in that position. This entire packet moves through space at the speed of light.

Basically, as you add multiple waves of different frequencies you can shape your wave into a little packet as you see above. The more frequencies you add the less certain you can be about your frequency but the sharper you can make that peak. So yea, technically photons can be everywhere at once. Even a billion miles to the right of this packet the packet still won't be zero, it still has a very very small amplitude.

[N_las]

The wave itself is called photon. Its not like there is an ocean of photons, and an em-wave is a wave on that ocean.

[Seret]

I don't think you want to get too hung up on the "massless" thing. Photons might technically have no rest mass, but they're never at rest anyway, so they always have momentum. Since they're always trucking along at the speed of light they always have significant momentum, so for your purposes you can just think of them like an object with mass.

Some radiation is particles that genuinely do have mass btw. For example alpha particles which are neutrons and protons (basically a helium nucleus) and beta particles which are electrons.

[FREEFALL1984]

Just watched a few videos on this stuff on youtube and read your explanation and the whole prospect of probability waves baffles me

[Nuke]

the problem with shooting electrons out the back of your space ship is that your ship starts developing a positive charge, so the electrons that you shoot away tend to come back producing no net thrust. some types of electric engines also have this problem, but in reverse. positive ions fly out the back but that makes your ship negative, to keep the ions from coming back you usually have an electron gun next to the engine to fire electrons into the ion stream, so they meet up with the ions to neutralize them.

[lajoswinkler]

Ah ok I just thought radiation was just considered energy and is therefor without mass or momentum.

This is one of the things that, sadly, even popular science sources teach entirely wrong. "Light is pure energy", etc. Tons of crap almost everyone takes for granted.

Radiation is not energy. It contains energy. Energy is a property and can be quantified. It can not exist alone, by itself. It requires a carrier. Photons and protons can carry a certain amount of it, but unlike protons, photons behave differently. Photon disappears when it donates its energy to another system. Disappearing particles is something you should accept as a fact. Don't think about it.

[K^2]

Particles never disappear. Quantum Mechanics just doesn't work that way. There are a whole bunch of conserved quantities which have to keep going. You can have a neutrino encounter a W^- boson and continue on as an electron. Or a photon splitting off into a positron and an electron. But none of them just disappear. That's a very misleading thing to say.

[lajoswinkler]

Particles never disappear. Quantum Mechanics just doesn't work that way. There are a whole bunch of conserved quantities which have to keep going. You can have a neutrino encounter a W^- boson and continue on as an electron. Or a photon splitting off into a positron and an electron. But none of them just disappear. That's a very misleading thing to say.

This is semantics. Particles do disappear. If I throw a ball into a window, and the ball turns into mushroom, and the window breaks into several roses, mushroom and roses will possess the qualities of the ball (momentum, just to name one), but it isn't a ball anymore. The ball, as a round, rubbery, bouncy object is gone. QM works exactly that way.

[K^2]

Fine. Can you tell me the exact time at which a particle stopped existing? No? How about an exact time after which you are certain it no longer exists. Also no? Ok, how about this. We have a positively charged pion. Its up quark can annihilate with the d-bar to produce a W⁺ which then interacts with an anti-neutrino it pulled out of vacuum to make a positron. That's pretty straight forward. But there are also a bunch of quark and anti-quark fluctuations in vacuum. So say, instead of the valence d-bar, the up quark pulled a d-bar from vacuum, leaving an extra down quark to continue on with a valence d-bar. So it still spits out a positron, but now it continues on as a neutral pion until that decays. This is absolutely equivalent to a process where the up quark simply decides to spit out the W⁺, and pion becomes neutral. All quarks of the same flavor are indistinguishable, after all. So did the original pion decay, leaving a neutral pion as a product, or did the pion simply let go of its charge? It's a Ship of Theseus question certainly, and you'll say, it's because pion is a composite particle. But a photon behaves exactly the same way. It propagates as a an electron-positron pair. And the electrons/positrons keep getting swapped out with ones from vacuum constantly. So at which point, exactly, should the photon stop existing? Or did it even exist to begin with, under your definitions?

[Seret]

K^2, I think you're tilting at windmills here. The OP clearly doesn't operate at the level of physics knowledge that requires making that fine-grained level of distinction.

[lajoswinkler]

Fine. Can you tell me the exact time at which a particle stopped existing? No? How about an exact time after which you are certain it no longer exists. Also no? Ok, how about this. We have a positively charged pion. Its up quark can annihilate with the d-bar to produce a W⁺ which then interacts with an anti-neutrino it pulled out of vacuum to make a positron. That's pretty straight forward. But there are also a bunch of quark and anti-quark fluctuations in vacuum. So say, instead of the valence d-bar, the up quark pulled a d-bar from vacuum, leaving an extra down quark to continue on with a valence d-bar. So it still spits out a positron, but now it continues on as a neutral pion until that decays. This is absolutely equivalent to a process where the up quark simply decides to spit out the W⁺, and pion becomes neutral. All quarks of the same flavor are indistinguishable, after all. So did the original pion decay, leaving a neutral pion as a product, or did the pion simply let go of its charge? It's a Ship of Theseus question certainly, and you'll say, it's because pion is a composite particle. But a photon behaves exactly the same way. It propagates as a an electron-positron pair. And the electrons/positrons keep getting swapped out with ones from vacuum constantly. So at which point, exactly, should the photon stop existing? Or did it even exist to begin with, under your definitions?

I can't define the moment it stopped existing, but I can say I can't detect it anymore, can't I? Instead, I have a bunch of other particles.

Let's say that two protons collide at extreme velocities and we get a burst of exotic particles. Where are those protons? They're gone. Nothing is lost except the concept. If you tear down a building, the building is gone. You have a rubble. Mass is conserved (let's ignore the infinitesimally small change), energy is conserved, but the building ceases to exist. It boils down to semantics, really.

[K^2]

I can't define the moment it stopped existing, but I can say I can't detect it anymore, can't I? Instead, I have a bunch of other particles.

Let's say that two protons collide at extreme velocities and we get a burst of exotic particles. Where are those protons? They're gone. Nothing is lost except the concept. If you tear down a building, the building is gone. You have a rubble. Mass is conserved (let's ignore the infinitesimally small change), energy is conserved, but the building ceases to exist. It boils down to semantics, really.

You do understand the concept of indistinguishable particles, don't you? As soon as you explain to me how you want to tell the difference between not being able to detect a particle because it stopped existing and between being unable to detect it because it tunneled out of the ion trap, we can continue discussing this particular line of reasoning.

And again, there is no such thing as two protons colliding and you being able to say that they were disintegrated. A collision between two protons can happen without a single valence quark being struck. With exactly the same shower produced. More importantly, that's always part of the process. Heck, the very distinction between valence and non-valence quarks gets really hazy at this point.

This is precisely because composition of the proton doesn't matter. A proton is 99% just vacuum fluctuations. What's important about the proton is that it has a +1 baryon number, +1 electric charge, spin 1/2, and whatever energy and momentum it happened to carry. That's what really makes a proton. And all of these numbers are conserved. They might end up being distributed among different excitations of various fields, but what exactly stops existing that existed in the first place?

[KSK]

Particles never disappear. Quantum Mechanics just doesn't work that way. There are a whole bunch of conserved quantities which have to keep going. You can have a neutrino encounter a W^- boson and continue on as an electron. Or a photon splitting off into a positron and an electron. But none of them just disappear. That's a very misleading thing to say.

OK, genuine question then. What happens when an atom absorbs a photon? My understanding was that the photon disappears and the atom is placed into some kind of excited state. It's either spinning faster (if it absorbs a microwave photon), vibrating faster (infra-red photon), etc. etc. Energy is conserved, other quantum numbers are conserved but the photon has gone.

Various processes can happen to that excited atom and some of them involve emitting another photon but the original photon has gone.

I'm approaching this as a chemist rather than a physicist so it's quite possible that I'm missing something.

[lajoswinkler]

You do understand the concept of indistinguishable particles, don't you? As soon as you explain to me how you want to tell the difference between not being able to detect a particle because it stopped existing and between being unable to detect it because it tunneled out of the ion trap, we can continue discussing this particular line of reasoning.

And again, there is no such thing as two protons colliding and you being able to say that they were disintegrated. A collision between two protons can happen without a single valence quark being struck. With exactly the same shower produced. More importantly, that's always part of the process. Heck, the very distinction between valence and non-valence quarks gets really hazy at this point.

This is precisely because composition of the proton doesn't matter. A proton is 99% just vacuum fluctuations. What's important about the proton is that it has a +1 baryon number, +1 electric charge, spin 1/2, and whatever energy and momentum it happened to carry. That's what really makes a proton. And all of these numbers are conserved. They might end up being distributed among different excitations of various fields, but what exactly stops existing that existed in the first place?

I do understant that concept, of course, but I don't understand how you're defending something completely opposite from what my professors of physics were talking to me.

Where are those particles if you can't detect them anymore? Do you have one evidence to support your claim that they're "somewhere"? You've written a lot of words, yet none of them explained this.

If a photon is ejected from a system, from our reference frame it travels for some time until it hits the target, where it disappears, leaving the target in an excited state. From its reference point, zero time has passed. In every literature I've read it was mentioned that photons appear and disappear, leaving excited states of the target systems they hit.

If you're saying that we can't say they disappear because we have no proof of that, that would be fundamentally true, but that's on the order of saying we can't say all Al-27 atoms in cosmos have the same chemical properties because we haven't tested all of them. For all intents and purposes, we can consider them disappearing.

[Dodgey]

So much knowledge. Lajoswinkler I think you are using disappear the wrong way. A more accurate statement would be that it breaks down. Just a layman here but I think that would be more correct.

[Beowolf]

I do understant that concept, of course, but I don't understand how you're defending something completely opposite from what my professors of physics were talking to me.

I had a similar conversation with my son recently. He's in grad school specializing in quarks and the force holding them together. He said his professors now consider the whole concept of "particle" to be obsolete and merely an illusion. They don't even think in quarks. They see the universe as probability fields, each representing a single property, bound by forces represented by other probability fields.

This changed because those particles aren't nearly as permanent as we thought. It isn't just electrons that move around, what you and I were taught were individual properties of a particle can move around independently too! So the photon that left Andromeda isn't necessarily the same one that hit my retina. At times, due to vacuum fluctuations, there wasn't a photon at all. Those properties represented by the photon mixed with something the vacuum did to form an entirely different "particle" for a while.

They've even "split" an electron in a lab! Working near absolute zero, they somehow separated it into an orbiton, a holon, and a spinon, each containing a single property.

That's the perspective K^2 uses, if I understand him/her correctly. Wonder if he knows my son.