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Atoms and the electron

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bdewoody

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When I was in school they taught us that an atom of any given element has an equal number of protons and electrons and that the electrons orbit the nucleus at various levels creating a spherical shell of sorts. Recently I heard on a Science Channel program that the electrons pop in and out of existance and don't necessarily orbit the nucleus in the traditional sense. Can anyone here expand on this?
 
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MeteorWayne

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Well, first, the shells are not spherical.

http://en.wikipedia.org/wiki/Electron_configuration

Second, yes at the quantum level, strange stuff can happen, including electrons tunneling throught the nucleus (just read that today). The orbitals are just the location of the highest likelyhood at any given time (quantumly speaking).
MW
 
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bdewoody

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Well I did say of a sort meaning that I knew the shell was not technically spherical. I barely understood the commonly held atomic theory when I was in school, I guess that's one reason why I didn't pursue a career in physics but now with quantum physics I'm totally baffled by it all. I do recall that like 99% of an atom is open space so I really ponder why things appear solid. And I always had a picture of electricity being the movement of electrons along a wire.
 
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Mee_n_Mac

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bdewoody":12zjv9v3 said:
Well I did say of a sort meaning that I knew the shell was not technically spherical. I barely understood the commonly held atomic theory when I was in school, I guess that's one reason why I didn't pursue a career in physics but now with quantum physics I'm totally baffled by it all. I do recall that like 99% of an atom is open space so I really ponder why things appear solid. And I always had a picture of electricity being the movement of electrons along a wire.
Last things first ... electricity is the movement of charges, electrons, through space or a conductor. In this sense you can model an electron as a tiny little ball of charge. So your understanding is still good ... so far as I know.

As to empty space and things being solid ... what do you mean by "appear" ? If it's the pasage of light through the material ... well some things absorb the photons as they impinge onto the electron shell and reradiate this as EM below our ability to see. They appear solid since the light doesn't pass through or get reradiated in our visible passband.

If by solid you mean I can put a cup on a table and it doesn't fall through ... well the outer electron shell(s) of the cups atoms gets close to the outer shell(s) of the tables and it's simple electrostatic repulsion as the electrons carry the same - charge(s). It's analagous to trying to push the same ends of 2 magnets (ie : north - north) together.

As for the Bohr model of an atom ... it's what I learned waaaaay back in prehistory but it's been superceeded. Short of doing the QM math, I don't have a good narrative, that makes "commonsense", to describe electrons about the nucleus.
 
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csmyth3025

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bdewoody":2xyhwhel said:
Well I did say of a sort meaning that I knew the shell was not technically spherical. I barely understood the commonly held atomic theory when I was in school, I guess that's one reason why I didn't pursue a career in physics but now with quantum physics I'm totally baffled by it all. I do recall that like 99% of an atom is open space so I really ponder why things appear solid.
The video in the link below may help:
http://www.teachersdomain.org/asset/phy03_vid_atoms/

Chris
 
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emperor_of_localgroup

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As for the Bohr model of an atom ... it's what I learned waaaaay back in prehistory but it's been superceeded. -MAC
Oh really?
How would you explain mechanism for laser light or even ordinary light then? Bohr's model fits perfectly for those.

In my totally unproven opinion, eletron is not a particle but a fundamental charge. The mass of electron is the minimum mass that can hold a charge (or we can say mass of a charge). A neutron may be a proton with a captured electron. Eventhough combined mass of electron and proton do not match the mass of neutron at this moment, someday we may find the mismatch can be attributed to the binding energy of electron and proton.

Erratic behavior of electrons around a nucleus may be caused by non-uniform electromagnetic field around a nucleus. If anyone claims 'electrons can go through a nucleus', the scenario may be different than as it appears. An orbiting electron may be captured by a proton and then emitting an electron by a neutron, all in the nucleus.

Let's not forget:

p + e[super]-[/super]--> n + neutrino stuff
n --> e[super]-[/super] + p + neutrino stuff


I know, my theory has lots of holes somewhere.
 
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darkmatter4brains

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Even the description of the electron as a point particle more or less breaks down on distance scales smaller than the compton length.

Because of the Uncertainty Principle, the tighter you constrain position, the larger the momentum uncertainty and, as an effect, the larger the energy uncertainty.

When the distance at which you try to pin down the electron size shrinks below the compton length, the energy uncertainty will be twice as large as the rest mass of the electron, which make it possible for other particles to pop into existence.

So, if you try to pin down an electron to determine whether it is a point particle, you'll suddenly end up with three, or possibly more particles, and your little experiment will fall apart.

Also, in the view of quantum field theory, what we normally consider an electron, can really be viewed as superposition of many particles. The single electron state holds the largest amplitude.

And, if you want to get really weird look at the Unruh Effect for describing the mechanism behind Black Hole Evaporation.

Lots of weird stuff in Quantum Theory ....
 
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darkmatter4brains

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MeteorWayne":1k5g2rwn said:
Well, first, the shells are not spherical.

http://en.wikipedia.org/wiki/Electron_configuration

Second, yes at the quantum level, strange stuff can happen, including electrons tunneling throught the nucleus (just read that today). The orbitals are just the location of the highest likelyhood at any given time (quantumly speaking).
MW
Just a nit pick. The orbitals are related to discrete energy levels - the only ones allowed for a given atom. The position of the electron, while in a certain oribital/energy state, is represented as a fuzzy probablistic shell. But, the ground state of hydrogen, for example, will always be -13.6 eV.

(Actually, there are even caveats with the energy, due to the energy-time uncertainty relation - but, that's opening up another whole can of worms!)
 
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MeteorWayne

Guest
This is progressing beyond the realm of Science 101, so is moved to Physics.
 
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emperor_of_localgroup

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darkmatter4brains":54g4yifz said:
Even the description of the electron as a point particle more or less breaks down on distance scales smaller than the compton length.

Lots of weird stuff in Quantum Theory ....
I understand electrons do weird things on quantum level.
What surprises and amazes me is the consistency of electric current. If the basic definition of electric current is correct, we may then come to a conclusion, like photons, free electrons act like particles and electrons bound to atoms act like waves or are better described by wave functions. IMO, huge number of free electrons and their motions in a conducting wire can not be precisely represented by wavefunctions.
 
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darkmatter4brains

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emperor_of_localgroup":2ydrn522 said:
darkmatter4brains":2ydrn522 said:
Even the description of the electron as a point particle more or less breaks down on distance scales smaller than the compton length.

Lots of weird stuff in Quantum Theory ....
I understand electrons do weird things on quantum level.
What surprises and amazes me is the consistency of electric current. If the basic definition of electric current is correct, we may then come to a conclusion, like photons, free electrons act like particles and electrons bound to atoms act like waves or are better described by wave functions. IMO, huge number of free electrons and their motions in a conducting wire can not be precisely represented by wavefunctions.
Well, many things that are quantized do look continous at a macroscopic level. Thanks to the very, very small magnitude of the fundamental constant of quantum mechanics (hbar), most discrete levels are extremely tiny. So, many things that are quantized and discrete, look virtually continuous at the macroscopic level.

Also, a current is (very roughly) like water in a garden hose. Picture a hose that is full of water. When you turn on the spicket, you almost instantly have water popping out one end. That's not because the water molecules at the spicket end instantly traveled all the way through the hose, because they didn't. But, you can very roughly picture one water molecule bumping into the next, which in turn bumps into the next, all the way down the line, until the one near the end gets pushed out. Same with current. Most of the individual electrons are taking some erratic path through the wire, and hardly getting anywhere. Check out the drift speed section (a somewhat bad description, but it gets the point across) on this Wiki page:

http://en.wikipedia.org/wiki/Electric_current

Also, within quantum mechanics the wave function description is always valid. But the description of wave-like nature in Quantum Mechanics is different than what we're used to from Classical Mechanics. In quantum mechanics we're dealing with a probability wave, which isn't really a physical wave, like standing waves on a string are, or a sound wave. (Although, there is still some debate on this) The square of the amplitude of the wave function gives the probabilities of where you are likely to find the particle (or the value of some other observable, like spin) upon measurement.

Let's not forget about wave/particle duality, either. All quantum phenomenon can act like either a particle or a wave, and most often as both, depending on what's going on. You can also picture a wave function shaped like a delta dunction, which moves around upon time evolution, and that's basically a description of a what we normally picture for a classical particle, at least from the perspective of position.

Quantum Mechanics of course is only valid in it's domain. It also doesn't allow the creation/annihiliation of particles. Quantum Field Theory takes over when relativistic effects are important, but is once again only valid to certain energy levels. At high energies we need a new theory, and String Theory looks like the most likely candidate. But, one thing these all share, is a quantum description of the world, which does not have the deterministic descriptions we're used to in Classical Mechanics. Determinism is gone for good, I think.
 
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Jimmyboy

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darkmatter4brains":1dy1hzab said:
Also, a current is (very roughly) like water in a garden hose. Picture a hose that is full of water. When you turn on the spicket, you almost instantly have water popping out one end. That's not because the water molecules at the spicket end instantly traveled all the way through the hose, because they didn't. But, you can very roughly picture one water molecule bumping into the next, which in turn bumps into the next, all the way down the line, until the one near the end gets pushed out. Same with current. Most of the individual electrons are taking some erratic path through the wire, and hardly getting anywhere. Check out the drift speed section (a somewhat bad description, but it gets the point across) on this Wiki page:
From what I have learnt this is the true deffinition of current:

The ampere is defined as that current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in a vaumuum, would produce between these conductors a force of 2 x 10-7 newtons per metre of length. Attractive if currents in same direction, repel if currents in opposite direction.
 
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drwayne

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Moderator Note:

Please don't copy and paste a spam link, depending on how things are spaced, it may get missed and allow
the spam to live on after the spammer and the original spam has been removed.

Thanks!

drwayne
 
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darkmatter4brains

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Jimmyboy":2o9h7vp0 said:
darkmatter4brains":2o9h7vp0 said:
Also, a current is (very roughly) like water in a garden hose. Picture a hose that is full of water. When you turn on the spicket, you almost instantly have water popping out one end. That's not because the water molecules at the spicket end instantly traveled all the way through the hose, because they didn't. But, you can very roughly picture one water molecule bumping into the next, which in turn bumps into the next, all the way down the line, until the one near the end gets pushed out. Same with current. Most of the individual electrons are taking some erratic path through the wire, and hardly getting anywhere. Check out the drift speed section (a somewhat bad description, but it gets the point across) on this Wiki page:
From what I have learnt this is the true deffinition of current:

The ampere is defined as that current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in a vaumuum, would produce between these conductors a force of 2 x 10-7 newtons per metre of length. Attractive if currents in same direction, repel if currents in opposite direction.
That's the definition of an Ampere, which is a SI unit of measure for currents. It may also be changed:

"Rather than a definition in terms of the force between two current-carrying wires, it has been proposed to define the ampere in terms of the rate of flow of elementary charges.[13] Since a coulomb is approximately equal to 6.24150948×1018 elementary charges, one ampere is approximately equivalent to 6.24150948×1018 elementary charges, such as electrons, moving past a boundary in one second. The proposed change would define 1 A as being the current in the direction of flow of a particular number of elementary charges per second. In 2005, the International Committee for Weights and Measures (CIPM) agreed to study the proposed change, and, depending on the outcome of experiments over the next few years, to formally propose the change at the 24th General Conference on Weights and Measures (CGPM) in 2011.[14]"

I was trying to give a physical description of the movementds of the individual charge carrying paricles that produce a current
 
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Jimmyboy

Guest
I suppose it all depends on what one uses the definition for, and the uses for it. Im an electrical engineer so the def i usesd would be more useful in my line of work such as electric motors/generators and the like..
 
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darkmatter4brains

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Jimmyboy":27hnva0l said:
I suppose it all depends on what one uses the definition for, and the uses for it. Im an electrical engineer so the def i usesd would be more useful in my line of work such as electric motors/generators and the like..
Hey Jimmy, that's fine and all. I wasn't trying trying to define a current, or an Amp, or what have you. I was just giving an illustration to show what the motion of individual particles are like that constitute a current. In the OP, it sounded like it was described as free electrons hurtling freely through a wire. When it's not like that at all.

It's like in Serway's classic general physics text, he calculates the drift speed of electrons in a particular copper wire. The answer is: 2.46 * 10^-4 m/s. The individual electrons are hardly progessing down the wire at all. It would take 68 mins to travel 1 m at that speed!

So, that's not a free electron. However, in a conductor, the electric field that drives the current travels through the wire close to the speed of light. So, when the on switch is flipped the message for electrons to start moving throughout the wire, is close to instantaneous. So, although it takes a LONG time for an indivual electron to get anywhere within the wire, a current is set up all along the wire very quickly. That current is a result of the "average" movement of all those erratic slow paths followed by the electrons.

Now, for as how strong that current is, now we can define the Ampere, etc, to quantify the strength of the current.

Hopefully that all makes sense. See Serway page 274, if not ;)
 
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