The Naked Scientists

The Naked Scientists Forum

Author Topic: Does conservation of angular momentum apply to electrons orbiting atoms?  (Read 9107 times)

Andrejs Skuja

  • Guest
Andrejs Skuja  asked the Naked Scientists:
   
Hi Chris,

I have a query I'd love answered if possible.

I was recently posed the question, 'Why do our solar system's planets sit on a flat plane'. I understand this is due to the conservation of angular momentum. Upon discovering this the first image to jump to mind was of an atom with it's electrons zipping around it in random trajectories not adhering to a single plane. Why does the law that binds our solar system seem not to apply to atoms, or is my view based on flawed cartoons from my childhood? (Up and at 'em, Atom Ant!)

Thanks very much,

Andrejs Skuja

What do you think?
« Last Edit: 24/11/2010 01:30:03 by _system »


 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Excellent question!  Electrons in atoms do have angular momentum.  The difference is their orbits are these fuzzy quantum clouds called orbitals rather than nice classical orbits like the planets.  These orbitals aren't random, and they do have different shapes which are specified by their energy and angular momentum.  Without going into details, there are some pictures here: http://en.wikipedia.org/wiki/Atomic_orbital#The_shapes_of_orbitals
 

Offline Bill S

  • Neilep Level Member
  • ******
  • Posts: 1802
  • Thanked: 11 times
    • View Profile
Isn't it also something to do with the idea that when not being observed the electron is a standing wave rather than an orbiting particle?
 

Offline maffsolo

  • Sr. Member
  • ****
  • Posts: 280
    • View Profile
Isn't it also something to do with the idea that when not being observed the electron is a standing wave rather than an orbiting particle?

Is it that a standing wave is the radiation energy resultant of the obiting partical?
Electrons do have a mass where a standing wave is radiation containing photons I think ?
Or is there a photon to partical swapping action here?
Is my thinking incorrect here?

Excellent question!  Electrons in atoms do have angular momentum.  The difference is their orbits are these fuzzy quantum clouds called orbitals rather than nice classical orbits like the planets.  These orbitals aren't random, and they do have different shapes which are specified by their energy and angular momentum.  Without going into details, there are some pictures here: http://en.wikipedia.org/wiki/Atomic_orbital#The_shapes_of_orbitals

From my minds eye, deriving from that site, am I correct to say, that the electrons are revolving around the nucleus on a plane and the plane is also revolving on an concentric axes, centrally pivoting on the nucleus?

====
Maybe that the size of the universe prohibits any reference point to permit use to see the solar system is also in a wabble of sorts? Maybe the duration of the wabble is to extreme
« Last Edit: 23/11/2010 15:48:34 by maffsolo »
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Isn't it also something to do with the idea that when not being observed the electron is a standing wave rather than an orbiting particle?

Yeah.  That's basically exactly what's happening, and waves can have angular momentum just like particles can.  Interestingly, in order for light to cause an electron to jump from one energy level to another, the photon that gets absorbed or emitted has to have the right angular momentum so that total angular momentum of the energy level + photon is conserved.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
From my minds eye, deriving from that site, am I correct to say, that the electrons are revolving around the nucleus on a plane and the plane is also revolving on an concentric axes, centrally pivoting on the nucleus?

No, that's not quite correct.  The electron is smeared out over the whole orbital shape at one instant.  Part of quantum mechanics says that you have to use this smeared-out model and that you  simply can't use the model of a particle zipping around an orbit.  Angular momentum determines the shape of the orbital, though.

Both the planet and the electron have angular momentum, but the electron behaves like a wave that's smeared out over the whole orbital at once, while the planet behaves like a mass that takes up only one point on the orbit at any time and moves around the orbit as time goes on.
 

Offline Bill S

  • Neilep Level Member
  • ******
  • Posts: 1802
  • Thanked: 11 times
    • View Profile
On the subject of an electron that jumps from one energy level to another, the thing I have difficulty getting my head round is that one cannot think of it as being anywhere while it is making the transition.  What happens to the angular momentum during the transition? 
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
I was recently posed the question, 'Why do our solar system's planets sit on a flat plane'. I understand this is due to the conservation of angular momentum. Upon discovering this the first image to jump to mind was of an atom with it's electrons zipping around it in random trajectories not adhering to a single plane. Why does the law that binds our solar system seem not to apply to atoms, or is my view based on flawed cartoons from my childhood? (Up and at 'em, Atom Ant!)
Conservation of angular momentum still holds, but microscopic objects cannot have a precise trajectory, or this would violate Heisenberg uncertainty principle; in the microscopic domain the exact position and velocity of a particle are replaced by an entire function of space x and time t, called "wavefunction" which square modulus gives you the probability per unit volume to find the particle in a region of space around x at the time t. You cannot know its position and velocity, and so its trajectory, better than this, and not because of instrumental inefficiency, but because of theoric reasons.
« Last Edit: 23/11/2010 19:59:07 by lightarrow »
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
On the subject of an electron that jumps from one energy level to another, the thing I have difficulty getting my head round is that one cannot think of it as being anywhere while it is making the transition.  What happens to the angular momentum during the transition? 

During the transition it's in both A and B.  I read this somewhere after someone asked how long the transition takes.  There's a finite but very short time it spends straddling the two states.  (If it instantly hopped from one to the other, it would move faster than the speed of light.)  If you try to measure its angular momentum during the transition, you'd find it would either be in state A with some probability or state B with some other probability.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8648
  • Thanked: 42 times
    • View Profile
On the subject of an electron that jumps from one energy level to another, the thing I have difficulty getting my head round is that one cannot think of it as being anywhere while it is making the transition.  What happens to the angular momentum during the transition? 
Angular momentum, in the form of "spin" is transferred to or from the photon.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
On the subject of an electron that jumps from one energy level to another, the thing I have difficulty getting my head round is that one cannot think of it as being anywhere while it is making the transition.  What happens to the angular momentum during the transition? 
Angular momentum, in the form of "spin" is transferred to or from the photon.

The photon can also have angular momentum on top of spin.  For example, certain beams tend to "rotate," which gives the photons angular momentum.
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3812
  • Thanked: 19 times
    • View Profile
If an antenna is receiving a circular polarised signal is a torque transferred to it albeit very small?
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
If an antenna is receiving a circular polarised signal is a torque transferred to it albeit very small?
I would propose this as "The question of the year".
Good question!
(I mean, it surely has an answer, but I don't know it).
 

Offline Bill S

  • Neilep Level Member
  • ******
  • Posts: 1802
  • Thanked: 11 times
    • View Profile
Thanks for the responses.  A thought, which may well be completely iconoclastic, is struggling to gain a foothold in my mind.  It concerns the positions of quantum objects, and looking back at the start of this thread, I think it may be a somewhat off subject, so I shall think about it a bit and, perhaps, start a fresh thread. [8D]
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
If an antenna is receiving a circular polarised signal is a torque transferred to it albeit very small?
I would propose this as "The question of the year".
Good question!
(I mean, it surely has an answer, but I don't know it).

I agree. It's certainly worth starting a new topic.

Syhprum, would you like to do that? We don't want to pinch your question.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
If an antenna is receiving a circular polarised signal is a torque transferred to it albeit very small?
I would propose this as "The question of the year".
Good question!
(I mean, it surely has an answer, but I don't know it).

The answer is almost certainly yes.  Circular polarization is a relatively small effect (and is closely related to photon spin).  Beams with what's called angular momentum impart much more angular momentum to objects they strike.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
If an antenna is receiving a circular polarised signal is a torque transferred to it albeit very small?
I would propose this as "The question of the year".
Good question!
(I mean, it surely has an answer, but I don't know it).

The answer is almost certainly yes.  Circular polarization is a relatively small effect (and is closely related to photon spin).  Beams with what's called angular momentum impart much more angular momentum to objects they strike.
But it sounds strange to me: the electric and magnetic field are still oscillating along a single direction and with respect to the same point; to have a torque there should be two field vectors not applied in the same point and along different action lines.  ???

How can this happen in our case?
« Last Edit: 25/11/2010 16:10:57 by lightarrow »
 

Offline granpa

  • Sr. Member
  • ****
  • Posts: 118
    • View Profile
I guess it isnt oscillating back and forth along 1 dimension.
looks like the electric field must be rotating
 

Offline QuantumClue

  • Hero Member
  • *****
  • Posts: 613
    • View Profile
I guess it isnt oscillating back and forth along 1 dimension.
looks like the electric field must be rotating

By rotation, what is meant? Their is a relative case of rotation, but I am not sure what context you are proclaiming this in.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
I guess it isnt oscillating back and forth along 1 dimension.
looks like the electric field must be rotating
Of course... :)
What I meant is: it oscillates back and fort along a direction + this direction rotates, but however it oscillates in a direction; the electric (or magnetic) vectors don't create a couple. Nonetheless, angular momentum conservation implies there must be a torque...
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
I would like to return to the original question. The fact that the solar system (and galaxies) tend to occupy a flat plane is nothing to do with the conservation of angular momentum.  This is always conserved for orbits in any direction and at any scale right down to the absolute limits.  This is a requirement (via Noether's theorem) for physical laws to be consistent whatever direction you are facing.

The reason for the flat disc like structures is due to The multi-particle dynamics of collapsing rotating objects containing angular momentum.  If many bodies are orbiting in random orbits of all shapes and directions they will occasionally collide link up break up and share orbital energy and change their orbits sometime bodies will gain a lot of energy and be ejected from the system.  If bodies are orbiting in a flat plane in orbits that are close to being circular the probability that they will collide at high speed is very greatly reduced so given time a flat disc structure is likely to develop.  There is also a suggestion that the extremely weak "gravitomagnetic" forces also act on orbits in a way that helps this disc structure to form and stay stable.

The charge and electromagnetic forces on a conceptual electron particle allow the formation of the stable three dimensional orbital patterns even if the individual paths of electrons in this structure are not observable.

As to the question of the reaction of an antenna to the receipt of a signal the reaction of atoms when emitting or absorbing high energy photons is observable and the reaction of a mirror reflecting light is observable so an antenna receiving radio waves will react to the signal it is receiving so there will definitely be a reaction in the opposite direction to the signal coming in.  Next we have the question if this is a circularly polarised signal with the matched circularly polarised antenna. will there be a torque on the antenna?  I cannot think of an equivalent experiment at high energy where this sort of waves could be generated and reactions measured but by extending the analogy my guess is yes there will be a torque.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Next we have the question if this is a circularly polarised signal with the matched circularly polarised antenna. will there be a torque on the antenna?  I cannot think of an equivalent experiment at high energy where this sort of waves could be generated and reactions measured but by extending the analogy my guess is yes there will be a torque.

If you tune the energy of your light beam way down, you're basically hitting the antenna with single photons, all with the same spin.  The spin would impart angular momentum onto the antenna.  I suspect you could do a similar thing by taking electrons, for example, all with spin +1/2, and sending them at a detector.  It's probably not practical, though.
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3812
  • Thanked: 19 times
    • View Profile
I thought it was a relatively trivial matter but enter it as a new question by all means (I am boosting up the mounting of my weather satellite antenna) just in case!
 

Offline jartza

  • Sr. Member
  • ****
  • Posts: 230
    • View Profile

Let's calculate the angular momentum of circularly polarized radio wave that has frequency of 1 Hz, and energy of 1 J.

The number of photons is the inverse of planck's constant.
Each photon has angular momentum of planck's constant / 2 pi

So answer is 1 / 2*pi Nms   (0.16 Newton meter seconds)

 

Offline jartza

  • Sr. Member
  • ****
  • Posts: 230
    • View Profile
So ... let's take 50 left circularly polarized photons,  and 50 right circularly polarized photons.  Now we mix these photons together.  Now what we have is linearly polarized light.

Any objections?
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums