Argument with physics teacher


by ΔxΔp≥ћ/2
Tags: argument, physics, teacher
ΔxΔp≥ћ/2
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#1
Nov15-07, 05:57 PM
P: 56
Today, I had an argument with my physics teacher about the movement of electrons around the nucleus. I have read way more quantum mechanics than any normal high school student and my teacher is trained as an engineer, not a physicist, but I am not sure if I'm right.

His argument was something like the following:
Electrons move around the nucleus much like planets around the sun. They move in an elliptical orbit. The centrifugal force is what keeps them from crashing into the nucleus.

My response was:
We cannot know the precise position of an electron around the nucleus because of the uncertainty principle (note my name). It is therefore impossible to establish the electron as orbiting (elliptically) the nucleus.

Other thoughts:
If we approach the electron as a wave, I doubt any of his classical stuff makes sense. The centrifugal force thing even seems to be in conflict with the erroneous Rutherford and Bohr models.

I am not too familiar with the following concepts but I believe they also have a role:
The electron cannot fall into the nucleus because of the exclusion principle.
If the electron glued itself to the nucleus, its position would be more or less certain, giving it an enormous momentum.
These classical concepts where an issue after the discovery of the Rutherford model and the only way that an electron would stay out of the nucleus was if it accelerated because it would radiate energy.

Anyways, those are some arguments that come to mind.

Could someone please sort this out for me?
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nrqed
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#2
Nov15-07, 06:11 PM
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Quote Quote by ΔxΔp≥ћ/2 View Post
Today, I had an argument with my physics teacher about the movement of electrons around the nucleus. I have read way more quantum mechanics than any normal high school student and my teacher is trained as an engineer, not a physicist, but I am not sure if I'm right.

His argument was something like the following:
Electrons move around the nucleus much like planets around the sun. They move in an elliptical orbit. The centrifugal force is what keeps them from crashing into the nucleus.
He/she is completely wrong!
My response was:
We cannot know the precise position of an electron around the nucleus because of the uncertainty principle (note my name). It is therefore impossible to establish the electron as orbiting (elliptically) the nucleus.
Correct!
Other thoughts:
If we approach the electron as a wave, I doubt any of his classical stuff makes sense. The centrifugal force thing even seems to be in conflict with the erroneous Rutherford and Bohr models.
Exactly
I am not too familiar with the following concepts but I believe they also have a role:
The electron cannot fall into the nucleus because of the exclusion principle.
The electron and nucleus are different types of particles so exclusion principle does not apply here
If the electron glued itself to the nucleus, its position would be more or less certain, giving it an enormous momentum.
correct so it cannot be "glued" there. Same thing prevents white dwarves or neutron stars from collapsing
These classical concepts where an issue after the discovery of the Rutherford model and the only way that an electron would stay out of the nucleus was if it accelerated because it would radiate energy.
This sounds a bit confused. It was that a classical electric charge in an elliptical orbit would necessarily radiate away energy and so a classical nucleus would be unstable.
russ_watters
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#3
Nov15-07, 06:35 PM
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Wow, that's painful.

ΔxΔp≥ћ/2
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#4
Nov15-07, 07:30 PM
P: 56

Argument with physics teacher


Feels good to know I'm right.

Quote Quote by nrqed View Post
This sounds a bit confused. It was that a classical electric charge in an elliptical orbit would necessarily radiate away energy and so a classical nucleus would be unstable.
Yeah, thats what I meant, but I'm not a physicist... ...yet.

What kind of scared me though was when my teacher flatly said something along the lines of "No you're wrong." and then to justify himself, "I have a course prepared on nuclear physics/ I teach nuclear physics."

Guess that sometimes happens when engeneers teach physics. Still respect the dude though.

Anyone with a clear, reputable, simple reference that I can print to prove my point to him?
CPL.Luke
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#5
Nov15-07, 09:03 PM
P: 444
http://en.wikipedia.org/wiki/Hydrogen_atom

simplest case of atomic theory

granted wikipediaisn't the best reference, but any book or set of lecture notes will say the same. do a google search on hydrogen atom to get more info.

also a highschool chem book should contain similar information at a simpler level
DaveC426913
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#6
Nov15-07, 09:06 PM
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Electrons move around the nucleus much like planets around the sun. They move in an elliptical orbit. The centrifugal force is what keeps them from crashing into the nucleus.
Yep. Dead wrong.
DaveC426913
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#7
Nov16-07, 08:13 AM
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Quote Quote by ΔxΔp≥ћ/2 View Post
Anyone with a clear, reputable, simple reference that I can print to prove my point to him?
How about you just show him a couple of diagrams of electron orbitals and ask him to identify the ellliptical orbit for you?

Or ask him to explain why a water molecule is bent? No classical explanation does this. Orbitals do with ease.
cks
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#8
Nov16-07, 08:26 AM
P: 167
I'm not sure whether the conditions below justify what your teacher said. Actually, I don't know what is your teacher thinking. It's inappropriate to think of an electron as a body revolves around the nucleus same as planet moving around the Sun and says its path is deterministic. But, I think it's appropritate to say that the centripetal force balances the electric potential. CORRECT ME IF I AM WRONG! Your answer to your teacher sayings sounds like your teacher is asking a question where you answer other things. What you said regarding uncertainty principle is correct, but he's saying other thing.

Let's do a semiclassical way of approaching hydrogen atom.

[tex] T=\frac{1}{2}m v^2-\frac{e^2}{ r} [/tex]

Well, you should have no question about this, the total energy is the sum of kinetic and potential energy.

[tex] \frac{m v^2}{r}=\frac{e^2}{r^2} [/tex]

Well, it's saying the centripetal force is equal to the electric force!!! surprise

[tex] m vr=n\hbar [/tex]

The third equation expresses the quantization condition, introduced empirically by Bohr in order to explain the existence of discrete energy levels. he postulated that only circular orbits satisfying this condition are possible trajectories for the electron.

[tex] E_n=-\frac{1}{n^2}E_1 [/tex]

[tex] r_n=n^2a_0 [/tex]

Where E_1 and a_0 are ground state energy and Bohr radius.
ZapperZ
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#9
Nov16-07, 08:30 AM
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Quote Quote by cks View Post
I'm not sure whether the conditions below justify what your teacher said. Actually, I don't know what is your teacher thinking. It's inappropriate to think of an electron as a body revolves around the nucleus same as planet moving around the Sun and says its path is deterministic. But, I think it's appropritate to say that the centripetal force balances the electric potential. CORRECT ME IF I AM WRONG!
Er.. you are wrong.

The centripetal force IS the "electric force"!

"Centripetal force" is a GENERIC term given to a cental force. ANY force can be a centripetal force. Gravity is the centripetal force in planetary motion. The Lorentz force is the centripetal force when a charged particle is moving in a magnetic field. In the case of a charge being pulled in due to coulombic force, then the electric field IS the centripetal force.

Think about it, if what you claim is true (" ....centripetal force balances the electric potential..."), then what is your NET force? Zero! They balance out, as you claim. Then why would it move in a circular path in the first place? That would violate Newton's first law, wouldn't it?

Zz.
cks
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#10
Nov16-07, 08:31 AM
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I just think it's inappropriate to put the analogy of revolving around the sun is same as electron revolving around the nucleus. His argument that says centripetal force acts and balanced by electric potential is correct to some sense.
cks
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#11
Nov16-07, 08:33 AM
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Ok, I see, I admitted I didn't use the word centripetal force correctly. I agreed with what you said that centripetal force is indeed electric force. How about centrifugal force balanced by electric force.
ZapperZ
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#12
Nov16-07, 08:38 AM
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Quote Quote by cks View Post
Ok, I see, then I maybe used the wrong words, how about centrifugal force balanced by electric force.
Er.. invoking a fictitious force doesn't make it any better. There is no "centrifugral force" unless you intend to sit in the electron's reference frame. I thought this whole issue here is that there is no such ability at tracking an electron's path when we solve for the atom's orbital?

Why are we making this more complicated than it is? Write down the free-body diagram of a body in a simple circular motion, and that's that.

Zz.
Parlyne
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#13
Nov16-07, 11:17 AM
P: 546
Quote Quote by cks View Post
I'm not sure whether the conditions below justify what your teacher said. Actually, I don't know what is your teacher thinking. It's inappropriate to think of an electron as a body revolves around the nucleus same as planet moving around the Sun and says its path is deterministic. But, I think it's appropritate to say that the centripetal force balances the electric potential. CORRECT ME IF I AM WRONG! Your answer to your teacher sayings sounds like your teacher is asking a question where you answer other things. What you said regarding uncertainty principle is correct, but he's saying other thing.

Let's do a semiclassical way of approaching hydrogen atom.

[tex] T=\frac{1}{2}m v^2-\frac{e^2}{ r} [/tex]

Well, you should have no question about this, the total energy is the sum of kinetic and potential energy.

[tex] \frac{m v^2}{r}=\frac{e^2}{r^2} [/tex]

Well, it's saying the centripetal force is equal to the electric force!!! surprise

[tex] m vr=n\hbar [/tex]

The third equation expresses the quantization condition, introduced empirically by Bohr in order to explain the existence of discrete energy levels. he postulated that only circular orbits satisfying this condition are possible trajectories for the electron.

[tex] E_n=-\frac{1}{n^2}E_1 [/tex]

[tex] r_n=n^2a_0 [/tex]

Where E_1 and a_0 are ground state energy and Bohr radius.
This line of reasoning fails once you try to explain why there can be multiple angular momentum states corresponding to each energy level. In a sense you can trace the problem to the assumption of circular orbits.

The fact that this model leads to correct prediction for the energy levels, however, suggests that, mingled in with the oversimplifications, there must be some bit of good physical intuition. In this case, it can be found in a physical justification for the introduction of the Bohr quantization condition. If you start by assuming that the electron has wave properties and will interfere with itself if it's orbit is not an integral number of wavelengths, you can actually derive that condition. And, as it happens, this is essentially an over-simplified version of what you're doing when you solve the Schroedinger equation for the hydrogen atom.
Crosson
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#14
Nov16-07, 12:36 PM
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Teacher's who want to use Bohr's semi-classical model of the hydrogen atom as a teaching tool should at least be clear that this was "thought at one time to be a complete description, but it has its shortcomings, and has since been replaced by quantum mechanics...but the mathematics for that is very difficult, so we are going to work with Bohr's model".
strangerep
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#15
Nov16-07, 03:52 PM
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Quote Quote by ΔxΔp≥ћ/2 View Post
Today, I had an argument with my physics teacher about the movement of electrons around the nucleus. [...]

His argument was something like the following:
Electrons move around the nucleus much like planets around the sun. They move in an
elliptical orbit. The centrifugal force is what keeps them from crashing into the nucleus.
That's incorrect, as others have already said. Electromagnetic radiation from the
accelerating electron would make it lose energy and spiral inwards.

But perhaps the best thing to do (to maintain a constructive teacher/student relationship)
is to point your teacher at this forum and get him to say what he was really thinking (in
case there was some misunderstanding about the debate). I'm sure the heavy-hitters
around here will quickly set things straight.
blechman
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#16
Nov18-07, 04:30 PM
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I have taught physics at all levels, from high school tutorials to graduate courses. Let me say that as a physics teacher and researcher, I am deeply disturbed by this post. When a student asks for justification, and the teacher's response is to pull rank ("I teach physics, you don't, so shut up!"), that is criminal! If any teachers are reading this: if your students ask questions that you can't answer, then be honest - admit that you don't know the answer and encourage them to find out, as DpDx>hbar/2 (who I'll refer to as HUP) did. HUP: I cannot praise you enough! I truly feel very sorry for your colleagues who aren't as inquisitive as you are, and I very much hope that they turn out alright. As a active member of APS, I truly worry for them. Keep up the great work - don't trust anyone!

Especially me!
Count Iblis
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#17
Nov18-07, 05:39 PM
P: 2,159
Quote Quote by Crosson View Post
Teacher's who want to use Bohr's semi-classical model of the hydrogen atom as a teaching tool should at least be clear that this was "thought at one time to be a complete description, but it has its shortcomings, and has since been replaced by quantum mechanics...but the mathematics for that is very difficult, so we are going to work with Bohr's model".
I thought that Bohr-Sommerfeld theory led to an extremely awkward formalism and that quantum mechanics is much simpler.
Parlyne
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#18
Nov18-07, 05:57 PM
P: 546
Quote Quote by Count Iblis View Post
I thought that Bohr-Sommerfeld theory led to an extremely awkward formalism and that quantum mechanics is much simpler.
The simple Bohr model (with circular orbits) can be derived with a couple of simple physical assumptions and algebra. The full quantum version requires one to solve a non-linear partial differential equation (which can, in this particular case, be done analytically). I think there's really no question that the Bohr model is simpler. It's also quite wrong.


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