dolimitless
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When does an electron have kinetic energy as opposed to potential energy?
Do electrons only exhibit potential energy when they are confined a stable electron energy level? I am reviewing Bohr's model of the atom in my
textbook and there is an equation that shows the potential energy of an electron in a certain energy level is given by the equation e = - Rhc / n^2.
Is there no kinetic energy factor to an electron when it is in a bound state, but only when in an unbound, free state? I am confused.
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bquirky
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im not relly qualified to answer but wouldn't it be that an electron cant really kick around at high speed while its stuck to an atom ?
an electron darting down a CRT tube whould surly have some notable kinetic energy but it isnt draging a whole atom along for the ride.
whereas a bunch of electrons sitting on a charged capacitor plate arnt moving and whould mainly have potential energy
thats my guess 
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hodges
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When an electron is part of an atom, it is not really "moving" in the classical sense. The old "solar system" model of electrons orbiting the nucleus
is not correct. A better model is to imagine the electron "smeared out" over the surface of each orbit.
Since the electron is not moving, it has no kinetic energy. Once the atom is ionized and the electron thus removed, it can then acquire kinetic
energy by being accelerated (for example, by an electric field).
Hodges
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entropy51
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Why would you think that an electron moving with velocity v had no kinetic energy?
The KE in the nth orbit is K.E = mv**2/2 = mZ**2e**4/[8(eps) 2h**2n**2 ]
where eps is the permittivity of free space
Quantum mechanics says that you have to take an average over the wave function to find the expectation value of v, it doesn't say that v = 0
[Edited on 28-6-2009 by entropy51]
[Edited on 28-6-2009 by entropy51]
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dolimitless
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Is the kinetic energy associated with electrons the photon it releases when jumping back to its energy level, and thus assuming a fixed, lower
potential energy?
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entropy51
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The photon energy is the difference in the TOTAL energy of the initial and final states. The total energy is E = K + U, where K = kinetic energy and
U = potential energy.
See here https://www.sciencemadness.org/whisper/viewthread.php?tid=12...
This forum is devoted to amateur science, and your questions seem more general and somewhat off topic. You might want to ask general questions at a
forum devoted to answering general questions, such as http://www.chemicalforums.com/
[Edited on 29-6-2009 by entropy51]
[Edited on 29-6-2009 by entropy51]
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dolimitless
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I am sorry if the "begginings" section of this forum is too accelerated and amatueur for my basic questions. Why would you direct me to another
forum? Why would you post a link to a thread advocating a homework section for this website? This is not a homework question. I am a beginner in
chemistry and merely had a question.
And you didn't even answer my question as posed by my first post, thanks.
[Edited on 29-6-2009 by dolimitless]
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dolimitless
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So can anybody clairfy for me.... Is there no kinetic energy factor to an electron when it is in a bound state, but only when in an unbound, free
state? I am confused. 
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12AX7
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In QM, there is neither KE nor PE, only E.
Tim
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dolimitless
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So neither? lol if this is such a basic question, why cant anyone give a straight answer?
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entropy51
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That's an interesting point of view. Last I looked the Schrodinger equation contains a term V(r), the potential energy. KE has also not been
forgotten in QM.
You might want to refresh your memory of Schrodinger's equation, at Wiki for example. I wasn't able to copy the link to post here.
But the original poster asked the question in the context of the Bohr atom, I do believe.
[Edited on 29-6-2009 by entropy51]
[Edited on 29-6-2009 by entropy51]
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chemrox
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An electron always has kinetic energy. When it ceases to have kinetic energy it ceases to be an electron. This is so fucking obvious I shouldn't
have to post it.
[Edited on 29-6-2009 by chemrox]
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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PainKilla
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If you are approaching Bohr's model, you need to understand the underlying physical principles and assumptions behind this model (not outlined in any
real detail here).
Realize that the Bohr model was not developed specifically to explain all aspects of atoms, rather it was developed to explain the emission spectrum
of hydrogen (based on the Rydberd formula, which was empirical in nature) - and it worked well for this, but not for the other atoms. The quantization
of energy based on the conclusions of Einstein with respect photoelectric effect had Bohr postulate that electrons jump in discrete intervals (hence
the energy levels). This also provided a foundation for the discrete spectral lines of hydrogen (and other atoms). The major step forward was the
quantization of orbits by making discrete the values of angular momentum that an electron could have.
Read Bohr's original paper for the assumptions taken in his model, but long story short, the kinetic energy is equal to potential energy, because the
centripetal velocity of the electron is equal to the Coulombic attraction of the electron and the proton... IE:
electrostatic force = (k*charge1*charge2) / (r^2) , where k is Coulomb's constant and r is radius.
centripetal force = (m*(v^2)) / r , where m is mass, v is velocity, r is radius
and then the actual energies given by:
PE = k*charge1*charge2 / r
and
KE = (1/2)*m*(v^2)
and KE = PE
You can read Bohr's original derivation here.
To answer the thread's original question, an electron will always have kinetic energy in principle, since the Coulombic attraction will exist
infinitely far away, just the kinetic energy will be infinitely small.
[Edited on 29-6-2009 by PainKilla]
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dolimitless
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Painkilla, thanks for the great explanation. Cheers to you.
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chemrox
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The above is indeed very nice but I don't understand why anyone would still want to place an electron in a Newtonian framework. Explaining electrons
was the beginning of quantum chemistry.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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12AX7
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Because Bohr did it before QM. It is astonishing that his model manages to produce the correct results.
Tim
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