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Square well/barrier

by failexam
Tags: square, well or barrier
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failexam
#1
Apr5-11, 05:20 PM
P: 339
Square potentials (finite or infinte, well or barrier) are used in intro Quantum Mechanics courses. My question is:

What does the word 'square' signify?
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failexam
#2
Apr5-11, 05:28 PM
P: 339
My other question:

In the infinite square potential well model, the wavefunction outside the well is zero. Two reasons are usually put forward:

1. V is infinite. So Vu is infinite unless u = 0. (I don't understand what's still wrong with Vu being infinite.)

2. The particle can't be in a region of infinite potential if it lacks infinite energy. (Why not? Why should classical principles dictate quantum rules? Moreover, in the finite square well model, there is a finite probability of the particle being within the potential barrier? Isn't that a violation of the classical principle we are using in the above argument?) Therefore, u = 0.

Thanks for any help.
Rap
#3
Apr5-11, 05:42 PM
P: 789
Square refers to the appearance of the plot of V versus x.

Quote Quote by failexam View Post
My other question:

In the infinite square potential well model, the wavefunction outside the well is zero. Two reasons are usually put forward:

1. V is infinite. So Vu is infinite unless u = 0. (I don't understand what's still wrong with Vu being infinite.)

2. The particle can't be in a region of infinite potential if it lacks infinite energy. (Why not? Why should classical principles dictate quantum rules? Moreover, in the finite square well model, there is a finite probability of the particle being within the potential barrier? Isn't that a violation of the classical principle we are using in the above argument?) Therefore, u = 0.

Thanks for any help.
Energy is conserved in both classical mechanics and quantum mechanics. Assuming that the particle inside the well has finite energy, and the potential outside the well is infinite, then when the particle is outside the well, it will have infinite energy, which will violate the conservation of energy. Vu represents the potential energy of a particle outside the well. The other term in the Schroedinger equation represents the kinetic energy. Assuming that the total energy is finite and positive, if Vu were infinite, the kinetic energy would have to be negative infinity (steady state), which is impossible.

failexam
#4
Apr5-11, 07:18 PM
P: 339
Square well/barrier

Quote Quote by Rap View Post
Square refers to the appearance of the plot of V versus x.
There is nothing of the plot of V versus x that reminds me of a square, actually!

Quote Quote by Rap View Post
Energy is conserved in both classical mechanics and quantum mechanics. Assuming that the particle inside the well has finite energy, and the potential outside the well is infinite, then when the particle is outside the well, it will have infinite energy, which will violate the conservation of energy.
But isn't that exactly what happens in quantum tunneling? How does the particle get the extra energy to jump into a potential barrier. Isn't that in violation of the principle of conservation of energy?

Quote Quote by Rap View Post
Vu represents the potential energy of a particle outside the well. The other term in the Schroedinger equation represents the kinetic energy. Assuming that the total energy is finite and positive, if Vu were infinite, the kinetic energy would have to be negative infinity (steady state), which is impossible.
Howcome Vu is potential energy? I thought V was?
How can the other term represent the kinetic energy?
How can positive and negative infinity add to give a finite positive number?
What does 'steady state mean'?

Thanks for all the help!
alxm
#5
Apr6-11, 12:12 PM
Sci Advisor
P: 1,866
Quote Quote by failexam View Post
Moreover, in the finite square well model, there is a finite probability of the particle being within the potential barrier? Isn't that a violation of the classical principle we are using in the above argument?)
You're not using any classical principles. Since you've apparently solved the finite square well, why don't you just let [tex]V \rightarrow \infty[/tex] and see what happens?

How does the particle get the extra energy to jump into a potential barrier. Isn't that in violation of the principle of conservation of energy?
You're thinking about it classically. How do you know its potential energy (i.e. the location) and the kinetic energy (i.e. the momentum) at the same time?
The_Duck
#6
Apr6-11, 01:04 PM
P: 846
Quote Quote by failexam View Post
There is nothing of the plot of V versus x that reminds me of a square, actually!
Square refers to the fact that the potential jumps discontinuously from one value to another, so that the graph of V(x) has a "square" corner. Compare to e.g. a harmonic potential well, whose graph is a parabola.
Rap
#7
Apr6-11, 03:11 PM
P: 789
Quote Quote by failexam View Post
There is nothing of the plot of V versus x that reminds me of a square, actually!
When the potential is finite, the "well" forms three corners of a rectangle. I wouldn't worry too much about the fact that it doesn't form a square. Its just a name.

Quote Quote by failexam View Post
But isn't that exactly what happens in quantum tunneling? How does the particle get the extra energy to jump into a potential barrier. Isn't that in violation of the principle of conservation of energy?
I may be wrong here, but as I remember, the time that a particle spends outside the well (delta t) is such that the Heisenberg uncertainty principle is obeyed. In other words, the energy deviation times the time interval is less than Planck's constant/2.


Quote Quote by failexam View Post
Howcome Vu is potential energy? I thought V was?
How can the other term represent the kinetic energy?
A better way to say it is that they are the potential energy and kinetic energy terms in the Schroedinger equation.

Quote Quote by failexam View Post
How can positive and negative infinity add to give a finite positive number?
Its a limiting process. For example, as n grows larger, (n+1)^2- (n^2+2n) equals 1. Each term on the left grows larger without bound.

Quote Quote by failexam View Post
What does 'steady state mean'?
It means no change in time.

Thanks for all the help![/QUOTE]


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