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A harmonic oscillator with mass \(m\) and force constant \(k^{\prime}\) is in an excited state that has quantum number \(n\). (a) Let \(p_{\max }=m v_{\max }\), where \(v_{\max }\) is the maximum speed calculated in the Newtonian analysis of the oscillator. Derive an expression for \(p_{\max }\) in terms of \(n, \hbar, k^{\prime},\) and \(m\). (b) Derive an expression for the classical amplitude \(A\) in terms of \(n, \hbar, k^{\prime},\) and \(m .\) (c) If \(\Delta x=A / \sqrt{2}\) and \(\Delta p_{x}=p_{\max } / \sqrt{2},\) what is the uncertainty product \(\Delta x \Delta p_{x} ?\) How does the uncertainty product depend on \(n ?\)

Short Answer

Expert verified
a) \(p_{max} = \sqrt{2m(n+1/2)\hbar \omega}\) b) \(A=\sqrt{2\hbar(n+1/2)/k'}\) c) The uncertainty product \(\Delta x \Delta p_{x} = \hbar(n+1/2)\), which shows that the uncertainty product depends linearly on \(n\).

Step by step solution

01

Define Maximum Momentum

The quantum number \(n\) describes the energy state of a harmonic oscillator and is given by the equation \(E=(n+1/2) \hbar \omega\), where \(\omega = \sqrt{k'/m}\) is the angular frequency. The maximum momentum \(p_{max}\) is given by \(p_{max}=\sqrt{2mE}\). By substituting the equation of \(E\) into \(p_{max}\), we get \(p_{max} = \sqrt{2m(n+1/2)\hbar \omega}\)
02

Determine the classical amplitude

The maximum possible displacement A of a harmonically oscillating particle is classically given by the equation \(A = p_{max}/\sqrt{m \omega}\). Substituting our expression for \(p_{max}\) from Step 1, we get \(A=\sqrt{2\hbar(n+1/2)/k'}\)
03

The Uncertainty Product

\(\Delta x = A/\sqrt{2}\) and \(\Delta p_{x} = p_{max}/\sqrt{2}\), so their product \(\Delta x \Delta p_{x}\) is equal to \(A \times p_{max}/2 = (\hbar(n+1/2)\). The uncertainty product depends linearly on the quantum number \(n\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Quantum Mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. Central to quantum mechanics is the idea that energy is quantized; it can only exist in discrete amounts called 'quanta'. This means that particles like electrons in an atom can only occupy specific energy levels, and not any value in between.

Quantum mechanics also introduces the concept of wave-particle duality, stating that every particle or quantum entity can be described as both a particle and a wave. This dual nature is evident in experiments like the double-slit experiment, which shows the interference pattern that is characteristic of waves, even when particles are fired one at a time.

In the context of the quantum harmonic oscillator from the exercise, a classical analogy would be a mass attached to a spring that can oscillate back and forth. In quantum mechanics, this oscillator can only have specific energy levels, which are determined by the quantum number 'n'. The higher the quantum number, the higher the energy state of the oscillator.
Uncertainty Principle
The Heisenberg Uncertainty Principle is a fundamental limit to the precision with which certain pairs of physical properties, such as position and momentum, can be known simultaneously. It arises from the intrinsic quantum fluctuations of a particle's wavefunction, and it's one of the key differences between classical and quantum physics.

Formally, the uncertainty principle can be expressed as \(\Delta x \Delta p_x \geq \frac{\hbar}{2}\), where \(\Delta x\) is the uncertainty in position, \(\Delta p_x\) is the uncertainty in momentum, and \(\hbar\) is the reduced Planck's constant. In simple terms, the more precisely you know a particle's position, the less precisely you can know its momentum, and vice versa.

In the exercise, the uncertainty principle is represented by the uncertainty product \(\Delta x \Delta p_x\), which shows a clear dependence on the quantum number \(n\). As \(n\) increases, indicating higher energy states, both the position and momentum of the quantum harmonic oscillator can be predicted with less certainty.
Quantum Numbers
Quantum numbers are sets of numerical values that give acceptable solutions to the Schrödinger equation and provide important information about the energy and spatial distribution of a particle's probability amplitude. There are several types of quantum numbers, including:

  • The principal quantum number \(n\): Determines the energy level of an electron in an atom and its rough distance from the nucleus.
  • The orbital angular momentum quantum number \(l\): Describes the shape of an electron's orbital.
  • The magnetic quantum number \(m_l\): Determines the orientation of the orbital in space.
  • The spin quantum number \(s\) or \(m_s\): Describes the angular momentum of an electron's spin.
In the case of the quantum harmonic oscillator, the quantum number in the exercise relates specifically to the energy levels that the oscillator can take. The principal quantum number \(n\) identifies the specific energy state, where \(n=0\) is the lowest energy state, also known as the ground state. Each increase in \(n\) steps up to a higher energy state, demonstrating the quantized nature of energy in quantum systems.

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Most popular questions from this chapter

Protons, neutrons, and many other particles are made of more fundamental particles called quarks and antiquarks (the antimatter equivalent of quarks). A quark and an antiquark can form a bound state with a variety of different energy levels, each of which corresponds to a different particle observed in the laboratory. As an example, the \(\psi\) particle is a low-energy bound state of a so-called charm quark and its antiquark, with a rest energy of \(3097 \mathrm{MeV} ;\) the \(\psi(2 \mathrm{~S})\) particle is an excited state of this same quark-antiquark combination, with a rest energy of \(3686 \mathrm{MeV}\). A simplified representation of the potential energy of interaction between a quark and an antiquark is \(U(x)=A|x|,\) where \(A\) is a positive constant and \(x\) represents the distance between the quark and the antiquark. You can use the WKB approximation (see Challenge Problem 40.64 ) to determine the bound- state energy levels for this potential-energy function. In the WKB approximation, the energy levels are the solutions to the equation $$ \int_{a}^{b} \sqrt{2 m[E-U(x)]} d x=\frac{n h}{2} \quad(n=1,2,3, \ldots) $$ Here \(E\) is the energy, \(U(x)\) is the potential-energy function, and \(x=a\) and \(x=b\) are the classical turning points (the points at which \(E\) is equal to the potential energy, so the Newtonian kinetic energy would be zero). (a) Determine the classical turning points for the potential \(U(x)=A|x|\) and for an energy \(E\). (b) Carry out the above integral and show that the allowed energy levels in the WKB approximation are given by $$ E_{n}=\frac{1}{2 m}\left(\frac{3 m A h}{4}\right)^{2 / 3} n^{2 / 3} \quad(n=1,2,3, \ldots) $$ (Hint: The integrand is even, so the integral from \(-x\) to \(x\) is equal to twice the integral from 0 to \(x .\) ) (c) Does the difference in energy between successive levels increase, decrease, or remain the same as \(n\) increases? How does this compare to the behavior of the energy levels for the harmonic oscillator? For the particle in a box? Can you suggest a simple rule that relates the difference in energy between successive levels to the shape of the potential-energy function?

(a) The wave nature of particles results in the quantummechanical situation that a particle confined in a box can assume only wavelengths that result in standing waves in the box, with nodes at the box walls. Use this to show that an electron confined in a onedimensional box of length \(L\) will have energy levels given by $$ E_{n}=\frac{n^{2} h^{2}}{8 m L^{2}} $$ (Hint: Recall that the relationship between the de Broglie wavelength and the speed of a nonrelativistic particle is \(m v=h / \lambda .\) The energy of the particle is \(\left.\frac{1}{2} m v^{2} .\right)\) (b) If a hydrogen atom is modeled as a onedimensional box with length equal to the Bohr radius, what is the energy (in electron volts) of the lowest energy level of the electron?

An electron is bound in a square well that has a depth equal to six times the ground-level energy \(E_{1-\mathrm{IDW}}\) of an infinite well of the same width. The longest-wavelength photon that is absorbed by this electron has a wavelength of \(582 \mathrm{nm}\). Determine the width of the well.

Ground-Level Billiards. (a) Find the lowest energy level for a particle in a box if the particle is a billiard ball \((m=0.20 \mathrm{~kg})\) and the box has a width of \(1.3 \mathrm{~m}\), the size of a billiard table. (Assume that the billiard ball slides without friction rather than rolls; that is, ignore the to what speed does this correspond? How much time would it take at this speed for the ball to move from one side of the table to the other? (c) What is the difference in energy between the \(n=2\) and \(n=1\) levels? (d) Are quantum- mechanical effects important for the game of billiards?

An electron is in a box of width \(3.0 \times 10^{-10} \mathrm{~m} .\) What are the de Broglie wavelength and the magnitude of the momentum of the electron if it is in (a) the \(n=1\) level; (b) the \(n=2\) level; (c) the \(n=3\) level? In each case how does the wavelength compare to the width of the box?

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