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Consider an electron of mass \(9.11 \times 10^{-31} \mathrm{~kg}\) which is localized in a cube of volume \(10^{-6} \mathrm{~m}^{3}\). The wavefunction of the particle vanishes at the faces of the cube. What is the energy eigenvalue of the single-particle state with quantum numbers up to \(n_{1}^{2}+n_{2}^{2}+n_{3}^{2}=14\) ? What is the degeneracy of each energy level ignoring the spin of the electron?

Short Answer

Expert verified
The energy eigenvalue of the single-particle state is \(1.981 \times 10^{-20}\) J and the degeneracy of each energy level (ignoring the spin of the electron) is 6.

Step by step solution

01

Calculation of Energy Eigenvalue

The energy eigenvalue, expressed in terms of the quantum numbers, can be calculated via the formula \[E=\frac{h^{2}}{8mL^{2}}(n_{1}^{2}+n_{2}^{2}+n_{3}^{2})\], where \(h\) is the reduced Planck鈥檚 constant, \(m\) is the mass of the electron, \(L\) is the length of the cubic box, and \(n_1, n_2, n_3\) are the quantum numbers. By substituting \(h = 1.055 \times 10^{-34}\) kgm虏s鈦宦, \(m = 9.11 \times 10^{-31}\) kg, \(L = (10^{-6}\) m\(^{3})^\frac{1}{3}\) (cube root to obtain length), and sum of squares of quantum numbers being 14, calculate the energy eigenvalue using above equation.
02

Calculation of Degeneracy

Degeneracy means the number of states that have the same energy. It can be found by counting the number of different ways we can distribute three positive integers \(n_1, n_2, n_3\) such that \(n_{1}^{2}+n_{2}^{2}+n_{3}^{2}=14\). There are wilful ways to distribute these numbers: (1,2,3), (1,3,2), (2,1,3), (2,3,1), (3,1,2) and (3,2,1). Thus, the degeneracy of this level is 6.

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Key Concepts

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

Energy Eigenvalue
In quantum mechanics, the energy eigenvalue is a critical concept used to describe the energy levels of a quantum system. For an electron confined in a cubic box, the energy eigenvalues are determined by the quantum numbers associated with the system. The formula for the energy eigenvalue in a cube is given by:
  • \(E = \frac{h^2}{8mL^2}(n_1^2 + n_2^2 + n_3^2)\)
Here, \(h\) is Planck's constant, \(m\) is the electron's mass, and \(L\) is the box's side length. The terms \(n_1, n_2, n_3\) are quantum numbers that signify the state of the electron inside the cube. Each combination of \(n_1, n_2, n_3\) provides a unique energy level for the system.
Upon substituting the known values:
  • \(h = 1.055 \times 10^{-34}\) kgm虏s鈦宦
  • \(m = 9.11 \times 10^{-31}\) kg
  • \(L = (10^{-6} \text{m}^3)^{1/3}\)
And considering the constraint that \(n_1^2 + n_2^2 + n_3^2 = 14\), we can solve this equation to find the energy eigenvalue associated with the quantum state.
Quantum Numbers
Quantum numbers are essential tools in quantum mechanics for describing the quantized states of an electron within a potential well, like a cube. They are integer values and play a fundamental role in defining the energy and physical properties of quantum systems.In the context of an electron confined in a cubic box:
  • Each state is characterized by the triplet of quantum numbers \(n_1, n_2, n_3\).
  • They determine the vibrational modes the electron can adopt within the box.
These quantum numbers are not arbitrary but are subject to constraints. For the problem at hand:
  • The sum of their squares is fixed: \(n_1^2 + n_2^2 + n_3^2 = 14\).
This condition defines which combinations of \(n_1, n_2, n_3\) are permissible and subsequently determines the corresponding energy levels. It confines the electron to discrete energy states, profoundly impacting its behavior.
Wavefunction
The wavefunction is a pivotal concept in quantum mechanics, describing the probability distribution of a particle's position and momentum.For an electron in a cubic box:
  • The wavefunction must vanish at the boundaries of the cube, implying that the electron cannot exist outside the box.
  • Inside the box, the wavefunction forms standing waves, with nodes at the cube's faces.
Mathematically, the wavefunction \(\psi\) for the electron in the cube is a product of sinusoidal functions, represented as:
  • \(\psi(x, y, z) = A\sin\left(\frac{n_1\pi x}{L}\right)\sin\left(\frac{n_2\pi y}{L}\right)\sin\left(\frac{n_3\pi z}{L}\right)\)
Here\(,A\) is the normalization constant, ensuring that the total probability of finding the electron within the cube is 1. The nodes formed by each sinusoidal component in the wavefunction confer additional constraints on the system and reflect the quantized nature of the electron's states.

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Most popular questions from this chapter

The average kinetic energy \(\left(=3 k_{\mathrm{B}} T / 2\right)\) of hydrogen atoms in a stellar gas is \(1 \mathrm{eV}\). What is the ratio of the number of atoms in the second excited state \((n=3)\) to the number in the ground state \((n=1)\) ? The energy levels of the hydrogen atom are \(\varepsilon_{n}=-\alpha / n^{2}\) where \(\alpha=13.6 \mathrm{eV}\), and the degeneracy of the \(n\)th level is \(2 n^{2}\).

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