115年 - 115 台灣聯合大學系統_碩士班招生考試_物理類：近代物理#139678

1. Let $S$ and $S'$ be two inertial frames of reference. $S'$ moves relative to $S$ with velocity $v$ in the $z$-direction. In $S$, two events occur at the same location with a time separation of 4s. What is the spatial distance of these two events in $S'$, given that they occur with a time separation of 5s in that frame? (A) 1cs (B) 2cs(C)2.42cs (D) 3cs (E) 4cs

2. Two particles, each of mass $m$, move directly toward one another at a speed of $v = 0.866c$. After the head-on collision, all that remains is a new particle of mass $M$. What is the mass of this new particle? (At $v = 0.866c$, $\sqrt{1 - v^2/c^2} = 0.5$). (A) $M = 2.0m$ (B) $M = 4.0m$ (C) $M = m$ (D) $M = 1.5m$ (E) $M = 5.0m$

3. The momentum of a photon is $2.000 \, \mathrm{MeV} / c$. What is its total energy? (A) 2.000 MeV (B) 2.200 MeV(C) 4.000 MeV (D) 0.970 MeV (E) 3.100 MeV

4. Certain surface waves in a fluid travel with phase velocity $v_{p} = \sqrt{b / \lambda}$, where $b$ is a constant and $\lambda$ is the wavelength. Find the group velocity of a packet of surface waves, in terms of the phase velocity $v_{p}$. (A) $v_{p}$ (B) $3v_{p} / 2$ (C) $3v_{p} / 4$ (D) $2v_{p}$ (E) $5v_{p} / 2$

5. Including the electron spin, what is the degeneracy of the $n = 5$ energy level of hydrogen? (A) 25 (B) 16 (C) 24 (D) 50 (E) 60

6. In a certain region of space, a particle of mass $m$ is described by the wave function $\psi(x) = Cxe^{-bx}$ where $C$ and $b$ are real constants. The potential $V(x)$ in this region is proportional to $1/x$, with no constant term $(V(x) \propto 1/x)$. By substituting this into the time-independent Schrödinger equation, find the energy of the particle, assuming the energy is constant throughout the region and independent of $x$. (A) $\hbar^2 b^2 / m$ (B) $-\hbar^2 b^2 / m$ (C) $\hbar^2 b / m$ (D) $-\hbar^2 b^2 / 2m$ (E) $\hbar b / 2m$ 

7. Which of the following physical quantities has the same dimension as the Planck constant? (A) momentum (B) angular momentum (C) energy (D) power (E) photon frequency

8. Which of the following physical quantities has the same dimension as $\sqrt{\hbar(m\omega)^{-1}}$? (m: mass, $\omega$: angular frequency) (A) momentum (B) time (C) energy (D) speed (E) distance

9. A particle of mass $m$ is subject to a one-dimensional attractive delta potential given by: $$ V(x) = -\alpha \delta(x), $$ where $\alpha > 0$. This potential supports exactly one bound state. Determine the energy eigenvalue $E$ and the normalized eigenfunction $\psi(x)$ for this state. We define $\kappa = \sqrt{-2mE} / \hbar$. (A) $E = -m\alpha^2 / 2\hbar^2$ ; $\psi(x) = \sqrt{\kappa} e^{-\kappa |x|}$ (B) $E = -m\alpha^2 / 2\hbar^2$ ; $\psi(x) = \sqrt{\kappa} e^{\kappa |x|}$ (C) $E = -m\alpha^2 / 2\hbar^2$ ; $\psi(x) = \sqrt{\kappa} e^{-\kappa x^2}$ (D) $E = +m\alpha^2 / 2\hbar^2$ ; $\psi(x) = \sqrt{\kappa} e^{-\kappa x^2}$ (E) $E = -\hbar^2 / 2m\alpha^2$ ; $\psi(x) = \sqrt{\kappa} e^{+\kappa |x|}$

10. In quantum mechanics, which of the following pairs of operators share simultaneous eigenstates? (A) Position along the $x$-axis $(\hat{x})$ and linear momentum along the $x$-axis $(\hat{p}_x)$. (B) The $x$-component of angular momentum $(\hat{L}_x)$ and the $y$-component of angular momentum $(\hat{L}_y)$. (C) Position along the $x$-axis $(\hat{x})$ and the $z$-component of angular momentum $(\hat{L}_z)$. (D) The squared magnitude of orbital angular momentum $(\hat{L}^2)$ and the $z$-component of orbital angular momentum $(\hat{L}_z)$. (E) The $x$-component of spin angular momentum $(\hat{S}_x)$ and the $z$-component of spin angular momentum $(\hat{S}_z)$. 

11. Consider a quantum particle in a one-dimensional infinite square well with potential $V(x) = 0$ for $-L < x < L$ and $V(x) = \infty$ for $|x| \geq L$. The particle has an initial wave function $\psi(x, t = 0) = \frac{1}{\sqrt{3}} \psi_1(x) + \sqrt{\frac{2}{3}} \psi_2(x)$, where $\psi_1$ and $\psi_2$ are the ground state and the first excited state of the system with eigenenergies $E_1$ and $E_2$, respectively. $\psi_{1,2}$ are real functions. What are the probability density $|\psi(x,t)|^2$ and the expectation value of energy $\langle H \rangle$ at later times $t > 0$? (A) $\langle H\rangle = \frac{1}{3} E_1 + \frac{2}{3} E_2$ ; $|\Psi (x,t)|^2 = \frac{1}{3}\psi_1^2 +\frac{2}{3}\psi_2^2 +\frac{2\sqrt{2}}{3}\psi_1\psi_2\cos \left(\frac{(E_2 - E_1)t}{\hbar}\right)$ (B) $\langle H\rangle \neq \frac{1}{2} (E_1 + E_2)$ ; $|\Psi (x,t)|^2 = \frac{1}{3}\psi_1^2 +\frac{2}{3}\psi_2^2 +\psi_1\psi_2\cos \left(\frac{(E_2 - E_1)t}{\hbar}\right)$ (C) $\langle H\rangle = \frac{1}{3} E_1 + \frac{2}{3} E_2$ ; $|\Psi (x,t)|^2 = \frac{1}{3}\psi_1^2 +\frac{2}{3}\psi_2^2$ (D) $\langle H\rangle = \frac{1}{3} E_1 + \frac{2}{3} E_2\cos (t)$ ; $|\Psi (x,t)|^2 = \frac{1}{3}\psi_1^2 +\frac{2}{3}\psi_2^2 +\frac{\sqrt{2}}{3}\psi_1\psi_2\cos \left(\frac{(E_2 - E_1)t}{\hbar}\right)$ (E) $\langle H\rangle = E_1 + \sqrt{2} E_2$ ; $|\Psi (x,t)|^2 = \psi_1^2 +2\psi_2^2 +2\sqrt{2}\psi_1\psi_2\cos \left(\frac{(E_2 - E_1)t}{\hbar}\right)$

12. The Pauli-X matrix, denoted by $\hat{\sigma}_x$, is represented as $$ \hat {\sigma} _ {x} = \left( \begin{array}{c c} 0 & 1 \\ 1 & 0 \end{array} \right). $$ Find the expectation value $\langle \hat{\sigma}_x\rangle$ in the spin state: $$ | \psi \rangle = \alpha_ {+} \left( \begin{array}{c} 1 \\ 0 \end{array} \right) + \alpha_ {-} \left( \begin{array}{c} 0 \\ 1 \end{array} \right), $$ where $\alpha_{+}$ and $\alpha_{-}$ are complex numbers. (A) 0 (B) $2\operatorname{Re}(\alpha_{+}^{*}\alpha_{-})$ (C) $2\operatorname{Im}(\alpha_{+}^{*}\alpha_{-})$ (D) $|\alpha_{+}|^{2} - |\alpha_{-}|^{2}$ (E) $|\alpha_{+}|^{2} + |\alpha_{-}|^{2}$

13. Consider a free electron gas in $d$ spatial dimensions. Let $n$ be the electron number density (defined as the number of electrons per unit length, area, or volume, depending on the dimension). Which of the following correctly describes the dependence of the Fermi energy $E_{F}$ on the density $n$? (A) $E_{F}\propto n^{2 / d}$ (B) $E_{F}\propto n^{d / 2}$ (C) $E_{F}\propto n$ (D) $E_{F}\propto n^{2}$ (E) $E_{F}\propto n^{1 / d}$

14. Consider a spinless particle in a three-dimensional isotropic simple harmonic oscillator potential. The particle is in an energy eigenstate with eigenenergy: $$ E = \left(99 + \frac{3}{2}\right) \hbar \omega $$ What is the degeneracy of this energy level? (A) 2500 (B) 5151 (C) 10100 (D) 100 (E) 5050

15. The total wavefunction of two electrons is written as a product of a spatial part $\psi(\vec{r}_1, \vec{r}_2)$ and a spin part $\chi_{spin}(s_1, s_2)$. Which of the following combinations correctly describes the symmetry of the spatial and spin parts specifically for the ground state of helium? (A) Spatial part is symmetric; Spin part is antisymmetric. (B) Spatial part is symmetric; Spin part is symmetric. (C) Spatial part is antisymmetric; Spin part is symmetric. (D) Spatial part is antisymmetric; Spin part is antisymmetric. (E) The Pauli principle forbids two electrons from occupying the same 1s spatial orbital, regardless of spin state.

16. Which of the following mathematical expressions correctly represents Planck's formula for the energy density per unit frequency $\rho(\omega)$ of a blackbody at absolute temperature $T$? ($k_B$: Boltzmann constant, $\omega$: angular frequency of light.) (A) $\frac{\hbar \omega^3}{\pi^2 c^3} \frac{1}{e^{\hbar \omega / (k_B T)} - 1}$ (B) $\frac{\hbar \omega^3}{\pi^2 c^3} k_B T$ (C) $\frac{\hbar \omega^3}{\pi^2 c^3} e^{-\hbar \omega / (k_B T)}$ (D) $\frac{\hbar \omega^3}{\pi^2 c^3} \frac{1}{e^{\hbar \omega / (k_B T)} + 1}$ (E) $\frac{\hbar \omega}{e^{\hbar \omega / (k_B T)} - 1}$

17. A harmonic oscillator with charge $q$ is initially in an eigenstate $|n\rangle$. It is subjected to a time-dependent electric field $\mathcal{E}(t)$ along the $x$-axis, resulting in the perturbing Hamiltonian $\hat{H}'(t) = -q\mathcal{E}(t)\hat{x}$. Based on the properties of the position operator $\hat{x}$, which of the following transitions are allowed in first-order perturbation theory? (A) $n \to n + 1$ (B) $n \to n - 1$ (C) $n \to n + 2$ (D) $n \to n - 2$ (E) $n \to n$

18. In a Compton scattering experiment, the spectrum of scattered X-rays displays two distinct peaks: a modified peak at wavelength $\lambda'$ and an unmodified peak at the incident wavelength $\lambda$. The Compton shift is defined as $\Delta \lambda = \lambda' - \lambda$. Which of the following statements regarding this experiment are correct? (A) The modified peak is due to scattering from loosely bound electrons. (B) The Compton shift $\Delta \lambda$ is independent of the wavelength $\lambda$ of the incident X-ray. (C) The Compton shift $\Delta \lambda$ is always smaller than or equal to zero ($\Delta \lambda \leq 0$). (D) The Compton shift $\Delta \lambda$ depends strongly on the atomic number $Z$ of the target material. (E) The Compton shift $\Delta \lambda$ varies with the scattering angle $\theta$.

19. A Light Emitting Diode (LED) is a p-n junction diode that emits light when activated. Which of the following statements correctly describe the physics and operation of a standard LED? (A) The device must be forward-biased to function properly. (B) The wavelength (color) of the emitted light is primarily determined by the band gap energy $(E_{g})$ of the semiconductor material. (C) The emission process relies on the spontaneous recombination of electron-hole pairs. (D) Indirect band gap semiconductors, such as pure Silicon (Si) or Germanium (Ge), are the most efficient materials for manufacturing visible light LEDs. (E) The intensity of the emitted light decreases as the forward current increases.

20. Which of the following statements regarding the Stern-Gerlach experiment are correct? (A) A non-uniform magnetic field (field gradient) is essential to exert a net force on the particles. (B) The experiment results in a continuous distribution of particles on the detector, as predicted by classical mechanics. (C) The experiment provides direct evidence for the quantization of angular momentum (spin). (D) A beam of neutral silver atoms in their ground state sent through a Stern-Gerlach apparatus will split into two discrete beams. (E) Silver atoms were ideal for the Stern-Gerlach experiment because they possess a single, unpaired valence electron.