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

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