2016-68.md

course	course_year	question_number	tags	title	year
Quantum Mechanics	IB	68	IB 2016 Quantum Mechanics	Paper 3, Section I, B	2016

(a) Consider a quantum particle moving in one space dimension, in a timeindependent real potential $V(x)$. For a wavefunction $\psi(x, t)$, define the probability density $\rho(x, t)$ and probability current $j(x, t)$ and show that

$$\frac{\partial \rho}{\partial t}+\frac{\partial j}{\partial x}=0$$

(b) Suppose now that $V(x)=0$ and $\psi(x, t)=\left(e^{i k x}+R e^{-i k x}\right) e^{-i E t / \hbar}$, where $E=\hbar^{2} k^{2} /(2 m), k$ and $m$ are real positive constants, and $R$ is a complex constant. Compute the probability current for this wavefunction. Interpret the terms in $\psi$ and comment on how this relates to the computed expression for the probability current.