Building Confidence in the Dirac $\delta$-function

Abstract

In this note we present an example from undergraduate quantum mechanics designed to highlight the versatility of the Dirac $\delta$-function. Namely, we compute the expectation value of the Hamiltonian of a free-particle in a state described by a triangular wave function $\psi(x)$. Since the first derivative of $\psi(x)$ is piecewise constant, and because this Hamiltonian is proportional to the second order spatial derivative, students often end up finding the expectation value to be zero --an unphysical answer. This problem provides a pedagogical application of the Dirac $\delta$-function. By arriving at the same result via alternate pathways, this exercise reinforces students' confidence in the Dirac $\delta$-function and highlights its efficiency and elegance.