  The Dirac delta function is a standard mathematical tool that appears
repeatedly in the undergraduate physics curriculum in multiple topical areas
including electrostatics, and quantum mechanics. While Dirac delta functions
are often introduced in order to simplify a problem mathematically, students
still struggle to manipulate and interpret them. To characterize student
difficulties with the delta function at the upper-division level, we examined
students' responses to traditional exam questions and a standardized conceptual
assessment, and conducted think-aloud interviews. Our analysis was guided by an
analytical framework that focuses on how students activate, construct, execute,
and reflect on the Dirac delta function in the context of problem solving in
physics. Here, we focus on student difficulties using the delta function to
express charge distributions in the context of junior-level electrostatics.
Common challenges included: invoking the delta function spontaneously,
translating a description of a charge distribution into a mathematical
expression using delta functions, integrating 3D or non-Cartesian delta
function expressions, and recognizing that the delta function can have units.
We also briefly discuss implications of these difficulties for instruction.
