Many students who have completed a full year of calculus continue to demonstrate a weak understanding of the role of the infinitesimal in an integral. This lack of understanding has implications for learning in introductory calculus-based physics, where students must integrate contributions from spatially-distributed sources. Symbolically, a student who constructs such an integral in terms of small chunks of sources, like dm or dq, will need to relate their infinitesimal to a small chunk of space, like dx. In order to better understand how students use the integration infinitesimal, we collected and analyzed written solutions from 39 upperdivision physics majors who solved for the electric field at an asymmetric point in space above a homogeneously charged bar. We have identified several categories of ways students handled the infinitesimal when constructing an integral. We present evidence, supporting prior work, that a small segment understanding of the infinitesimal leads students to a correct translation from dq to dx.