The main purpose of the study was to examine college student’ individual mental models of electric field and understand how these mental models evolved and modified through instruction and group interaction. Using a hybrid methodology of case study and grounded theory, this study utilized 8 case participants in a cohort of 24 students for video-taped interviews, focus group discussions, and in-depth analysis of concept maps, drawings, and other student-generated outputs. Data analysis was conducted using the inductive procedure of grounded theory. Findings of the study revealed that students generally represented and understood electric field using a one-size-fits all “attraction-and-repulsion core mental model”. For case students, the electric field exhibited attraction and repulsion characteristics as represented by negative and positive charges. This core mental model was evident prior to instruction which was modified towards an increased level of sophistication after instruction and group interaction. Instruction and group interaction greatly impacted the development of college students’ mental models of electric field. Implications for teaching electric field were advanced in the study.