We present two studies documenting the effectiveness of the use of a computer simulation with Tutorials in Introductory Physics1 in a transformed college physics course.2 An interactive computer simulation, entitled the Circuit Construction Kit (CCK),3,4 was introduced to investigate its possible impact on students' conceptual understanding. The first study compared students using either CCK or real laboratory equipment to complete two Tutorials on DC circuits. The second study investigated the impact of the simulation's explicit representation for visualizing current flow by removing this feature for a subset of students. In the first study, students using CCK with Tutorials performed slightly better on measures of conceptual understanding compared to real equipment, as measured by exam performance soon after the intervention. In the second study, students using CCK with and without the explicit visualization of current performed similarly to students using real equipment, though on some specific questions we note significant variation in student performance. We discuss the implications of adding (or removing) such representations within computer simulations.