Traditional multiple-choice concept inventories measure students' critical conceptual understanding and are designed to reveal students' naive or alternate ideas. The overall scores, however, give little information about the state of students' knowledge and the consistency of reasoning. This study investigates whether students have consistent alternate models when reasoning about Newton's third law principle in the context of electromagnetics (EM), and whether these possible models are related to conceptual change and overall performance. Students' conceptual understanding is evaluated with The Conceptual Survey of Electricity and Magnetism (CSEM) multiple-choice test. The data (N = 118) are collected from an undergraduate static field theory course at the Helsinki University of Technology, Finland. The data are analysed using frequency distributions, Fisher's exact test, and One-Way ANOVA analysis. The study shows that every fifth student has a consistent or partially consistent alternate model of Newton's third law principle in the context of EM prior to instruction. Students with this alternate model perform significantly (p = 0.01) better on the overall concept test and are more likely to change conceptual understanding towards a correct model compared to students in an inconsistent mixed model state.