Learning physics is often difficult for students because concepts such as electricity, magnetism and sound, cannot be seen with the naked eye. Emerging technologies such as Augmented Reality (AR) can transform education by making challenging concepts visible and accessible to novices. We present a Hololens-based augmented reality system where collaborators learn about the invisible electromagnetism phenomena involved in audio speakers, and we measure the benefits of AR technology through quantitative and qualitative methods. Specifically, we measure learning (knowledge gains and transfer) and collaborative knowledge exchange behaviors. Our results indicate that, while AR generally provides a novelty effect, specific educational AR visualizations can be both beneficial and detrimental to learning - they helped students to learn spatial content and structural relationships, but hindered their understanding of kinesthetic content. Furthermore, AR facilitated learning in collaborations by providing representational common ground, which improved communication and peer teaching. We discuss these effects, as well as identify factors that have positive impact (e.g., co-located representations, easier access to resources, better grounding) or negative impact (e.g., tunnel vision, overlooking kinesthetic feedback) on student collaborative learning with augmented reality applications.