Thermal physics courses have received relatively less attention from the field of physics education research than other core physics courses like quantum mechanics or electromagnetism. This thesis is composed of two projects which look at thermal physics courses from different perspectives. The first looks qualitatively at student reasoning in think-aloud interviews on a set of conceptual problems related to entropy. We use a conceptual resources framework to analyze and compare graduate and undergraduate student responses. The set of questions includes both new and previously studied problems and includes a novel system---a string waving in a bath of water---which could be used as a complementary way of introducing students to the concept of entropy. The second project quantitatively examines social networks of students working together on homework assignments in the upper-division thermal physics course at CU Boulder (as well as a middle-division math methods course at the Colorado School of Mines). We calculate the correlation between nodal centrality measures, which quantify how connected a node is to its larger network, and performance to quantify the relationship between collaboration and course grades. Also, we studied the possible effects of systematic errors caused by missing data within networks to better understand the significance of our results.</p
