The present study was carried out with 46 teacher candidates taking the course of "Thermodynamics" in the Department of Physics Teaching. The purpose of the study was to determine the difficulties that teacher candidates experienced in explaining the heat, work and internal energy relationships in the processes of adiabatic compression and expansion of an ideal gas. By examining both the results the teacher candidates found and the related interpretations made by the teacher candidates, the difficulties they experienced in understanding the subject were determined. The difficulties determined were gathered under two categories: a) discriminating the concepts (heat, work, internal energy and temperature) and b) application of the first law of thermodynamics to the adiabatic processes. It was seen that most of the teacher candidates experienced difficulty in understanding the fact that there was no difference between the functions of the concepts of heat and work in the microscopic scale.