The effects of both heat flow and work are combined in the first law of thermodynamics. Thermodynamics is the study of the relationships between heat and work. When heat is added to a system, it can do two things; increase the internal energy of the system or do mechanical work.
This study of heat transformations into other forms of energy began with the building of steam engines by eighteenth-century engineers. Today, the concepts of thermodynamics are used widely in many different ways. The laws of thermodynamics are used continuously to develop better automobile engines, refrigerators and many other machines.