Thermodynamics is the study of heat, a discipline that has its roots in the early 1800s. The first law of thermodynamics is the application of the law of conservation of energy to thermal systems. A system is any group of "things" under defined conditions. For example, a system may be the steam in a steam engine, the components of the human body, or all of the plants in a garden. Anything outside of the set boundaries defined by the system is said to be a part of the surroundings. A thermal system is any system that involves the transfer of heat.

Heat was originally thought of as a fluid that flowed from hot to cold objects. This fluid was called caloric. This theory of heat, called the caloric theory, was the basis for the study of thermal systems in the eighteenth century...