In its common usage, the word heat refers to both thermal energy and its transfer from a warmer object to a cooler object. Thermodynamics is a branch of physics that studies heat transfer between systems. This field has observed the laws of thermodynamics which define how heat, within a system, flows and does work. In any system, when two objects with different temperatures are brought into contact with one another, they will eventually establish thermodynamic equilibrium. As heat moves from one object to the other, physical changes will take place: the balloon filled with gas will grow or shrink, the roadway will expand or contract, the electrical resistance in the circuit will increase or decrease, and these changes are predictable and can be measured. Engineers and scientists take these laws into account when they design projects and experiments.
Use these resources to learn more about thermodynamics.