during chemical reactions when a fuel like methane, cooking
gas or coal burns in air. The chemical energy may also be
used to do mechanical work when a fuel burns in an engine
or to provide electrical energy through a galvanic cell like
dry cell. Thus, various forms of energy are interrelated and
under certain conditions, these may be transformed from
one form into another. The study of these energy
transformations forms the subject matter of thermodynamics.
The laws of thermodynamics deal with energy changes of
macroscopic systems involving a large number of molecules
rather than microscopic systems containing a few molecules.