E is the capacity to do work or to supply heat.
E is detected only bec of its effects. The kinds of atoms and the arrangement of the atoms in a subs determines the amt of E stored in the subs. Heat, rep by q, is a form of E that always flows from a warmer obj to a cooler obj. In chem rxns bonds are broken, atoms are rearranged and new bonds are formed. This E must be supplied in order for the rxn to occur.