internal energy present in a system in a state of thermodynamic equilibrium by virtue of its temperature. Thermal energy cannot be converted to useful work as easily as the energy of systems that are not in states of thermodynamic equilibrium. A flowing fluid or a moving solid, for example, possesses energy that can be converted to work in some mechanical device, such as a windmill or a waterwheel, but the same fluid or solid in a thermodynamic equilibrium state having the same energy (as thermal energy) can do no work unless it is combined with another substance at a different temperature, as in a heat engine.