Is there a distinction between thermal energy and internal energy? Which term do physicists prefer? Thermal energy is outside a substance, whereas internal energy is inside. Physicists prefer internal energy. Internal energy emphasizes heat flow, whereas thermal energy is the grand total of all energies outside a substance. Physicists prefer thermal energy. Thermal energy emphasizes heat flow, whereas internal energy is the grand total of all energies inside a substance. Physicists prefer internal energy. Thermal energy and internal energy are the same. Physicists prefer thermal energy