Question

What happens to the thermal energy during thermionic emission?

Answer 1

In a hot metal, the electrons are at the same temperature as the lattice. The thermal energy is the random kinetic energy of the metal ions and the electrons. The (small) fraction of electrons in the metal with sufficient thermal energy to surmount the metal surface potential barrier can be emitted. This is called thermionic emission.The emitted electrons just carry away the energy they had inside the metal before emission.