Question

which best describes thermal energy?
it is the difference between internal energies of two or more substances.
it is the sum of internal energies of two or more substances.
it is the portion of internal energy that can be transferred from one substance to another.
it is the portion of potential energy that can be transferred from one substance to another.

Explanation:

Thermal energy is the portion of internal energy that can be transferred from one substance to another. Internal energy is the total energy of a substance's particles (kinetic and potential). Thermal energy specifically refers to the part that can be transferred (as heat). The first option (difference between internal energies) is incorrect as it's not about difference. The second (sum of internal energies) is wrong as it's not the total sum but a transferable portion. The fourth (portion of potential energy) is incorrect as thermal energy involves kinetic energy of particles too (not just potential).

Answer:

It is the portion of internal energy that can be transferred from one substance to another.