My Chemistry textbook says that in free expansion there is no external pressure and thus the change in internal energy of the system is equal to the heat that we supply to it.
But i think that the only way to apply heat to a system is to make the external molecules loose their kinetic energy to the molecules of the system. ( NOTE - I am not talking about increasing the heat by adding radiant energy to the system )
If the external molecules loose some of their KE then they must exert a force on the system and thus the external pressure must not be equal to zero!
Is this correct? If not, then please tell me where I am wrong.