Question

What is the difference betweeen external and internal pressure of a system?

Answer 1

If the system is in thermodynamic equilibrium then the internal and external pressures must be equal. If the system is undergoing a reversible change then it must be arbitrarily close to a state of thermodynamic equilibrium and so the internal and external pressures can only differ infinitesimally. If the system is undergoing an irreversible change thermodynamics does not deal with it directly and so you must either focus entirely on the end points of the process or use a more involved theory like fluid dynamics –