Free expansion (or unresisted expansion) process. A free expansion occurs when a fluid is allowed to expand suddenly into a vaccum chamber through an orifice of large dimensions. In this process, no heat is supplied or rejected and no external work is done. Hence the total heat of the fluid remains constant. This type of expansion may also be called as constant total heat expansion. It is thus obvious, that in a free expansion process,
Q1-2 = 0, W1-2 = 0 and dU = 0.