Express the change in internal energy of a system when
(i) No heat is absorbed by the system from the surroundings, but work (w) is done on the system. What type of wall does the system have ?
ii) No work is done on the system, but q amount of heat is taken out from the system and given to the surroundings. What type of wall does the system have?
(iii) w amount of work is done by the system and q amount of heat is supplied to the system. What type of system would it be?

Explanation

(i) Here, `q = 0 :. Delta U = q+w= 0 + w_(ad) = w_(ad)`
As no heat is absorbed by the system, the wall id adiabatic.
(ii) Here, ` w= 0 , q= -q :. Delta U = q + w= -q+0= -q`
As heat is taken out, the system must be having thermally conducting walls.
(iii) `w = -w , q= +q` `:. Delta u = q+ w = q-w`
As work is done by the system on absorbing heat, it must be a closed system.