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) Since no heat is absorbed by the system,
q = 0
`therefore` `DeltaU = q + w = 0 + w_(adiabatic)`
`therefore DeltaU = w_(adiabatic)`
As no heat is absorbed by the system, the wall is adiabatic.
(ii) Here, w = 0 and q = -q
`therefore` `DeltaU = q + w = -q + 0 = -q`
Since heat is taken out, the system must be having thermally conducting wall.
(iii) Here, `w = -w, q = +q`
`therefore DeltaU = q + w = q +(-w) = q - w`
Since work is done by the system on absorbing heat, it must be a closed system.