Express the change in internal energy of a system when
1) 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 a 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) `triangleU =w_(ad)` wall is adiabatic
ii) `triangleU=-q,` thermally conducting walls
iii) `triangleU=-q-w, ` closed system.