Question

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

Answer 1

Hint: The energy associated with the spontaneous, disordered motion of molecules is known as internal energy. It is distinct in size from the macroscopic ordered energy associated with moving objects; it refers to the microscopic energy on the atomic and molecular scale that is invisible to the naked eye.

Complete answer:
(i) If no heat is absorbed by the system from the surroundings, but work is done on the system, then the system has an adiabatic wall.
 $ {{\Delta U}}\,\,{\text{ = }}\,\,{{\text{w}}_{{\text{ad}}}} $
An adiabatic wall is a thermodynamic barrier that prevents heat transfer from one side to the other. Heat cannot escape or enter through an adiabatic wall.
(ii) If no work is done on the system, but $ {\text{q}} $ amount of heat is taken out from the system and given to the surroundings, then the system has thermally conducting walls.
 $ {{\Delta U}}\,\,{\text{ = }}\,\,{\text{ - q}} $
Thermally conducting walls are a form of conducting wall that allows heat to flow between two systems.
(iii) If $ {\text{w}} $ amount of work is done by the system and $ {\text{q}} $ amount of heat is supplied to the system, then the system is a closed system.
 $ {{\Delta U}}\,\,{\text{ = }}\,\,{\text{q}}\,{\text{ - }}\,{\text{w}} $
A closed system will exchange energy (as heat or work) but not matter with its surroundings.

Note:
Internal energy is an equivalent representation of entropy, both cardinal state functions of only extensive state variables, which represents the entire thermodynamic information of a system. As a result, its value is solely determined by the current state of the system, rather than by a particular choice among the many possible processes by which energy may enter or exit the system.