# Equilibrium and non-equilibrium thermodynamic states?

I have the following two states. I have classified them as equilibrium and non-equilibrium and I am not sure if that is the right category my problem is I need to understand what parameters to look into to classify them any suggestions

• A. The system is a Daniell cell on open circuit .This is Non Equilibrium State- Because Potential difference at end of cell is more then potential difference of other part of this system.

• B. The system is a sphere which is falling through a viscous liquid and has reached its thermal velocity. This is Equilibrium - As in this state the viscous force (Drag Force) is equal to Gravitational Force.

## 1 Answer

System A is a thermodynamic equilibrium state as long as the circuit is open. The open circuit is a constraint which maintains the system in thermodynamic equilibrium. This system is analogous to two chambers of gas at different pressures separated by a rigid barrier. As long as the barrier (constraint) is in place, each chamber is in thermodynamic equilibrium. Once the barrier is removed, however, the system is no longer in thermodynamic equilibrium, and a spontaneous change takes place.

System B is in mechanical equilibrium (forces balanced), but not in thermodynamic equilibrium. Viscous dissipation of mechanical energy is occurring within the liquid, and its temperature is rising in the vicinity of the sphere as the sphere advances to encounter new fluid. Also, from the frame of reference of an observer moving with the sphere (so that the fluid appears to be flowing past), the system of fluid and sphere is at steady state.