As per my understanding, all adiabatic processes carried out in absence of an adiabatic wall need to be fast enough so that there is no heat exchange with the surrounding. Similarly, all isothermal processes are slow so that thermal equilibrium can be established with the surroundings in order to maintain a constant temperature in the system.
But then since isothermal processes are slow, how are isothermal irreversible processes possible?
Is my notion that 'irreversible processes are fast' wrong?
Answer
There are some irreversible processes that can be slow. However, when we refer to an isothermal irreversible process, what we really mean is that the boundary of the system with its surroundings are maintained at a constant temperature during the process, even if the process is fast. So, although the temperature at the boundary is constant, the temperature distribution within the system is not spatially uniform, and irreversible heat conduction is occurring within the system. Also, typically in an isothermal irreversible process, the boundary temperature is assumed to be held at the initial uniform temperature of the system in its initial thermodynamic equilibrium state, and the temperature of the system again becomes uniform at the boundary temperature in the final equilibrium state of the system.
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