# Does a fast process always have to be adiabatic?

In common questions on thermodynamic processes, say for example a simple straight-forward question like "A gas at $T_1\ K$ and $P_1$ atm is suddenly released at atmospheric pressure. Find the final temperature of the gas", we assume the process to be adiabatic since no heat is exchanged between the system and the surroundings in that small interval of time. So my question is, do all quick processes have to be adiabatic, and similarly, is a slow process always isothermal?

• No, a process can be so fast it can be a nonequilibrium process, which is not adiabatic. But most of the time in high school exams, 'fast' means adibatic. Apr 3, 2018 at 22:36
• Is a slow process always isothermal? Apr 3, 2018 at 23:20
• It really depends, again. If there’s “ideal” insulation, then no. Or if you have some weird setup, like a gas slowly expanding inside a container inside an oven that slowly is heating up. But for an exam it’ll probably be isothermal unless they’re trying to be tricky. Apr 3, 2018 at 23:22
• Could you give an example of the first bit? Apr 3, 2018 at 23:26
• A slow process doesn't have to be isothermal. For example, if you heat a gas in a closed container, its temperature will rise. If we expand a gas, adding a greater amount of heat to it than is necessary to hold its temperature constant, its temperature will rise. There are a zillion other examples. Apr 3, 2018 at 23:26