# How does expanding gas in isobaric process move a piston?

If we have a simple adiabatic cylinder with a piston on top and we heat the gas from the inside, the gas starts to expand and move the piston up. This process is isobaric; but if the pressure on the piston doesn't change, how does it move up?

In reality, such a process is not truly isobaric; a small rise in temperature will cause the gas to build up an infinitesimally small amount of pressure, which moves the piston up an infinitesimally small distance, which restores the pressure to its original value. If you assume that the piston can react quickly to the heat being applied, then it behaves nearly isobarically. Or, in mathematical terms, if $\Delta T$ is the rise in temperature from one state to the next, the process becomes isobaric as ${\Delta T \to 0}$. This is also referred to as a quasi-static process.