If I take a piston (such as the classic expanding gas piston) with a constant weight on the plate and liquid water beneath the plate. I understand by Pascal's principle that the pressure is constant (neglecting the change in pressure that occurs with depth) throughout the liquid. However, if I add heat so that this water becomes gas, the gas will expand. My textbook claims that this whole process occurs under constant pressure, but I am not sure why since Pascal's principle only applies to incompressible fluids (i.e. not water vapor). I would appreciate if anyone could shed some light on why the whole process occurs under constant pressure.
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3$\begingroup$ Pascal's principle says that at a given location in a fluid (whether compressible or incompressible), pressure acts equally in all directions. So the weight on the plate sets the downward pressure, and, for equilibrium, the upward pressure of the gas must match that downward pressure (assuming negligible weight of the plate). $\endgroup$– Chet MillerCommented Aug 8, 2020 at 1:41
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2$\begingroup$ Just a small point about the wording of your title... The water and/or water vapour is not "in a piston". It is in a cylinder fitted with a piston. The piston is the movable bit (that you refer to as "the plate"). $\endgroup$– Philip WoodCommented Aug 12, 2020 at 15:57
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Assume you suddenly provide so much heat that all water gets vapourized in an instant . Now at this instant there must be huge pressure inside the cylinder (vapour occupies far more volume than liquid ) this causes the Piston to rise until the pressure equals the initial pressure of water.
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My textbook claims that this whole process occurs under constant pressure.
The whole process can be assumed to occur at constant pressure only if heat is supplied at extremely low rate , such that the system is in equilibrium with surroundings(simply pressure inside the cylinder is equal to ${P_{atmosphere }}+{P_{weight}}$at each and every point of the curve of the process.
Assume you suddenly provide so much heat that all water gets vapourized in an instant . Now at this instant there must be huge pressure inside the cylinder (vapour occupies far more volume than liquid ) this causes the Piston to rise until the pressure equals the initial pressure of water.
This was done suddenly . If it was done infinitesimally slowly then , every time heat is supplied , the system has time to equalize pressure to initial pressure . The system always attains equilibrium and is at each moment at the initial pressure .
$\textit{Your book must be talking about a slow reversible process.}$ if not it can't be at constant pressure .
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$\begingroup$ I tried using mark down but it doesn't work so I used mathjax. Can somebody help? $\endgroup$– ProteinCommented Aug 8, 2020 at 5:09
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$\begingroup$ Is it necessary for process to be reversible ? For some reason reversible felt necessary , is it really necessary? $\endgroup$– ProteinCommented Aug 8, 2020 at 5:14