# Isothermal expansion of a piston with initial system and surrounding pressures equal.

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## Main Question or Discussion Point

In an isothermal expansion of an ideal gas, can the piston be made to expand isothermally if the initial system pressure is equal to the initial surrounding pressure?(In most of the books and video lectures whenever they explain isothermal expansion, they assume initial system pressure to be higher than surrounding pressure.A well known example is a piston cylinder arrangement with weights placed on the piston.They never assume the initial pressures to be equal.)

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Chestermiller
Mentor
In an isothermal expansion of an ideal gas, can the piston be made to expand isothermally if the initial system pressure is equal to the initial surrounding pressure?(In most of the books and video lectures whenever they explain isothermal expansion, they assume initial system pressure to be higher than surrounding pressure.A well known example is a piston cylinder arrangement with weights placed on the piston.They never assume the initial pressures to be equal.)
Why don't you precisely define a focus problem that we can work on together to help you get an understanding of these issues?

Why don't you precisely define a focus problem that we can work on together to help you get an understanding of these issues?
Let us consider a frictionless piston cylinder arrangement containing an ideal gas.Initially the gas(system) inside the arrangement is at atmospheric pressure.The surrounding is the atmosphere.So the piston is at rest.If we heat the cylinder(by placing it in contact with a slightly higher temperature source(infinitesimal temperature difference), can the gas expand isothermally?(If so, please tell me whether the pressure of the gas increases or decreases during expansion and whether the final system pressure value will be higher or lower than the initial value after attaining equilibrium(the piston comes to rest))

Chestermiller
Mentor
Let us consider a frictionless piston cylinder arrangement containing an ideal gas.Initially the gas(system) inside the arrangement is at atmospheric pressure.The surrounding is the atmosphere.So the piston is at rest.If we heat the cylinder(by placing it in contact with a slightly higher temperature source(infinitesimal temperature difference), can the gas expand isothermally?(If so, please tell me whether the pressure of the gas increases or decreases during expansion and whether the final system pressure value will be higher or lower than the initial value after attaining equilibrium(the piston comes to rest))
is the cylinder horizontal or vertical? If the cylinder is vertical, does the piston have mass, or is it massless?

is the cylinder horizontal or vertical? If the cylinder is vertical, does the piston have mass, or is it massless?
The piston doesn't have any mass.So I think there will not be any difference between the two orientations.

Chestermiller
Mentor
If the cylinder is in contact with a slightly higher temperature source during this scenario, then, in the end, the temperature of the gas will be at the slightly higher source temperature, and the volume will be slightly higher. In other words, not much expansion will happen. The equilibrium pressure of the gas in the end (as well as throughout this "expansion") will be atmospheric.

If the cylinder is in contact with a slightly higher temperature source during this scenario, then, in the end, the temperature of the gas will be at the slightly higher source temperature, and the volume will be slightly higher. In other words, not much expansion will happen. The equilibrium pressure of the gas in the end (as well as throughout this "expansion") will be atmospheric.
Thank you.So this means the process cannot be isothermal (i.e. if the initial system and surrounding pressures(before expansion) are equal, the process cannot be an isothermal expansion)I am not really sure whether my inference is right.Please comment on it.

Chestermiller
Mentor
Thank you.So this means the process cannot be isothermal (i.e. if the initial system and surrounding pressures(before expansion) are equal, the process cannot be an isothermal expansion)I am not really sure whether my inference is right.Please comment on it.
There are two ways to get the gas to expand: (a) increase the outside temperature or (b) decrease the outside pressure. Or you can use combinations of outside pressure and temperature changes, if, at the final state, the volume (calculated from the ideal gas law) is greater than the initial volume.