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- TL;DR Summary
- a figure in a textbook shows a cylinder filled with gas and one side of the cylinder has a movable piston which is open to the atmosphere, the textbook says "This ensures the gas in the cylinder is always at a constant pressure". how did the book deduce that?

im learning thermodynamics and currently in a lesson about thermal processes. one process has constant pressure and before diving into equations or any proof the book provides a figure of a gas cylinder. the cylinder has a movable piston/lid on one side. the book then says "...and the piston end of the cylinder is open to the atmosphere. This ensures that the gas in the cylinder is always at a constant pressure."

i dont know if this is relevant but the book talks about hot-air balloons and pressure in the previous paragraph:

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i really didnt understand this part so if anyone can help me out with this too i would really appreciate it.

**how was the book able to deduce that the cylinder had constant pressure just by the fact that it's piston is exposed to the atmosphere?**i dont know if this is relevant but the book talks about hot-air balloons and pressure in the previous paragraph:

"

*If you've ever seen a large hot-air balloon being inflated, then you know that the gas in the balloon expands as it is heated. The balloon is surrounded by the atmosphere as it is heated, and the atmosphere is at a constant pressure. Therefore, the heating and expansion of the balloon occur with no change in pressure*."i really didnt understand this part so if anyone can help me out with this too i would really appreciate it.