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At constant pressure,
$$ V\propto T$$

Letting the piston freely moving fixes the pressure of the gas to the atmospheric;
then heating the gas causes the $T$ to increase,
this causes the $V$ to increase by the same factor.

However, in the other direction, how can one maintain the pressure constant while changing volume? Feels it is impossible to change volume and maintain the pressure constant at the same time... (to observe how the temperature changes)

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However, in the other direction, how can one maintain the pressure constant while changing volume? Feels it is impossible to change volume and maintain the pressure constant at the same time

Think about it this way. The pressure against the walls of the vessel is proportional to the rate at which the gas molecules collide with the walls. If you increase the temperature of the gas the average speed of the gas molecules increases. That would increase the pressure if the volume was fixed since it would increase the collision rate.

However, if you allow the volume to increase at the same time you increase the temperature, the distance the molecules have to travel between collisions with the walls increases. The increase in distance between collisions with the wall compensates for the increase in the speed of the molecules, thus, the pressure of the gas remains constant.

Hope this helps.

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  • $\begingroup$ That means you're increasing the temperature to account for the decrease in pressure due to increase in volume. Following these steps in sequence: 1) Increase volume 2) Pressure decreases 3) Heat it to bring pressure to the previous level. Here I want to observe the change in temperature, but since heating the gas is same as increasing the temperature, how can I observe the same? I'm wondering if there's a way to keep the pressure constant without "manually/directly" tweaking temperature/heating... Thanks! @Bob $\endgroup$
    – across
    Commented Sep 9, 2020 at 13:40
  • $\begingroup$ @across I look at it differently. You're not manually "tweaking" the temperature. You are slowly adding heat and that increases the temperature slowly. If the kept the volume fixed (lock a piston in place) the pressure would increase. But by allowing the volume to naturally increase keeps bringing the pressure back down. $\endgroup$
    – Bob D
    Commented Sep 9, 2020 at 13:48

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