# What's the explanation for temperature decrease when I pull a piston upwards? And how energy leaves a system when I transfer heat to it?

I got 2 important and tricky questions that I couldn't find on internet the answers.

First:

If I got a piston cylinder, when I push it downards, I increase particles velocity, so it will increase the internal energy of the system. What about the reverse? What happens if I pull the piston upwards? Would the temperature gets lower? would it keeps constant? I don't see an explanation for it to gets lower. I think that If I pull it up, particles will simply have more room, but will move with the same kinetic energy, so the temperature would keep constant.

Second:

In an Isothermal process, in order to keep temperature constant, the energy that enters, must leave in the same instant. If I have a piston cylinder, and If I push down the piston, heat must leave the cylinder in the same instant. However, what would happen if first I transfer heat to my system? How would this energy disappear in order to temperature be constant? I know that the heat energy would converted into work, but why does particles lose kinetic energy? If they push the piston with some KE, the piston would raise, but shouldn't them keep moving with the same KE?

Thanks!

• In your first example understanding the first sentence—understanding why that is so—is exactly equivalent to understanding your question. Commented Apr 23, 2017 at 3:00
• @dmckee ? Didn't understand Commented Apr 23, 2017 at 3:28