I'm trying to explain this to my 8th grade brother. But I don't seem to understand this fully... I keep getting stuck because of my poor knowledge in thermodynamics. I'd like to ask a related question first :
1) I know that the energy decreases in an adiabatic expansion. This is because the gas molecues do the work on the piston and push it up, increasing the volume. But what happens if I push the piston up instead of the gas molecules ?(like, pulling a syringe) Would it still be called an adiabatic expansion ? I'm not adding any heat / matter to the system. So I think this should still be an adiabatic process. If so, the energy of the system shouldn't decrease as the gas didn't do any work. I did the work. Something wrong in my reasoning ?
2) Here is the actual question : Does the kinetic energy of an individual water molecule (vapor) decrease as it goes up in the atmosphere ? Or is it just the average kinetic energy that gets decreased due to low density at higher altitudes ? Also is this average, over space or is it over the number of molecules ? If the average is over space, then low density explains why the average kinetic energy decreases. If the average is over number of molecules, I don't get why the average kinetic energy decreases... Appreciate any help. Thank you !