change in volume and change in temp thermodynamics

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MrNeuro

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if you increase the volume you decrease the temperature? (TPR)

i thought that according to the first law of thermodynamics as you increase volume you add head to the system

volume increase does work on surroundings hence system must gain heat

Can someone walk me through why an increase in volume would lead to a decrease in temp? cant seem to wrap my head around that one....
 
So there's two different scenarios in which your system's volume could increase. Call #1 "balloon" and #2 "piston".

In a balloon, you have a volume that can respond to the conditions inside it. So if you heat the air in the balloon, it expands until the pressure is at equilibrium again. This expansion pushes the surrounding air out of the way, doing work.

Now, in a piston, you have a volume that you control directly, and the conditions inside it change in response. If you push down on the piston, thus reducing the volume, you haven't changed the kinetic energy of any of the particles, but you have made them closer to each other and reduced the surface area they have to bounce off of (inside the piston). The pressure in a system is the force by the contents on its containing walls. So in reducing the volume, you up the pressure.

The previous paragraph is about what happens at constant temperature: pressure changes inverse to volume. But sometimes we want to know about a system with constant pressure. The only way to make it so you don't up the pressure when you reduce the volume is to reduce the temperature of the contents of the piston. So, holding pressure constant, decreasing the volume you also have to decrease the temperature. And the inverse is also true: if you want to hold the pressure constant while increasing the volume, you need to put a bunch of heat into the system.

In summation: it's not that an increase in volume *leads* to an increase in temp. But if you want to keep the pressure constant, each increase in volume needs to be accompanied by an increase in temp.
 
if you increase the volume you decrease the temperature? (TPR)

i thought that according to the first law of thermodynamics as you increase volume you add head to the system

volume increase does work on surroundings hence system must gain heat

Can someone walk me through why an increase in volume would lead to a decrease in temp? cant seem to wrap my head around that one....

if you increase the volume of a container that contains a gas you decrease the amount of intramolecular interactions and collisions which would decrease heat.
 
if you increase the volume of a container that contains a gas you decrease the amount of intramolecular interactions and collisions which would decrease heat.

Yes, but in the situation you describe, you also decrease pressure. The weird case, and what I think is probably what's confusing the OP, is when you keep pressure constant.
 
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