# Pressure and temperature where volume is constant

## Main Question or Discussion Point

If I where to take a volume of water that is at atmospheric pressure and then add more water molecules to it without allowing the volume to increase I understand that the pressure will increase but does this also mean the temperature will increase?

My guess is that is does but I don't understand completely where the extra kinetic energy is sourced from. Would it be correct that the extra energy is coming from the force exerted on the liquid by the container to maintain a constant volume?

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Bystander
Homework Helper
Gold Member
Would it be correct that the extra energy is coming from the force exerted on the liquid by the container
No.
and then add more water molecules to it
You are doing the work as you add mass/matter to the system. It's not going in without effort on your part.

russ_watters
Mentor
Welcome to PF!

Yes, the temperature will increase, but it isn't caused by the constant force from the walls, it is caused by the actual act of inserting the extra liquid (you must have a force and a displacement to have an energy transfer). Then, after a while, the temeprature will drop back to ambient if it isn't well insulated.

Welcome to PF!

Yes, the temperature will increase, but it isn't caused by the constant force from the walls, it is caused by the actual act of inserting the extra liquid (you must have a force and a displacement to have an energy transfer). Then, after a while, the temeprature will drop back to ambient if it isn't well insulated.
So would this mean that say in this case water pump would give the water molecules a higher average kinetic energy than the molecules already present in the container. Then once these two 'groups' of particles are able to collide and the molecules with more energy will transfer energy to ther lower energy molecules therefore increasing the average kinetic energy of the molecules in the system so increasing the temperature.

Thanks for the help!

russ_watters
Mentor
Yes. By the way, since water is not very compressible, this effect is hard to demonstrate. But with air, it is very easy: pump up a bicycle tire (with a pump or non-tanked compressor, not a compressor with a tank) and feel the hose that supplied the air - it will be warm/hot.