Pressure and temperature where volume is constant

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Discussion Overview

The discussion centers around the relationship between pressure, temperature, and volume in a system where the volume is held constant, specifically in the context of adding water molecules to a container. Participants explore the implications of this scenario on temperature changes and the sources of energy involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that adding water molecules to a constant volume will increase pressure and questions whether this also leads to an increase in temperature.
  • Another participant asserts that the extra energy comes from the work done by the person adding the water, rather than from the container's walls.
  • Some participants agree that temperature will increase due to the act of inserting additional liquid, emphasizing the need for force and displacement for energy transfer.
  • A later reply elaborates that the water pump could impart higher average kinetic energy to the new water molecules, which would then transfer energy to the existing molecules, thus increasing the overall temperature.
  • One participant notes that while this effect is difficult to demonstrate with water due to its incompressibility, it is easily observable with air, using the example of a bicycle tire pump that heats up during use.

Areas of Agreement / Disagreement

Participants generally agree that adding water will increase temperature, but there is some disagreement about the source of the energy responsible for this increase, with different views on the role of external forces and work done.

Contextual Notes

Participants discuss the implications of incompressibility in water compared to air, highlighting limitations in demonstrating the effects described. The discussion also touches on the conditions under which temperature may return to ambient levels if not insulated.

Who May Find This Useful

This discussion may be useful for individuals interested in thermodynamics, fluid mechanics, and the behavior of gases versus liquids under varying conditions.

Alevelman
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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?

Thank you in advance.
 
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Alevelman said:
Would it be correct that the extra energy is coming from the force exerted on the liquid by the container
No.
Alevelman said:
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.
 
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.
 
russ_watters said:
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!
 
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.
 
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