#### cdub27

I understand that adiabatic cooling is the reason as to why, a valve releasing compressed air from a tire, would become cold. But I'm having a hard time grasping why this actually happens. I've been searching online, and I can't seem to find the type of simplified answer I'm looking for.

Could anyone explain this in a simple way, so I can understand the basic concept?

Related Introductory Physics Homework Help News on Phys.org

#### Andrew Mason

Homework Helper
I understand that adiabatic cooling is the reason as to why, a valve releasing compressed air from a tire, would become cold. But I'm having a hard time grasping why this actually happens. I've been searching online, and I can't seem to find the type of simplified answer I'm looking for.

Could anyone explain this in a simple way, so I can understand the basic concept?
Apply the first law of thermodynamics. Ask yourself whether the compressed air in the tire does work as the air is released from the tire and enters the atmosphere. Is any energy (eg. heat) being added to the tire? If the air does work, and no heat is being added to the tire, where does that energy come from? So what does that tell you about the internal energy of the air in/from the tire? And what does that tell you about the air's temperature?

AM

#### cdub27

Thank you AM,

Well from what I gather, the air does do work on the atmosphere by pushing it away against the atmpspheric pressure to make room for the expanded air. Which is caused by PV=nRT, which I believe means that the volume and nR remain constant; while P and T change. This relation means that the change in pressure will cause a change in temperature. Therefore when the valve releases the air, the air pressure becomes equalized to the surrounding air which causes it to cool down. Is this way off, or am I going in the right direction?

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving