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name: John
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 1999 


Question:
Why does a can of electronic airduster cool when you use it?


Replies:
It's a phenomenon called "Joule-Tompson cooling." What is happening is that the propellant molecules like to stick together a little bit; their potential energy is lower when molecules are close together (something akin to condensation). When the propellant gas is expanded, the propellant molecules move away from each other. This puts the molecules into a condition of higher potential energy, which comes at the expense of their kinetic energy. Since temperature IS molecular kinetic energy, this means that the gas will cool somewhat.

Not all gases cool upon expanding. In fact, most gases have a region of temperature and pressure in which they cool and a complementary region in which they get hotter. It depends on the potential energy between molecules as they approach each other - there is usually an optimum separation, with the potential energy increasing as the molecules move closer together or farther apart.

Richard Barrans Jr., Ph.D.


Air molecules attract each other weakly, so it takes energy to move them apart. When the compressed air comes out of the can, the molecules are on average farther apart than they were while still in the can. The energy required to move them apart is supplied by the kinetic energy of the molecules. In other words, some of kinetic energy is converted into potential energy. The average kinetic energy of the molecules is directly related to the temperature -- the faster, the hotter. (By the way, you probably weren't there when the can was filled, but if you had been, you would have noticed that it got warmer.)

The situation is analogous to throwing a ball into the air. As the ball goes higher, it acquires gravitational potential energy and it slows down. As it comes back down, gravitational potential energy is converted to kinetic energy and the ball speeds up. Tim Mooney



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