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Cooling
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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Update: June 2012
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