Illustration 20.4: Evaporative Cooling
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For a gas at a given temperature, there are particles with different speeds, and the particles' speed distribution follows the Maxwell-Boltzmann distribution. In this animation the two containers are separated by a "membrane" in the middle. Initially, no particles can cross the membrane. Once the particles are fairly evenly distributed in the left chamber, you are ready to allow for evaporation-that is, the fastest molecules can escape the left chamber and enter the right chamber. What is the approximate temperature of both sides initially? The light blue wall of the right chamber is at a constant chilly temperature of 20 K so that as particles hit it, they cool (slow down). Restart.
Try letting particles through the membrane. This animation only allows particles that hit the membrane at a speed of 25 or higher to pass through. This threshold is shown on the speed histogram. After some time passes, particles are no longer passing through the membrane as much. Notice what has happened to the speed distribution in the left chamber. (There may still be particles with speeds greater than the threshold because the speed distribution still follows a Maxwell-Boltzmann distribution.) What has happened to the temperature in the left chamber? This is what happens with evaporation: The fastest particles leave and so the temperature of what remains behind is cooler. This is why sweating cools you off—as the sweat evaporates off your skin, you are cooled down. Thus, evaporation is a cooling process.
Illustration authored by Anne J. Cox.