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Your explanation of the blow hot/blow cold question is, dare I say it, full of hot air. It’s all in how you blow. Here’s why. When you warm your hands, you blow steadily with your mouth open. This allows a greater volume of warm air to reach your cold hands. But to cool a hot cup of coffee, you first pucker. This causes the air to do interesting things before it reaches your cup: (1) The narrower opening reduces the volume of gas that escapes. (2) The increased pressure compresses the air. (3) The velocity of the escaping air increases. According to Boyle’s law . . . at a constant temperature the volume of a definite mass of gas is inversely proportional to the pressure [blah, blah]. When a gas is allowed to expand adiabatically through a porous plug the temperature of the gas changes. This rate of change is known as the Joule-Thomson differential [blah, blah]. As the rapidly expanding air leaves your mouth, it sweeps along neighboring molecules by adhesion (van der Waals force) [blah, blah]. Thus we . . . see that air that normally blows hot ceases to do so whenever it is compressed and allowed to expand. Isn’t science fun? [Two single-spaced pages, two equations, one poem deleted.] –Michael Godfrey, Cupertino, California
Pullara’s objection is more serious. I’ve gotten several similar notes. Here’s another: “You are forgetting that the latent heat of vaporization of water is very high. When you blow on your coffee, you replace the vapor-laden air above the coffee with dry air, allowing evaporation to proceed at the maximum possible rate. The temperature of your breath is not important. The coffee is not cooling by conduction of heat directly to the air but by shedding heat in the steam which comes off the coffee.”
