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 |   Tales from real life |
| Well, if they're not true, they oughta be! |
The Seattle area had a record overnight temperature Wednesday, the highest low ever for the date. The daytime high was in the 90's and clouds moved in during the afternoon. The fluffy sky blanket sealed in the heat like putting a lid on a boiling pot. It was an uncomfortable 80 degrees outside at 11 o'clock. As I lay awake in the darkness, I thought back to a long-running discussion at work. The subject then was also temperature, but it took place during a cold snap in January. One of the guys made the claim that his car was colder inside that morning than the temperature shown on his outdoor thermometer. We're all engineers and our expertise varies by degree, but the consensus was that Bill must be crazy. Either the sensor in his car or the thermometer must be defective. It seemed intuitive that the car was warm when it was parked, then gradually cooled overnight to match the ambient temperature outside. If it was really well insulated, then it might remain slightly warmer, but no way could it be colder! After a spirited discussion, we finally did some research and found that our consensus was defective. Bill was absolutely correct, and the laws of thermodynamics backed him up. To understand why, we need a quick review of heat transfer. Heat can be radiated like the rays of the sun, it can be conducted through direct touch like cuddling in bed, or it can be carried by a stream of gas or liquid like in a convection oven. The night of the freezing car was very still, so convection played no part in our mystery. We all thought that the heat inside the car would be conducted through the steel and glass to the surrounding air. When the temperature inside matched the outside, then heat transfer should stop. Our reasoning was correct, but we forgot about radiation. The radiation of heat depends on the temperature difference of the sending and receiving objects. Think of how direct sunlight feels on your face when you stand in front of your house on a cool day. The sun is the warmer object, and the temperature difference is immense. The sun's radiation easily overpowers conduction. Walk around to the back and stand near a shaded wall. Now your face is the warmer object, and the sensation is actually cooling. That's what was happening to Bill's car. The car was parked in an open driveway and the view through the windshield was unobstructed by buildings, trees, or clouds. The sky was crystal clear all the way to the infinity of deep space, and the temperature of that space is close to absolute zero. Bill's car was still 475 degrees warmer than outer space even when its interior temperature matched the chilly ambient of 20 degrees Fahrenheit. So, the car continued to radiate heat to outer space all night. The steel and glass of the car insulates the inside from the outside, so conduction doesn't replace the heat lost by radiation until there's a measurable temperature difference. Of course, this cooling effect isn't seen when the car is parked inside a garage or under tree cover. The cooling is similar but exactly opposite to parking your car in direct sunlight. Our 27 million-degree sun radiates a large amount of heat into your car through its windows. The interior temperature can rise to a lethal 120 degrees Fahrenheit before the process of conduction removes as much heat as is added by radiation. I like to tell this story because it demonstrates the power of science. I was humbled by my poor understanding of college physics. It reminds me to question what I think I know and remain open to learning. Certainty is not a substitute for knowledge. |
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