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mcat_premed3832
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 Posted: Tue Jul 10, 2007 2:58 am Post subject: |
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| In a pocket of air, you can have convection and you can have radiation but you can't have conduction. Conduction is the only one that requires physical contact. |
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admin
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| Posted: Wed Jul 25, 2007 9:06 pm Post subject: |
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| The question is asking which type of heat transfer do the air pockets limit. So we are looking for what does NOT work in the air pockets. Convection and radiation DO occur in the air pockets so they are the wrong answer. |
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admin
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| Posted: Mon Aug 20, 2007 9:20 am Post subject: |
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| Quote: | | (not pertaining to the question, well sort of) radiation is just electromagnetic waves, so it does not require a medium. |
Correct. |
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admin
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| Posted: Tue Apr 15, 2008 5:57 pm Post subject: |
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It is incorrect to say that air does NOT conduct, actually, air (after all, it does contain molecules in motion) is a very poor conductor.
"Air and water are relatively poor conductors": www.ucar.edu/learn/1_1_1.htm
"Some materials are better conductors of heat than others, metals are good conductors, air is poor.": www.aos.wisc.edu/~aos152/lesson1/content.html |
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mcat_premed3832
Joined: 19 Oct 2006
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| Posted: Mon Mar 08, 2010 9:30 pm Post subject: |
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ladywait of course you are correct that radiation requires no medium since it is electromagnetic radiation. And, if you have ever seen any of the Predator movies, you would have seen that humans can be tracked because we all emit infrared radiation.
And yes, convection and conduction are also means by which humans can interact with their environment including through sweating discussed more below.
But first I would like to address some earlier points:
| Quote: | | The point of a jacket or blanket is that it prevents the air around you from moving, and eventually that air equilibrates with your body temperature - thus preventing CONVECTION. I think the right answer should have been D. |
The problem with the above assessment is that the point of reference given in the question is not the environment, rather it is the pocket of air in the coat. Convection continually occurs in the air pockets of a coat because the body is warmer than the environment. The body warms up the cool air in the pocket closest to the body. That air rises and the cool air closest to the environment moves downward and circulates towards the body heating up as the warmer air that has moved to the other side cools down and so on. It is a dynamic process.
Note: the body will still lose some heat by radiation and convection. However, conduction is severely limited because air is a very poor conductor.
| Quote: | What if this question were the opposite? Without a down filled jacket, heat would be lost most through what? I'd say convection!
Heat loss through convection, is minimized through the jacket.
I think the answer to this question should be convection or at least both convection AND conduction should be correct. |
The question is about the air filled pockets. If there were no air filled pockets then, of course, radiation as expected would take place. And, if it is a thin jacket, you can touch the jacket and feel the warmth of the person's body indicating that conduction is taking place. And yes, there would be convection.
For perspective: conduction and radiation are by far the most important in terms of heat loss for humans. Convection becomes most important with a significant "wind chill factor" which must include windy conditions. And, even if there is wind, a k way jacket (a very common material used as a "windproof" outer shell) significantly reduces heat lost through convection. It works by preventing wind from penetrating your clothing and removing the body heat you have stored there.
Have as your mantra: "Air is a poor conductor of heat." Then you will get any similar question correct.
A word about sweating: sweating is a physiological process of cooling. The body essentially cools the surface of the skin by allowing, during a heat spell, the skin to sweat or vaporize its water at the surface and this allows in turn the skin and body to keep cool. Basically, our bodies use the endothermic nature of VAPORIZATION for cooling. So as you overheat, you sweat causing your skin to be covered with liquid water and as this water EVAPORATES, it absorbs heat from the body in turn, cooling the skin. |
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