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Tuesday, January 19, 2010
What the heck is Emissivity? (Part 2)
Fill up two soda cans with hot water and wrap one in scotch tape. Which one will cool down faster? Obvious, right? Check it out, you might be surprised!
Very nice demonstration ! Do I understand that the ability to absorb heat and the ability to emit it are basically the same physical property? If your two cans full of liquid started at room temp, and you put them in the refrigerator, will the taped one also get cold faster?
It's particularly interesting, because aluminum is such a good thermal conductor. How might you demonstrate this completely counter-intuitive difference between aluminum's high conductivity and its low emissivity ? What a great teaching tool !!
The ability to absorb thermal radiation and emit it are equal at the same IR wavelength. This is called Kirchoff's Law. So yes, the taped can would cool faster in the refrigerator.
In any heat transfer situation, one must think about all the heat tranfer mechanisms going on: conduction, convection, and radiation. Just because aluminum is a great conductor of heat, doesn't necessarily mean that it will always heat or cool the fastest.
Cool Interesting
ReplyDeleteVery nice demonstration ! Do I understand that the ability to absorb heat and the ability to emit it are basically the same physical property? If your two cans full of liquid started at room temp, and you put them in the refrigerator, will the taped one also get cold faster?
ReplyDeleteIt's particularly interesting, because aluminum is such a good thermal conductor. How might you demonstrate this completely counter-intuitive difference between aluminum's high conductivity and its low emissivity ? What a great teaching tool !!
The ability to absorb thermal radiation and emit it are equal at the same IR wavelength. This is called Kirchoff's Law. So yes, the taped can would cool faster in the refrigerator.
ReplyDeleteIn any heat transfer situation, one must think about all the heat tranfer mechanisms going on: conduction, convection, and radiation. Just because aluminum is a great conductor of heat, doesn't necessarily mean that it will always heat or cool the fastest.