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Materials and Thermal Emissivity
Name: Andre
Status: educator
Grade: 12+
Location: CA
Country: USA
Date: N/A
Question:
My students want more information as we
study reflectivity, absorption, emissivity, of
surfaces. I have located some tables showing values
for common materials such as metals and even paints.
What confuses them is that they cannot quickly 'get'
how a building, with its many elements, will be
affected during the cycle of the day when sunlight
strikes it, leaves it for night time, open sky is
available to surfaces, and then the cycle begins
again. Stored energy then affects the next day's
values of what the building's occupants experience.
I suspect, from my experience, that an intuitive
guess can be made of how a building, structure, or
even 'things' will behave when we are given the
right kind of graphs or stories or demonstrations.
It is a lot to ask. I think there is a lot to be
gained when we all 'get it'. Can you help? The
question was meant to be open ended so your special
skills might be noted.
Replies:
Hi Andre,
It is not clear to me exactly what you are asking here, nor exactly what
your students cannot "get".
It seems that you are attempting to determine temperature fluctuations
in a building over time, based on heat gain or loss caused by radiation,
since emissivity, absorptivity (the same phenomenon as emissivity),
and reflectivity are all criteria that relate only to radiation. But
radiation
is often only a smaller part of the total heat loss or heat gain of a
building. The majority of heat loss or gain is commonly due to
convection, and conduction (through the walls etc.). Emissivity,
absorptivity and reflectivity have nothing to do with convective or
conductive heat transfer.
If you are attempting to graph a building's internal temperature over
time, as it is influenced day and night by factors such as the sun, the
night sky, etc, you are missing the greater effect caused by convective
and conductive effects such as ambient temperature, wind, building
insulation and so on. I suspect that this may be a much more complex
question than you may have thought.
Regards,
Bob Wilson
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Update: June 2012
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