Where
is
the emissivity of the surface. Emissivity measures how well
a surface approximates a blackbody, and ranges in value
between 0 and 1, where a value of 1 is a blackbody.
Table of some emissivities of some materials (Çengel
2010)
Emissivity isn't the only important
radiation related property of a surface. Another important
property is absorption, which gives the fraction of
radiation energy that intercepts a surface that is absorbed
by the surface. Absorptivity (
) has
a value between 0 and 1, where a value of 1 is a perfect
absorber and a blackbody. So, a black body is a perfect
emitter with a emissivity of 1, and a perfect absorber with
an absorptivity of 1.
Emissivity and absorptivity are related by Kirchhoff's Law,
which states that for a body in thermodynamic equilibrium,
the emissivity is equal to the absorptivity.
The other ways that electromagnetic radiation can interact
with a material is through reflection and transmission.
Reflection occurs when radiation bounces off of a surface
and is redirected, and transmission occurs when radiation
passes through a target. Radiation from a source can be
reflected, absorbed, or transmitted when it intercepts a
surface, and the energy related to these is related with:
It can be difficult to determine what the rate of heat
transfer is between two objects because of all of the
variables that are involved, such as the properties of the
surfaces of the objects, the object's orientation relative
to each other, and the properties of the medium that
radiation passes through. However, the math to solve the
rate of heat transfer between two objects can be simplified
if: the emissivity and surface area are relatively low at a
temperature
and is completely enclosed by a much larger surface at
and the medium through which the radiation passes through
has a negligible effect on the radiation, which will lead to
the equation:
(W)
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