The conduction of heat through a material
depends on two things: the temperature difference
across the material, and a property of the
material called the thermal conductivity.
Heat flows more quickly through a material with a
high thermal conductivity, so a material with
a low thermal conductivity is good for
insulating to prevent heat loss (or for
preventing something cold from warming up). Some
plastic materials have low thermal conductivities
and make good insulators.
However, another way to make an insulator is
to have a low-density material or porous
material, with lots of small internal spaces
or cavities. These materials incorporate lots of
air into them, and air is also an effective
insulator, as long as it can’t flow. As soon as
air can flow, it carries off heat with it, which
causes the object to quickly cool. An example
of a low-density insulating material is fiberglass
insulation. This material is made of glass
that has been spun into fibers.
Fiberglass insulation has a thermal
conductivity that is 20 times lower than solid
glass, and is therefore a much better insulator,
just due to its low density!
Covalently bonded materials tend not to
conduct electricity very effectively, while
metals and salts dissolved in water conduct
electricity extremely well. This means that
electrical insulators are often made from organic
compounds: plastic, rubber, and so forth.
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