Download Conceptual Physics. Tenth Edition

A NICE ‘WARM’ COAT
By Nienke Adamse,
9/17/10
“Put a warm coat on, you might catch a cold”! How many times
must we have heard that from good wishing people? In this
warning there are two myths: first of all you don’t catch a cold
from being cold, but this will require another essay… Second, you
don’t want your coat to be warm, although that feels comfortable
when you put it on, but you want your coat to keep you warm!
How does this work according to the physics of heat transfer?
According to the second law of thermodynamics (Hewitt, 2007):
“Heat never spontaneously flows from a lower-temperature
substance to a higher-temperature substance”, transfer of heat is
always from warmer objects to colder objects. A person is warm,
because it uses energy to live and part of that energy is generated
to keeping the body warm. A normal healthy person’s temperature
is 37 C. When the surrounding of a person is colder than 37 C,
heat transfer from the body to it’s surrounding will take place
and will not stop until the body and it’s surrounding have a
common temperature (thermal equilibrium) See figure 2. (Hewitt,
2007): “Whenever heat flows into or out of a system, the gain or
loss of thermal energy equals the amount of heat transferred”.
And: “The energy lost in one place is gained in another”. The
body heat is transmitted by conduction, and radiation (see figure
1). At the same time the cooler surrounding, cools the body by
convection. A cold wind that cools the body is the result of
convection currents in the atmosphere. It causes to speed up the
heat transfer as it replaces the transferred body heat by cooler air.
So, heat transfer from the body to the cooler air outside occurs in
three ways: by conduction, by convection and by radiation.
Conduction is the process where heat is being transferred through
molecular movements of particles, mainly electrons. It occurs in
solids, liquids as well as gases and from one phase to the other.
Convection occurs in liquids and gases, for heat transfer to occur
between a solid and a liquid or gas, the solid needs to touch a
liquid or a gas. Convection is the result of changes in density in
parts of a liquid or gas. This change in density is the result of
change in temperature. This process is called natural convection
whereas in a forced convection the speed of the heat transfer is
being increased by wind or other forces. Most insulation materials
prevent heat transfer by conduction and convection because of the
cellular structure of the material. The other way of heat transfer,
by radiation, can be decreased by absorption of the radiation
and by reflection. Radiation is the process where heat is being
emitted as a form of wave energy. Radiation does not need a
medium, like air, to be transported. All objects emit radiation
energy; this emission is being determined by the temperature of
the surface and the ability of the material to emit energy (Dark,
mat surfaces are good radiation absorbers (receivers) and emitters,
while glossy, reflecting materials are poor radiation absorbers
and emitters).
Conduction of heat can be slowed down by using materials that
are poor conductors Materials that are good conductors of heat
have atoms or molecules with loosely held electrons. Poor
conductors, or insulators are materials with molecules that firmly
hold the electrons are called insulators. A coat that keeps the
body heat from transferring to the cold surroundings needs to be
a good insulator. See figure 3.
The coat needs to be made of poor conducting materials and the
structure of the materials is such that it holds small pockets (or
cells) of air. Coats made out of fur or feathers (biological
insulation) or materials such as polyester have these small
pockets of air.
To prevent the body from cooling off by convention, the coat needs
to be preventing movement from air outside the body. The coat
needs to be windproof.
To prevent heat loss through radiation the coat should be made
out of materials that reflect radiation energy from the body, back
to the body on the inside and it should absorb radiation energy
from the sun on the outside.
The common mistake of calling a coat “warm’ can be illustrated
by the fact that putting a coat on a (dead) cold body does not
warm the body. If the body does not constantly generate heat, the
coat does not keep the body warm. To show this proof in a
classroom situation you could place a cup of cold water in a
Styrofoam box and take the temperature of the water before and
after you place the cup in the box. If the water does not become
warmer after placing it in the Styrofoam box, the ‘coat’ is not
warm.
REFERENCES
Hewitt, P.G. (2009) Conceptual Physics. Tenth Edition
Glenview, IL: Pearson Education
Touger, J. (2006) Physics. Building Understanding
Hoboken, NJ: John Wiley &Sons Inc.