Objects having differing temperatures try to come into
equilibrium, by moving energy from the warmer to to cooler one.
There are several different ways to transport the energy:
Conduction through a material, which we studied in the last section
Convection of a fluid (a gas or a liquid)
Radiation of light
Changes of phase: evaporation or melting (these will be explored in another section)
Convection
Gases and liquids can move from place to place, and in the process
carry energy.
This is called convective heat transfer.
Again we can distinguish two important cases: forced and natural convection.
Using a fan, we can force the gas or liquid to move.
Very commonly buildings and houses are heated in winter by
adding energy to the air in the furnace (making it warmer),
and then blowing the air throughout the building to deliver
the energy elsewhere. In summer, the same building is
cooled by removing energy from the air in the airconditioner,
and then blowing the cooled air into the room, which displaces
the warm air (either outside or into the airconditioner).
Air expands and becomes less dense when it gets warmer, and then
warm air will float upwards through cooler air.
Likewise, warm water will float upwards through cooler water.
These are examples if
natural convection. Sometimes natural convection is used
to heat a house (the steam "radiators"
in older houses are devices to warm the air near them, with
the expectation that natural convection will then carry the
energy throughout the room).
Convection is very important
in the heating and cooling
of the atmosphere and oceans: the wind and ocean currents are natural convection at work.
Convection carries the smoke from your fireplace up the chimney
(and carries most of the energy the fireplace produces along with
it).
We learned in the previous section that air is a poor conductor.
Stationary air does not conduct energy well.
By having the air move, it can carry energy with it, and this
is a more effective way to move the energy around.
Fur and feathers
are means to immobilize air, and thus prevent convection.
Birds fluff up their feathers on cold days because this gives a
thicker layer of nonconducting, nonconvecting air.
Glass windows are intended to keep the wind from convecting
all our energy away, but glass is a rather good conductor of
heat, and so the air right next to a window is almost as cold as the
air outside.
The solution is to use
storm windows or other kinds of double-paned glass.
These
trap a thin layer of air that is not able to move easily;
this stops convection, and makes use of the insulating
properties of the air.
Curtains around the windows are another means to block convection --
they trap the cold air next to the window.