Amos -
Conduction, convection and radiation are the three major methods of
heat transfer. That is, they are three different ways that heat can
spread.
Conduction is probably the simplest to understand. If you say that
something is hot, then what you are really saying is that the atoms
inside of it are moving around a lot. Even in a solid like a metal, the
atoms can move back and forth quite a bit. When the atoms move, they
bump into the atoms next to them, making them move. This starts a chain
reaction and the heat spreads through the whole object. This is what's
happening if you've ever set a pan on the stove. The handle gets warm
even though only the bottom touches the burner because the metal
conducts the heat. As it turns out, in metals heat is conducted not so
much by wiggling atoms, but mostly by wiggling electrons (the idea is
the same, though).
Convection is a bit trickier. Convection is a way for heat to move
through a fluid, like water or air. You may have heard of the phrase
'hot air rises.' This is convection. Hot air (or hot water, etc.) is
less dense than cooler air. So if you heat up the air at ground level,
it will rise up into the sky to be replaced by cooler air moving
downwards. This creates something called 'convection currents,' which
are one of the things that birds use to rise up into the air. To see
this at home, you can get a clear ice-cube and drop it into a glass of
warm colored water (food coloring is fine). You should be able to see
how the warm (colored) water moves upwards while the cold (clear) water
melting off the ice cube moves down.
Radiation has to do with light. Sounds weird? Not really. The light
that you see is actually made up of something called electromagnetic
waves. The electromagnetic waves that carry the kind of heat that we
are most familiar with (from a heat lamp, for example) have longer
wavelengths than the waves that make up visible light and hence they
are often called "infra-red". When something is hot, like the burner on
your stove, it releases some of its energy in the form of these
infrared waves. The waves travel through the air until they hit
something, which gets heated up. This is why you can feel the heat from
your stove even if your hand isn't actually touching it. The burner
literally 'radiates' heat.
-Tamara
(published on 10/22/2007)
