|How does heat transfer?|
|Question Date: 2012-12-19|
Hello! I'm so glad you asked this question.
First of all, I'd like to make a clarification
about what heat is. The way you have worded it,
one might think that heat is a "thing" that can
be "transferred." This is a very common
misconception! But heat is not a "thing" -- it
is a process. Heat is the transfer of energy
from one system or body to another, and it
occurs because the systems are trying to come to
thermal equilibrium with one another. In other
words, to attain thermal equilibrium, the
systems will exchange energy, and this process
is what we call "heat."
If you're wondering why systems end up
attaining thermal equilibrium, it actually has
to do with the concept of maximizing the entropy
by maximizing the number of "states" available
to a system. This is true of "isolated" systems.
It turns out that in an isolated system,
maximizing the entropy corresponds to the
situation where the temperatures of the two
bodies are equal. I hope this helps!
Heat can transfer by conduction (hot
materials conduct heat to colder materials in
which they are in contact), by convection (hot
materials move into areas where it is less hot,
happens in gasses and liquids), and by radiation
(hot materials emit energy in the form of light,
which in turn heats up stuff that the light
Heat can transfer through three main ways:
conduction, convection, and radiation.
Conduction happens when there's physical
contact, such as when you grab a piece of hot
metal, and the heat flows into your hand.
Convection happens when a gas or liquid moves
and transfers heat. When the molecules in a gas
(like air) get heated, they speed up and collide
with other molecules. Often, the density will
change, and the gas will also move. These
collisions and movement of the molecules spread
the energy to other parts of the gas, and this
creates currents and spreads the heat.
Radiative heating happens when the molecules or
atoms create electromagnetic waves. These waves--
usually infrared radiation--can travel through
vacuum, which is how things cool off in space
(and is also why you put metal foil around a
turkey to keep the heat in! The foil reflects
the radiation.) Radiative heating is also how
many of the heaters work at bus stops or in
public locations where the heater is far
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