Heat Flow Direction
Name: Hans F.
"Heat passes from a warmer body to a cooler" -
I have taught 11 year old children, during their observations of melting
ice, the above statement. A colleague, however, argued that the reverse
was true! I have searched the web and found much to support my argument,
but could not find a definitive answer to point to an author of the
statement - as in 'Charles' Law" or 'Boyle's Law', etc.
In the usual definition of the terms, you are correct that heat "flows"
from a "hotter" zone to a "colder" zone. The reason you are finding it
difficult to find a "definitive answer" is that when one proceeds from the
"common sense" definition of temperature to a scientifically rigorous
definition, things get very subtle. The fact of the matter is that
"temperature" is not a simple concept, and there is a risk of
over-explaining it to adults much less an 11 year old. To give you some
inkling about the difficulty consider that we assign larger numbers to
hotter zones and smaller numbers to colder zones. This is purely
convention! It could just as well be the other way around and nothing
really changes. In addition, when one tries to "define" terms like "heat"
one gets caught up in circular definitions. I wish I had a really simple
answer, but the answer is simply not simple.
Your first statement is true. Heat spontaneously flows from warmer to
cooler temperatures, in any passive situation.
That is approximately the definition of temperature itself.
But heat can also be _pumped_ from a cooler body to a warmer one, by a
thermodynamic machine, at some cost in entropy and energy and complexity.
And at a molecular level, bits of kinetic energy are always going both
ways across any given boundary,
and what we consider heat flow is the difference between the leftwards and
averaged over a specific range of time and space.
I do not offhand know what idea your colleague was referring to.
Perhaps if you could have told me more of what he typically says, I could
address it more directly.
Actually, the answer lies in the laws of thermodynamics. Two bodies of
unequal temperature when in contact will move toward thermodynamic
equilibrium. Heat will move from the hotter body to the colder one.
One can view this as moving from an ordered state to a disordered one,
hence the second law that entropy always increases. In order to move
heat from a colder body to a warmer body, an external source of work
must be provided. This is why electricity must be used to power a heat
pump to heat your home, why you must plug your refrigerator in to keep
it cool. It's all based on the fundamentals of thermodynamics.
Given two identical gases at different temperatures are separated by a
barrier. The hotter gas consists of molecules with larger average kinetic
energy than the molecules of the cooler gas. When the gases are combined,
the temperature of the mixture settles at an equilibrium temperature that
is between the temperature of the hot gas and the temperature of the cold
gas. The heat flows from the warmer gas to the cooler gas until the
average kinetic energy of the two gases are the same. Also, the Second
Law of Thermodynamics states that in an isolated system (one that has no
interactions with the surroundings) internal portions at different
temperatures will always adjust to a single uniform temperature and thus
I hope that this helps.
Some information taken from Microsoft Encarta DVD, 2005
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Update: June 2012