Ask A Scientist© Environmental Earth Science Archive

name         Colleen
status       student
grade        6-8
location     N/A

Question -   Why does a heat sink work? I know that one material takes
the heat away from another material so that material does not expand.
I did a butter experiment with this and the butter fell off of each
type of metal after a number of seconds then we added a bulldog clip
and it took longer. But why does this work?
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One of the laws of thermodynamics is that there is an desire for 
equilibrium between two different temperature areas (a very simplified 
explanation).  That is to say, if you have on area that has a higher 
temperature next to an area that has a lower temperature, the heat will 
"flow" to the cooler region to help raise its temperature while 
simultaneously cooling itself off and eventually they will both be the 
same temperature.   Your butter experiment is exactly what is going 
on.  The butter is cooler and the metal is warmer.  The metal will give up 
some of its heat to the butter raising the temperature of the butter while 
cooling the temperature of the metal.  When you add the heat sink to the 
metal, you have created a third area into which the metal gives its 
heat.  Now instead of all the heat flowing to the butter, some of the heat 
is going to the butter AND the heat sink.
Heat sinks aren't just for stopping the expansion of metals, they are 
instrumental in whisking heat away from devices that are very sensitive to 
heat problems.  Electronics are the best example.  Processor chips in you 
computer are very sensitive to heat.  If they see too much heat, they will 
start to loose their number crunching abilities, so designers put heat 
sinks along with fans on the processor to whisk heat away.  That is one 
reason you always hear a fan going when you turn you computer on.
Hope this helped explain things a little better.  Thanks for using Newton.

Christopher Murphy, P.E.
AFRL
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Heat sinks are used for the purpose of preventing something from getting too
hot.  A typical example is an integrated circuit chip in a computer or a
transistor.  The chip is small but dissipates a considerable amount of power
for its size (tens of watts).

If the chip were simply connected to the rest of the computer with wires, it
would get hot and fail.  Instead, the chip is connected by special glue or
bonding onto a metal heat sink plate that makes a good connection to the
chip.  The purpose of the heat sink is to spread the heat over a larger area
enabling it keep cooler.  The heat flows from the chip to the large heat
sink (which might be cooled by air from a fan.)

Powerful super computers might have liquid-cooled heat sinks that the chips
are bonded to.

When people think of heat sinks they commonly think of a metal device that
has no moving parts, and the item that you want cooled is clamped onto it.

  There are other types of cooling systems, some of which may have parts or
be electrical, and these have other names.

A "sink" is also a concept in science.  In science, if you want to say that
there is a drain or a way out for something, you might call it a "sink."
You might have a sink for electrons,  a sink for heat, or a sink for
positrons.  You can even refer to a "phonon sink" in physics.

Bob Erck
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Not really my field.
But, my layman's opinion.  You hit the nail on the head, with the diversion
statement.
Since heat is not visible to the naked eye, imagine the heat as water.  My
experience with a heat sink was in soldering.  You place the sink beyond the
piece you are soldering.  The idea to protect the items, further down the
line.  So, let us assume the heat is water.  You would place a diversion pipe.
The heat, or water flowing from the soldering gun, is bypassed, thus
protecting the items further along the line.

Now, the way the heat sink works, it is a material that absorbs heat easily.
I would guess its molecules and composition have more room.  So, with the
water analogy, it would be like a sponge at the end of the pipe.  So, you can
see, it is not a sure thing, there would be a limit.  With the sponge, it
becomes full and loses some to the air, but ultimately, the path is reformed.
The folks at the Alamo held off Santa Anna, but were overwhelmed.  If you
solder too long, you either have to add more heat sinks, or wait to let the
heat dissipate to the air etc.  It appears your experiment showed there was
a limit.

James Przewoznik
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There are many kinds of heat sinks.  In photosynthesis experiments, my students
use a heat sink involving nothing more than a one gallon aquarium filled with
water.  The light used in the experiment often is an incandescent bulb and the
heat from the bulb will effect the results.  The aquarium water acts to absorb
the heat from the light bulb so only the light reaches the experiment.  This is
generally how all heat sinks work in simple terms.

Steve Sample
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There are 2 factors to consider in heat sinks.  how much heat can the heat
sink absorb, and how much heat can it dissipate.

The heat capacity of a piece of metal depends not only on how large it is
(large objects tend to be harder to change the temperature of), but also
what kind of metal it is.  Aluminum for example, takes a lot more energy to
heat up than copper or iron.  So part of the heat that was previously going
straight to the butter is now working to heat up the heat sink instead. So
imagine the difference between filling up one 5 gallon bucket with water, or
filling up two 5 gallon buckets from the same hose.

Dissipate just means "get rid of"  Heat sinks used in computers have to be
very good at getting rid of heat, and usually the only way to do that is to
heat the air around them instead.  So an object with a lot of surface area
touches a lot of air, and will be getting rid of heat as well as absorbing
it itself.  (Now imagine one of your buckets has holes in it.)

Ryan Belscamper
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In the simplest case, it is all about sharing: you have some total amount
of energy, and it is going to be shared by all the molecules that are
physically in contact. If you have more molecules, they each get less
energy, naturally.  [Think of ten kids with one bag of candy.  Now add
a few more kids.]

In more complicated cases, it is also important to consider the rate at
which heat energy moves through materials.  Suppose you did not actually
attach the bulldog clip, but only put it near the hot metal.  You would
still have a heat sink -- an object that will want its share of the total
energy -- but the heat energy would have to travel through air to get to
the bulldog clip, and air is relatively poor conductor of heat.  In this
case, the bulldog clip would get a smaller share of the energy.  [Think of
ten kids, with ten straws, drinking from one glass of water.  Now add a
few more kids, but give the new kids really narrow straws.]

Tim Mooney
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