It's usually a metallic finned component that can be joined to an object that generates heat in order to remove the excess heat from that object. For example if an electronic component generates enough heat to damage itself during operation, the heatsink dissipates the excess heat.
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+1 BTW, heatsinks come in all different shapes and sizes. If you've ever seen the inside of a personal computer, you've probably seen a heatsink. Some pictures:http://www.google.com/images?hl=en&q=heatsink
They are also not exclusive to electronics. See the fins on an air cooled motorcycle engine, or a large oil filled transformer.
The others pretty much have it wrapped up -- if you look at the root of the word, 'heat' and 'sink' -- think what both are.One is average kinetic energy of matter...The other, when not just a noun involves 'draining', 'removing', 'transferring' etc. So, any time you have something hot and you need to get the hot from the hot thing to the not hot thing, you want to use a heat sink. Thermodynamics (the study of crap like this) has almost countless rules which apply to how energy is transferred in different situations. Some of the obvious ones (in a nutshell):1) Heat always travels from a hot object to a less hot object. "cooling" is almost a misnomer, as it's really 'heat-removing'.2) The more surface area is involved, the faster the transfer (most systems strive to have the most possible surface area (more fins on a cpu cooler, etc)3) The higher heat conductivity of the materials, the faster the transfer (better sinks are made of good conductors, such as copper or aluminum)4) Depending on the system, the specific heat capacity can play a role (humid air holds more heat, thus it makes air-cooled heatsinks more effective at the same temperature difference), etc.
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