This is driving me crazy


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Subject: This is driving me crazy
Name: Hugh
Date: 3/28/2003 1:25:05 PM (GMT-7)
IP Address: 132.206.186.52
In Reply to: Re: say what? posted by Dan Druff
Message:

I don't want to take this the wrong way Dan Druff, but I really think that you are missing some fundamental concept here. I can't really be considered an expert in the field of thermodynamics, but I do have some clue of what I am talking about. After all, I am finishing up my Masters degree in physics. I'm not going to launch into any sort of rant though. Besides, I think that Ludicrous has already done an admirable job in explaining the workings of the heatsink.

After reading your posts (Dan Druff), I think that your problem lies in the fact that your viewing the heatsink as being apart from the CPU. For example, in your last post you said:

"So the main problem here is how fast the heatsink/fan removes the heat from the CPU"

Instead, you should view the heatsink as an extension of the CPU. You'll agree with me that a CPU die surface area of 100 cm^2 cool down a lot faster than one that only has a surface area of 1 cm^2. It's like when you have a serving of burning food, like mashed potatoes or something. If you want to cool it down faster, you'll instinctively spread it around. The reason it cools faster this way is because you've increased the surface area of the mashed potatoes that can dissipate heat. If instead you leave the potatoes in a tight clump, the heat on the inside will have more trouble getting out than if the potatoes were flattened on the plate.

So, let's come back to the heatsink now. Metals like aluminum and copper are extremely good thermal conductors. There's no comparison between their ability to transfer heat and that of air. The reason why goose down is such a warm thing to make winter jackets out of is because down is so fluffy. In other words, it is the air held by the down that keeps all of your heat inside the jacket. So air = thermal INSULATOR, aluminum and copper are thermal CONDUCTORS. What I'm getting at is that as long as there is a good contact between the heat sink and the cpu (i.e. no air between both), the heat sink will act like an extension of the cpu's surface. From a thermal point of view, they are one. So if I want the CPU to cool down as fast as possible, I'll want to increase the surface area as much as I can, just like with the mashed potatoes. It wouldn't really be convenient if CPU manufacturers made CPU's which had surface areas of 50cm by 50cm (=2500cm^2) in order to dissipate all of that heat they produce. But that's exactly what is achieved by placing a heat sink like the Zalman CNPS6000-Cu. This particular heat sink has a surface area in excess of 2600cm^2! Remeber, the reason why this works so well is that, compared to air, heat isn't hindered in any way when going from the CPU to the heatsink. So when you say:

"Everything you said about surface area,radiation and convection of heat etc. is true but only if the CPU/puter is functioning at idle. The whole scenario changes when the CPU is running at peak."

you are getting things all wrong. In this situation, where the CPU is running at peak strength, it's even MORE important to have a heat sink. When you argue that:

"The heat build up exceeds the removal of it causing problems."

you are indeed correct. CPU's overheat when the heat buildup exceeds it's removal, EXCEPT for the fact that without a heatsink, this heat will buidup A LOT faster than with a heat sink. If I remember correctly, rate of heat loss is proportional to surface area. Say you have a CPU with a surface area of 2cm^2. If you compare the heat dissipation rate of this CPU with and without a heat sink (say the Zalman, with A ~ 2000cm^2) you will see that the bare CPU looses heat about 1000X slower than with the heat sink. In other words, running a cpu without a heatsink is something that you want to avoid.

Anyway, I hope that you understand what I'm trying to say. It looks like this did turn into a rant after all! I don't want to insult anyone, just clear up a few things.

Cheers,

Hugh




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Zalman CNPS6000-Cu heat sink


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