Quote:
Originally Posted by lordsmurf
Again, TDP isn't a direct measure of heat expelled by a given system. But it's also not a measurement of mere power consumption. For example, given the TDP numbers against your heat measurement, I have to wonder if the CPU heatsink is inadequate, or needs a re-seat/re-paste. That's what the numbers tell me. It also tells me to expect the GPU to run hotter, though probably not 2x hotter realistically. Those are why the numbers exist.
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That's not how TDP works. That's not how any of this works!
Clearly, it's time for
Thermodynamics 101
with T. Fluff, PhD
(I hold a doctorate in The Science of Telling People they are Wrong on the Internet)
Let's review the fundamentals first, yeah? This is all high school physics, so you should probably know this. The laws of thermodynamics tell us two important things relevant to this discussion, namely that
a) energy can never be created or destroyed (so if we put some energy into a system we must get the same amount of energy out), and
b) entropy tends to increase, so if you have two bodies with different temperatures in thermic contact with each other energy will flow from the hotter one to the colder one until they reach equilibrium.
Now on to what this means in practice. In a computer, we input electrical energy. Some of this energy is converted to kinetic energy (to spin harddrive platters and fans), and some is converted to electromagnetic radiation mainly in the form of visible light (in LED's and in the monitor), but the vast majority of it eventually decays to thermal energy after being used to push some electrons around through a bunch of transistors. This heat has to go somewhere, and that somewhere eventually ends up being the air of the room. In a moment, we will calculate the magnitude of this effect, but first we need to clear up a misconception.
The TDP of a processor is an estimated ballpark number of the amount of thermal energy it generates in a given fictive scenario that's supposed to represent a typical peak workload. In any other scenario (such as most scenarios you'll find in reality), the actual amount of heat generated is different - the TDP is only supposed to be a rough estimate of the maximum sustained heat generation possible. The TDP number has absolutely nothing to do with
any of the following:
- amount of heat generated at idle
- amount of electrical power consumed at idle
- temperature of the silicon in any given situation
In practice, the amount of thermal energy generated by a processor is pretty much equivalent to its electrical power consumption because almost all of the electrical energy quickly decays to heat. The first law of thermodynamics also tells us that we cannot possibly get more thermal energy out of a processor than the amount of electrical energy we put into it. If you look at the processor's power consumption then, you will have a good idea of how much heat it's producing. Modern CPU's and GPU's are very good at clocking down (and more importantly, reducing the voltage) at idle and so you'll see a typical idle power consumption of 10-20 watts. The power consumption - and by extension, thermal energy generation - still doesn't have anything to do with the temperature of the chip, though. See, temperature is a measure of energy, but it's a measure of
stored energy. Two chips consuming the same amount of electrical energy will heat your room
exactly the same, even if one is twice as hot as the other. The only thing that's different in the hotter chip is that the energy stays in it for longer before dissipating into the room.
Speaking of energy storage, to calculate the heating effect of an idling CPU we first need to discuss specific heat capacity. Different substances can store different amounts of thermal energy, and the specific heat capacity is a measure of how much energy a substance can store per unit mass. Or, in more practical terms - heating one kilogram of water by one degree Kelvin takes about four times as much energy as heating one kilogram of air by one degree Kelvin. Many metals have very low specific heat capacity, meaning it takes little energy to heat them up, but conversely that also means they're bad at retaining that energy and they quickly cool down again. For example, copper (commonly used in heatsinks because of its excellent thermal conductivity) has a specific heat capacity of 0.385 J/gK (joules per gram kelvin difference - it takes 0.385 joules of energy to heat one gram of copper by one degree kelvin). Air at typical indoor conditions has a specific heat capacity of about 1.01 J/gK.
If we then assume a spherical CPU in a vacuum... uh, no, I mean, a small 30 square meter studio apartment with the minimum indoor ceiling height of 2.4 meters allowed by the building code in these parts, we can easily calculate that the 72 cubic meters of air inside weighs around 92 kilograms. Given the previously discussed specific heat capacity, heating 92 kilograms of air by one degree kelvin (or equivalently in this case, one degree celsius) takes 92.9 kilojoules of energy. Now, a watt is a joule per second, so an idling CPU consuming 10 watts of energy would take 9290 seconds (or close to 2 hours and 35 minutes) to heat the apartment by one degree kelvin. Do note though that this of course assumes completely unrealistic conditions, for example that the apartment is perfectly thermally insulated against the outside world, so it is of course necessary for there to be no ventilation whatsoever. The building code here demands that the ventilation of private dwellings should change the indoor air at
least once every two hours, making the job of that idling CPU a Sisyphean task.
So, in conclusion, no, my heatsink isn't inadequately fastened, and the TDP has nothing to do with this at all. I could easily transfer the heat out of the CPU quicker and thereby making it cooler by running the CPU fan faster, but why on earth would I? There's absolutely no reason to.