Graystar,
believe it or not, I think we might be making "progress".
I am now "refreshed" and ready for a long work filled day. Saturday I know, but finals are finals and it would be a shame to waste 20 years of education!
Quote:
|
Such is the difficulty of trying to explain things in text in a fluid discussion. What I type may make sense to me (at the time) but not to others. So, maybe if we keep this up we might all end up on the same page at some point.
|
This is SO true, but you need to try and step back and get into the head of the author. Spend time making sure you really understand what has been said before picking through it.
Back to the subject in hand.
Would it help if I tried to explain it from a slightly different perspective.
Firstly, lets assume that we have a lump of metal on the table. This piece of metal is 1kg and has a specific heat capacity of 5 J/g/K.
If 5 KJ of thermal energy are "injected" into the block of metal, the temp rise would be defined as
(energy input) divided by (mass x specific heat capacity)
With the numbers I have suggested, the block would rise in temp by 1 degree. Now if we insulated this block such that none of the thermal energy can escape, it would stay at 1 degree above ambient.
So for no thermal energy going into or out of the block (perfect insulator) it will stay at the same temperature.
Now imagine we bury a resistive heater in the block, which has a power rating of 70W, ie 70 Joules of thermal energy per second.
At the same time we put a water jacket around the block inside the insulation which only removes 70W.
So we have 70W in and 70W out, IE, every second 70 Joules in and 70 Joules out. But we still have the original 5KJ, and that aint changing.
So the net change to the thermal enery of the block is zero. So, according to the specific heat capacity, the temperature will not change.
To raise the temp of the block some more, we need only add a finite amount of energy. Another 5KJ would raise the temp by 1 degree again.
So lets try and apply this to a processor. I'll do this from the other side as you seem insistent on considering what happens to the energy from the cpu, rather than starting at the radiator as I have suggested.
Suppose we have a processor at a temperature T. Doesn't matter what temp this is! This processor has a power output of 70W. So going back to our metal block example. If we don't want the temperature of the cpu to go up or down, then we need 70W to be removed. Most of this will be removed by the water, but some will also be conducted away by the pins, though this is minor and we will assume it is negligible.
So we want 70W of heat to be conducted into the water.
As we established before, thermal energy is conducted down a thermal gradient. The rate at which it is conducted is proportional to the value of the thermal gradient. So we need a thermal gradient which will allow the conduction of 70 Joules per second. We also need to get the heat into the water which is where the efficiency comes in. But back to the cpu.
We still want to keep the cpu at the original temperature T, so we need the water to be at a temperature T-x such that the gradient conducts 70W.
It doesn't matter what the value T is, provided that the temperature difference, x, will allow the conduction of 70W.
So again, we have no net change in the thermal energy in the processor, so according to the equation relating specific heat capacity and energy, thus no temperature change.
Again, x is the impotant value, not T, as it is the temperature DIFFERENCE which is necessary to conduct 70W.
Now lets say that we know what the water temperature is, because we do. It is determined by the air flow, the air temperature, the heat load - 70W - and the efficiency of the radiator.
So we know T-x, and we know x, we can work out the processor temperature.
So I have tried to explain what happens to the thermal energy from the moment it is converted from electrical energy in the processor, and at the last minute fixed the water temp.
So to recap,
1. 70W of thermal energy converted from electrical energy.
2. Temperature difference, x, drives the conduction of 70W of thermal energy away from the cpu.
3. We fix the water temperature by fixing the air temperature. Thus we know the cpu temperature.
4. The cpu stays the same temperature - SO THERE IS NO NET CHANGE IN ENERGY from the equation
deltaQ = m c deltaT
Does this make sense. I should have introduced this fundamental equation earlier.
By its definition, if the cpu is at a constant temperature, there must be no net change in thermal energy (delta Q).
In order for there to be no net change in thermal energy, the water/pins must be conducting away the same power as is porduced by the cpu.
Any questions, fire away.
Anyone else, does this make sense?
8-ball